diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 650525a2f520..a219f1dc0329 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -2877,7 +2877,7 @@ pub enum ModKind {
     /// or with definition outlined to a separate file `mod foo;` and already loaded from it.
     /// The inner span is from the first token past `{` to the last token until `}`,
     /// or from the first to the last token in the loaded file.
-    Loaded(ThinVec<P<Item>>, Inline, ModSpans),
+    Loaded(ThinVec<P<Item>>, Inline, ModSpans, Result<(), ErrorGuaranteed>),
     /// Module with definition outlined to a separate file `mod foo;` but not yet loaded from it.
     Unloaded,
 }
diff --git a/compiler/rustc_ast/src/expand/autodiff_attrs.rs b/compiler/rustc_ast/src/expand/autodiff_attrs.rs
index 05714731b9d4..7ef8bc179738 100644
--- a/compiler/rustc_ast/src/expand/autodiff_attrs.rs
+++ b/compiler/rustc_ast/src/expand/autodiff_attrs.rs
@@ -6,7 +6,6 @@
 use std::fmt::{self, Display, Formatter};
 use std::str::FromStr;
 
-use crate::expand::typetree::TypeTree;
 use crate::expand::{Decodable, Encodable, HashStable_Generic};
 use crate::ptr::P;
 use crate::{Ty, TyKind};
@@ -79,10 +78,6 @@ pub struct AutoDiffItem {
     /// The name of the function being generated
     pub target: String,
     pub attrs: AutoDiffAttrs,
-    /// Describe the memory layout of input types
-    pub inputs: Vec<TypeTree>,
-    /// Describe the memory layout of the output type
-    pub output: TypeTree,
 }
 #[derive(Clone, Eq, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
 pub struct AutoDiffAttrs {
@@ -262,22 +257,14 @@ impl AutoDiffAttrs {
         !matches!(self.mode, DiffMode::Error | DiffMode::Source)
     }
 
-    pub fn into_item(
-        self,
-        source: String,
-        target: String,
-        inputs: Vec<TypeTree>,
-        output: TypeTree,
-    ) -> AutoDiffItem {
-        AutoDiffItem { source, target, inputs, output, attrs: self }
+    pub fn into_item(self, source: String, target: String) -> AutoDiffItem {
+        AutoDiffItem { source, target, attrs: self }
     }
 }
 
 impl fmt::Display for AutoDiffItem {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "Differentiating {} -> {}", self.source, self.target)?;
-        write!(f, " with attributes: {:?}", self.attrs)?;
-        write!(f, " with inputs: {:?}", self.inputs)?;
-        write!(f, " with output: {:?}", self.output)
+        write!(f, " with attributes: {:?}", self.attrs)
     }
 }
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 2c09059fe190..0e299c604790 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -1212,7 +1212,12 @@ impl WalkItemKind for ItemKind {
             ItemKind::Mod(safety, mod_kind) => {
                 visit_safety(vis, safety);
                 match mod_kind {
-                    ModKind::Loaded(items, _inline, ModSpans { inner_span, inject_use_span }) => {
+                    ModKind::Loaded(
+                        items,
+                        _inline,
+                        ModSpans { inner_span, inject_use_span },
+                        _,
+                    ) => {
                         items.flat_map_in_place(|item| vis.flat_map_item(item));
                         vis.visit_span(inner_span);
                         vis.visit_span(inject_use_span);
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index a7f7c37693a8..a6232b6dfaf8 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -380,7 +380,7 @@ impl WalkItemKind for ItemKind {
                 try_visit!(visitor.visit_fn(kind, span, id));
             }
             ItemKind::Mod(_unsafety, mod_kind) => match mod_kind {
-                ModKind::Loaded(items, _inline, _inner_span) => {
+                ModKind::Loaded(items, _inline, _inner_span, _) => {
                     walk_list!(visitor, visit_item, items);
                 }
                 ModKind::Unloaded => {}
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 7d6c41992eb0..d63131eacb55 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -238,7 +238,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 })
             }
             ItemKind::Mod(_, mod_kind) => match mod_kind {
-                ModKind::Loaded(items, _, spans) => {
+                ModKind::Loaded(items, _, spans, _) => {
                     hir::ItemKind::Mod(self.lower_mod(items, spans))
                 }
                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 64e91c91e2ca..290c2e529701 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1029,7 +1029,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     self.dcx().emit_err(errors::UnsafeItem { span, kind: "module" });
                 }
                 // Ensure that `path` attributes on modules are recorded as used (cf. issue #35584).
-                if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _))
+                if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _, _))
                     && !attr::contains_name(&item.attrs, sym::path)
                 {
                     self.check_mod_file_item_asciionly(item.ident);
diff --git a/compiler/rustc_builtin_macros/src/test_harness.rs b/compiler/rustc_builtin_macros/src/test_harness.rs
index b2048c534a48..e7ff65e08f9c 100644
--- a/compiler/rustc_builtin_macros/src/test_harness.rs
+++ b/compiler/rustc_builtin_macros/src/test_harness.rs
@@ -141,8 +141,10 @@ impl<'a> MutVisitor for TestHarnessGenerator<'a> {
 
         // We don't want to recurse into anything other than mods, since
         // mods or tests inside of functions will break things
-        if let ast::ItemKind::Mod(_, ModKind::Loaded(.., ast::ModSpans { inner_span: span, .. })) =
-            item.kind
+        if let ast::ItemKind::Mod(
+            _,
+            ModKind::Loaded(.., ast::ModSpans { inner_span: span, .. }, _),
+        ) = item.kind
         {
             let prev_tests = mem::take(&mut self.tests);
             walk_item_kind(
diff --git a/compiler/rustc_codegen_gcc/src/lib.rs b/compiler/rustc_codegen_gcc/src/lib.rs
index 452e92bffa23..096e9a596179 100644
--- a/compiler/rustc_codegen_gcc/src/lib.rs
+++ b/compiler/rustc_codegen_gcc/src/lib.rs
@@ -93,6 +93,7 @@ use gccjit::{CType, Context, OptimizationLevel};
 #[cfg(feature = "master")]
 use gccjit::{TargetInfo, Version};
 use rustc_ast::expand::allocator::AllocatorKind;
+use rustc_ast::expand::autodiff_attrs::AutoDiffItem;
 use rustc_codegen_ssa::back::lto::{LtoModuleCodegen, SerializedModule, ThinModule};
 use rustc_codegen_ssa::back::write::{
     CodegenContext, FatLtoInput, ModuleConfig, TargetMachineFactoryFn,
@@ -439,6 +440,14 @@ impl WriteBackendMethods for GccCodegenBackend {
     ) -> Result<ModuleCodegen<Self::Module>, FatalError> {
         back::write::link(cgcx, dcx, modules)
     }
+    fn autodiff(
+        _cgcx: &CodegenContext<Self>,
+        _module: &ModuleCodegen<Self::Module>,
+        _diff_fncs: Vec<AutoDiffItem>,
+        _config: &ModuleConfig,
+    ) -> Result<(), FatalError> {
+        unimplemented!()
+    }
 }
 
 /// This is the entrypoint for a hot plugged rustc_codegen_gccjit
diff --git a/compiler/rustc_codegen_llvm/messages.ftl b/compiler/rustc_codegen_llvm/messages.ftl
index 63c64269eb80..3982c37528df 100644
--- a/compiler/rustc_codegen_llvm/messages.ftl
+++ b/compiler/rustc_codegen_llvm/messages.ftl
@@ -1,3 +1,5 @@
+codegen_llvm_autodiff_without_lto = using the autodiff feature requires using fat-lto
+
 codegen_llvm_copy_bitcode = failed to copy bitcode to object file: {$err}
 
 codegen_llvm_dynamic_linking_with_lto =
@@ -47,6 +49,8 @@ codegen_llvm_parse_bitcode_with_llvm_err = failed to parse bitcode for LTO modul
 codegen_llvm_parse_target_machine_config =
     failed to parse target machine config to target machine: {$error}
 
+codegen_llvm_prepare_autodiff = failed to prepare autodiff: src: {$src}, target: {$target}, {$error}
+codegen_llvm_prepare_autodiff_with_llvm_err = failed to prepare autodiff: {$llvm_err}, src: {$src}, target: {$target}, {$error}
 codegen_llvm_prepare_thin_lto_context = failed to prepare thin LTO context
 codegen_llvm_prepare_thin_lto_context_with_llvm_err = failed to prepare thin LTO context: {$llvm_err}
 
diff --git a/compiler/rustc_codegen_llvm/src/back/lto.rs b/compiler/rustc_codegen_llvm/src/back/lto.rs
index f6d2ec24da6f..4adf99e91d08 100644
--- a/compiler/rustc_codegen_llvm/src/back/lto.rs
+++ b/compiler/rustc_codegen_llvm/src/back/lto.rs
@@ -604,7 +604,14 @@ pub(crate) fn run_pass_manager(
     debug!("running the pass manager");
     let opt_stage = if thin { llvm::OptStage::ThinLTO } else { llvm::OptStage::FatLTO };
     let opt_level = config.opt_level.unwrap_or(config::OptLevel::No);
-    unsafe { write::llvm_optimize(cgcx, dcx, module, config, opt_level, opt_stage) }?;
+
+    // If this rustc version was build with enzyme/autodiff enabled, and if users applied the
+    // `#[autodiff]` macro at least once, then we will later call llvm_optimize a second time.
+    let first_run = true;
+    debug!("running llvm pm opt pipeline");
+    unsafe {
+        write::llvm_optimize(cgcx, dcx, module, config, opt_level, opt_stage, first_run)?;
+    }
     debug!("lto done");
     Ok(())
 }
diff --git a/compiler/rustc_codegen_llvm/src/back/write.rs b/compiler/rustc_codegen_llvm/src/back/write.rs
index 66ca4e2b4738..499523699d4f 100644
--- a/compiler/rustc_codegen_llvm/src/back/write.rs
+++ b/compiler/rustc_codegen_llvm/src/back/write.rs
@@ -8,6 +8,7 @@ use libc::{c_char, c_int, c_void, size_t};
 use llvm::{
     LLVMRustLLVMHasZlibCompressionForDebugSymbols, LLVMRustLLVMHasZstdCompressionForDebugSymbols,
 };
+use rustc_ast::expand::autodiff_attrs::AutoDiffItem;
 use rustc_codegen_ssa::back::link::ensure_removed;
 use rustc_codegen_ssa::back::versioned_llvm_target;
 use rustc_codegen_ssa::back::write::{
@@ -28,7 +29,7 @@ use rustc_session::config::{
 use rustc_span::symbol::sym;
 use rustc_span::{BytePos, InnerSpan, Pos, SpanData, SyntaxContext};
 use rustc_target::spec::{CodeModel, RelocModel, SanitizerSet, SplitDebuginfo, TlsModel};
-use tracing::debug;
+use tracing::{debug, trace};
 
 use crate::back::lto::ThinBuffer;
 use crate::back::owned_target_machine::OwnedTargetMachine;
@@ -530,9 +531,38 @@ pub(crate) unsafe fn llvm_optimize(
     config: &ModuleConfig,
     opt_level: config::OptLevel,
     opt_stage: llvm::OptStage,
+    skip_size_increasing_opts: bool,
 ) -> Result<(), FatalError> {
-    let unroll_loops =
-        opt_level != config::OptLevel::Size && opt_level != config::OptLevel::SizeMin;
+    // Enzyme:
+    // The whole point of compiler based AD is to differentiate optimized IR instead of unoptimized
+    // source code. However, benchmarks show that optimizations increasing the code size
+    // tend to reduce AD performance. Therefore deactivate them before AD, then differentiate the code
+    // and finally re-optimize the module, now with all optimizations available.
+    // FIXME(ZuseZ4): In a future update we could figure out how to only optimize individual functions getting
+    // differentiated.
+
+    let unroll_loops;
+    let vectorize_slp;
+    let vectorize_loop;
+
+    // When we build rustc with enzyme/autodiff support, we want to postpone size-increasing
+    // optimizations until after differentiation. FIXME(ZuseZ4): Before shipping on nightly,
+    // we should make this more granular, or at least check that the user has at least one autodiff
+    // call in their code, to justify altering the compilation pipeline.
+    if skip_size_increasing_opts && cfg!(llvm_enzyme) {
+        unroll_loops = false;
+        vectorize_slp = false;
+        vectorize_loop = false;
+    } else {
+        unroll_loops =
+            opt_level != config::OptLevel::Size && opt_level != config::OptLevel::SizeMin;
+        vectorize_slp = config.vectorize_slp;
+        vectorize_loop = config.vectorize_loop;
+    }
+    trace!(
+        "Enzyme: Running with unroll_loops: {}, vectorize_slp: {}, vectorize_loop: {}",
+        unroll_loops, vectorize_slp, vectorize_loop
+    );
     let using_thin_buffers = opt_stage == llvm::OptStage::PreLinkThinLTO || config.bitcode_needed();
     let pgo_gen_path = get_pgo_gen_path(config);
     let pgo_use_path = get_pgo_use_path(config);
@@ -596,8 +626,8 @@ pub(crate) unsafe fn llvm_optimize(
             using_thin_buffers,
             config.merge_functions,
             unroll_loops,
-            config.vectorize_slp,
-            config.vectorize_loop,
+            vectorize_slp,
+            vectorize_loop,
             config.no_builtins,
             config.emit_lifetime_markers,
             sanitizer_options.as_ref(),
@@ -619,6 +649,83 @@ pub(crate) unsafe fn llvm_optimize(
     result.into_result().map_err(|()| llvm_err(dcx, LlvmError::RunLlvmPasses))
 }
 
+pub(crate) fn differentiate(
+    module: &ModuleCodegen<ModuleLlvm>,
+    cgcx: &CodegenContext<LlvmCodegenBackend>,
+    diff_items: Vec<AutoDiffItem>,
+    config: &ModuleConfig,
+) -> Result<(), FatalError> {
+    for item in &diff_items {
+        trace!("{}", item);
+    }
+
+    let llmod = module.module_llvm.llmod();
+    let llcx = &module.module_llvm.llcx;
+    let diag_handler = cgcx.create_dcx();
+
+    // Before dumping the module, we want all the tt to become part of the module.
+    for item in diff_items.iter() {
+        let name = CString::new(item.source.clone()).unwrap();
+        let fn_def: Option<&llvm::Value> =
+            unsafe { llvm::LLVMGetNamedFunction(llmod, name.as_ptr()) };
+        let fn_def = match fn_def {
+            Some(x) => x,
+            None => {
+                return Err(llvm_err(diag_handler.handle(), LlvmError::PrepareAutoDiff {
+                    src: item.source.clone(),
+                    target: item.target.clone(),
+                    error: "could not find source function".to_owned(),
+                }));
+            }
+        };
+        let target_name = CString::new(item.target.clone()).unwrap();
+        debug!("target name: {:?}", &target_name);
+        let fn_target: Option<&llvm::Value> =
+            unsafe { llvm::LLVMGetNamedFunction(llmod, target_name.as_ptr()) };
+        let fn_target = match fn_target {
+            Some(x) => x,
+            None => {
+                return Err(llvm_err(diag_handler.handle(), LlvmError::PrepareAutoDiff {
+                    src: item.source.clone(),
+                    target: item.target.clone(),
+                    error: "could not find target function".to_owned(),
+                }));
+            }
+        };
+
+        crate::builder::generate_enzyme_call(llmod, llcx, fn_def, fn_target, item.attrs.clone());
+    }
+
+    // FIXME(ZuseZ4): support SanitizeHWAddress and prevent illegal/unsupported opts
+
+    if let Some(opt_level) = config.opt_level {
+        let opt_stage = match cgcx.lto {
+            Lto::Fat => llvm::OptStage::PreLinkFatLTO,
+            Lto::Thin | Lto::ThinLocal => llvm::OptStage::PreLinkThinLTO,
+            _ if cgcx.opts.cg.linker_plugin_lto.enabled() => llvm::OptStage::PreLinkThinLTO,
+            _ => llvm::OptStage::PreLinkNoLTO,
+        };
+        // This is our second opt call, so now we run all opts,
+        // to make sure we get the best performance.
+        let skip_size_increasing_opts = false;
+        trace!("running Module Optimization after differentiation");
+        unsafe {
+            llvm_optimize(
+                cgcx,
+                diag_handler.handle(),
+                module,
+                config,
+                opt_level,
+                opt_stage,
+                skip_size_increasing_opts,
+            )?
+        };
+    }
+    trace!("done with differentiate()");
+
+    Ok(())
+}
+
 // Unsafe due to LLVM calls.
 pub(crate) unsafe fn optimize(
     cgcx: &CodegenContext<LlvmCodegenBackend>,
@@ -641,6 +748,8 @@ pub(crate) unsafe fn optimize(
         unsafe { llvm::LLVMWriteBitcodeToFile(llmod, out.as_ptr()) };
     }
 
+    // FIXME(ZuseZ4): support SanitizeHWAddress and prevent illegal/unsupported opts
+
     if let Some(opt_level) = config.opt_level {
         let opt_stage = match cgcx.lto {
             Lto::Fat => llvm::OptStage::PreLinkFatLTO,
@@ -648,7 +757,20 @@ pub(crate) unsafe fn optimize(
             _ if cgcx.opts.cg.linker_plugin_lto.enabled() => llvm::OptStage::PreLinkThinLTO,
             _ => llvm::OptStage::PreLinkNoLTO,
         };
-        return unsafe { llvm_optimize(cgcx, dcx, module, config, opt_level, opt_stage) };
+
+        // If we know that we will later run AD, then we disable vectorization and loop unrolling
+        let skip_size_increasing_opts = cfg!(llvm_enzyme);
+        return unsafe {
+            llvm_optimize(
+                cgcx,
+                dcx,
+                module,
+                config,
+                opt_level,
+                opt_stage,
+                skip_size_increasing_opts,
+            )
+        };
     }
     Ok(())
 }
diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
index b5bb7630ca6c..6a2c84f61082 100644
--- a/compiler/rustc_codegen_llvm/src/builder.rs
+++ b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -5,6 +5,7 @@ use std::{iter, ptr};
 use libc::{c_char, c_uint};
 use rustc_abi as abi;
 use rustc_abi::{Align, Size, WrappingRange};
+use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, DiffActivity, DiffMode};
 use rustc_codegen_ssa::MemFlags;
 use rustc_codegen_ssa::common::{IntPredicate, RealPredicate, SynchronizationScope, TypeKind};
 use rustc_codegen_ssa::mir::operand::{OperandRef, OperandValue};
@@ -24,17 +25,276 @@ use rustc_span::Span;
 use rustc_target::callconv::FnAbi;
 use rustc_target::spec::{HasTargetSpec, SanitizerSet, Target};
 use smallvec::SmallVec;
-use tracing::{debug, instrument};
+use tracing::{debug, instrument, trace};
 
 use crate::abi::FnAbiLlvmExt;
 use crate::attributes;
 use crate::common::Funclet;
 use crate::context::CodegenCx;
-use crate::llvm::{self, AtomicOrdering, AtomicRmwBinOp, BasicBlock, False, True};
+use crate::llvm::AttributePlace::Function;
+use crate::llvm::{self, AtomicOrdering, AtomicRmwBinOp, BasicBlock, False, Metadata, True};
 use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
 
+fn get_params(fnc: &Value) -> Vec<&Value> {
+    unsafe {
+        let param_num = llvm::LLVMCountParams(fnc) as usize;
+        let mut fnc_args: Vec<&Value> = vec![];
+        fnc_args.reserve(param_num);
+        llvm::LLVMGetParams(fnc, fnc_args.as_mut_ptr());
+        fnc_args.set_len(param_num);
+        fnc_args
+    }
+}
+
+/// When differentiating `fn_to_diff`, take a `outer_fn` and generate another
+/// function with expected naming and calling conventions[^1] which will be
+/// discovered by the enzyme LLVM pass and its body populated with the differentiated
+/// `fn_to_diff`. `outer_fn` is then modified to have a call to the generated
+/// function and handle the differences between the Rust calling convention and
+/// Enzyme.
+/// [^1]: <https://enzyme.mit.edu/getting_started/CallingConvention/>
+// FIXME(ZuseZ4): `outer_fn` should include upstream safety checks to
+// cover some assumptions of enzyme/autodiff, which could lead to UB otherwise.
+pub(crate) fn generate_enzyme_call<'ll>(
+    llmod: &'ll llvm::Module,
+    llcx: &'ll llvm::Context,
+    fn_to_diff: &'ll Value,
+    outer_fn: &'ll Value,
+    attrs: AutoDiffAttrs,
+) {
+    let inputs = attrs.input_activity;
+    let output = attrs.ret_activity;
+
+    // We have to pick the name depending on whether we want forward or reverse mode autodiff.
+    // FIXME(ZuseZ4): The new pass based approach should not need the {Forward/Reverse}First method anymore, since
+    // it will handle higher-order derivatives correctly automatically (in theory). Currently
+    // higher-order derivatives fail, so we should debug that before adjusting this code.
+    let mut ad_name: String = match attrs.mode {
+        DiffMode::Forward => "__enzyme_fwddiff",
+        DiffMode::Reverse => "__enzyme_autodiff",
+        DiffMode::ForwardFirst => "__enzyme_fwddiff",
+        DiffMode::ReverseFirst => "__enzyme_autodiff",
+        _ => panic!("logic bug in autodiff, unrecognized mode"),
+    }
+    .to_string();
+
+    // add outer_fn name to ad_name to make it unique, in case users apply autodiff to multiple
+    // functions. Unwrap will only panic, if LLVM gave us an invalid string.
+    let name = llvm::get_value_name(outer_fn);
+    let outer_fn_name = std::ffi::CStr::from_bytes_with_nul(name).unwrap().to_str().unwrap();
+    ad_name.push_str(outer_fn_name.to_string().as_str());
+
+    // Let us assume the user wrote the following function square:
+    //
+    // ```llvm
+    // define double @square(double %x) {
+    // entry:
+    //  %0 = fmul double %x, %x
+    //  ret double %0
+    // }
+    // ```
+    //
+    // The user now applies autodiff to the function square, in which case fn_to_diff will be `square`.
+    // Our macro generates the following placeholder code (slightly simplified):
+    //
+    // ```llvm
+    // define double @dsquare(double %x) {
+    //  ; placeholder code
+    //  return 0.0;
+    // }
+    // ```
+    //
+    // so our `outer_fn` will be `dsquare`. The unsafe code section below now removes the placeholder
+    // code and inserts an autodiff call. We also add a declaration for the __enzyme_autodiff call.
+    // Again, the arguments to all functions are slightly simplified.
+    // ```llvm
+    // declare double @__enzyme_autodiff_square(...)
+    //
+    // define double @dsquare(double %x) {
+    // entry:
+    //   %0 = tail call double (...) @__enzyme_autodiff_square(double (double)* nonnull @square, double %x)
+    //   ret double %0
+    // }
+    // ```
+    unsafe {
+        // On LLVM-IR, we can luckily declare __enzyme_ functions without specifying the input
+        // arguments. We do however need to declare them with their correct return type.
+        // We already figured the correct return type out in our frontend, when generating the outer_fn,
+        // so we can now just go ahead and use that. FIXME(ZuseZ4): This doesn't handle sret yet.
+        let fn_ty = llvm::LLVMGlobalGetValueType(outer_fn);
+        let ret_ty = llvm::LLVMGetReturnType(fn_ty);
+
+        // LLVM can figure out the input types on it's own, so we take a shortcut here.
+        let enzyme_ty = llvm::LLVMFunctionType(ret_ty, ptr::null(), 0, True);
+        let ad_fn = llvm::LLVMRustGetOrInsertFunction(
+            llmod,
+            ad_name.as_ptr() as *const c_char,
+            ad_name.len().try_into().unwrap(),
+            enzyme_ty,
+        );
+        // Otherwise LLVM might inline our temporary code before the enzyme pass has a chance to
+        // do it's work.
+        let attr = llvm::AttributeKind::NoInline.create_attr(llcx);
+        attributes::apply_to_llfn(ad_fn, Function, &[attr]);
+
+        // first, remove all calls from fnc
+        let entry = llvm::LLVMGetFirstBasicBlock(outer_fn);
+        let br = llvm::LLVMRustGetTerminator(entry);
+        llvm::LLVMRustEraseInstFromParent(br);
+
+        let builder = llvm::LLVMCreateBuilderInContext(llcx);
+        let last_inst = llvm::LLVMRustGetLastInstruction(entry).unwrap();
+        llvm::LLVMPositionBuilderAtEnd(builder, entry);
+
+        let num_args = llvm::LLVMCountParams(&fn_to_diff);
+        let mut args = Vec::with_capacity(num_args as usize + 1);
+        args.push(fn_to_diff);
+
+        let enzyme_const = llvm::create_md_string(llcx, "enzyme_const");
+        let enzyme_out = llvm::create_md_string(llcx, "enzyme_out");
+        let enzyme_dup = llvm::create_md_string(llcx, "enzyme_dup");
+        let enzyme_dupnoneed = llvm::create_md_string(llcx, "enzyme_dupnoneed");
+        let enzyme_primal_ret = llvm::create_md_string(llcx, "enzyme_primal_return");
+
+        match output {
+            DiffActivity::Dual => {
+                args.push(llvm::LLVMMetadataAsValue(llcx, enzyme_primal_ret));
+            }
+            DiffActivity::Active => {
+                args.push(llvm::LLVMMetadataAsValue(llcx, enzyme_primal_ret));
+            }
+            _ => {}
+        }
+
+        trace!("matching autodiff arguments");
+        // We now handle the issue that Rust level arguments not always match the llvm-ir level
+        // arguments. A slice, `&[f32]`, for example, is represented as a pointer and a length on
+        // llvm-ir level. The number of activities matches the number of Rust level arguments, so we
+        // need to match those.
+        let mut outer_pos: usize = 0;
+        let mut activity_pos = 0;
+        let outer_args: Vec<&llvm::Value> = get_params(outer_fn);
+        while activity_pos < inputs.len() {
+            let activity = inputs[activity_pos as usize];
+            // Duplicated arguments received a shadow argument, into which enzyme will write the
+            // gradient.
+            let (activity, duplicated): (&Metadata, bool) = match activity {
+                DiffActivity::None => panic!("not a valid input activity"),
+                DiffActivity::Const => (enzyme_const, false),
+                DiffActivity::Active => (enzyme_out, false),
+                DiffActivity::ActiveOnly => (enzyme_out, false),
+                DiffActivity::Dual => (enzyme_dup, true),
+                DiffActivity::DualOnly => (enzyme_dupnoneed, true),
+                DiffActivity::Duplicated => (enzyme_dup, true),
+                DiffActivity::DuplicatedOnly => (enzyme_dupnoneed, true),
+                DiffActivity::FakeActivitySize => (enzyme_const, false),
+            };
+            let outer_arg = outer_args[outer_pos];
+            args.push(llvm::LLVMMetadataAsValue(llcx, activity));
+            args.push(outer_arg);
+            if duplicated {
+                // We know that duplicated args by construction have a following argument,
+                // so this can not be out of bounds.
+                let next_outer_arg = outer_args[outer_pos + 1];
+                let next_outer_ty = llvm::LLVMTypeOf(next_outer_arg);
+                // FIXME(ZuseZ4): We should add support for Vec here too, but it's less urgent since
+                // vectors behind references (&Vec<T>) are already supported. Users can not pass a
+                // Vec by value for reverse mode, so this would only help forward mode autodiff.
+                let slice = {
+                    if activity_pos + 1 >= inputs.len() {
+                        // If there is no arg following our ptr, it also can't be a slice,
+                        // since that would lead to a ptr, int pair.
+                        false
+                    } else {
+                        let next_activity = inputs[activity_pos + 1];
+                        // We analyze the MIR types and add this dummy activity if we visit a slice.
+                        next_activity == DiffActivity::FakeActivitySize
+                    }
+                };
+                if slice {
+                    // A duplicated slice will have the following two outer_fn arguments:
+                    // (..., ptr1, int1, ptr2, int2, ...). We add the following llvm-ir to our __enzyme call:
+                    // (..., metadata! enzyme_dup, ptr, ptr, int1, ...).
+                    // FIXME(ZuseZ4): We will upstream a safety check later which asserts that
+                    // int2 >= int1, which means the shadow vector is large enough to store the gradient.
+                    assert!(llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Integer);
+                    let next_outer_arg2 = outer_args[outer_pos + 2];
+                    let next_outer_ty2 = llvm::LLVMTypeOf(next_outer_arg2);
+                    assert!(llvm::LLVMRustGetTypeKind(next_outer_ty2) == llvm::TypeKind::Pointer);
+                    let next_outer_arg3 = outer_args[outer_pos + 3];
+                    let next_outer_ty3 = llvm::LLVMTypeOf(next_outer_arg3);
+                    assert!(llvm::LLVMRustGetTypeKind(next_outer_ty3) == llvm::TypeKind::Integer);
+                    args.push(next_outer_arg2);
+                    args.push(llvm::LLVMMetadataAsValue(llcx, enzyme_const));
+                    args.push(next_outer_arg);
+                    outer_pos += 4;
+                    activity_pos += 2;
+                } else {
+                    // A duplicated pointer will have the following two outer_fn arguments:
+                    // (..., ptr, ptr, ...). We add the following llvm-ir to our __enzyme call:
+                    // (..., metadata! enzyme_dup, ptr, ptr, ...).
+                    assert!(llvm::LLVMRustGetTypeKind(next_outer_ty) == llvm::TypeKind::Pointer);
+                    args.push(next_outer_arg);
+                    outer_pos += 2;
+                    activity_pos += 1;
+                }
+            } else {
+                // We do not differentiate with resprect to this argument.
+                // We already added the metadata and argument above, so just increase the counters.
+                outer_pos += 1;
+                activity_pos += 1;
+            }
+        }
+
+        let call = llvm::LLVMBuildCall2(
+            builder,
+            enzyme_ty,
+            ad_fn,
+            args.as_mut_ptr(),
+            args.len().try_into().unwrap(),
+            ad_name.as_ptr() as *const c_char,
+        );
+
+        // This part is a bit iffy. LLVM requires that a call to an inlineable function has some
+        // metadata attachted to it, but we just created this code oota. Given that the
+        // differentiated function already has partly confusing metadata, and given that this
+        // affects nothing but the auttodiff IR, we take a shortcut and just steal metadata from the
+        // dummy code which we inserted at a higher level.
+        // FIXME(ZuseZ4): Work with Enzyme core devs to clarify what debug metadata issues we have,
+        // and how to best improve it for enzyme core and rust-enzyme.
+        let md_ty = llvm::LLVMGetMDKindIDInContext(
+            llcx,
+            "dbg".as_ptr() as *const c_char,
+            "dbg".len() as c_uint,
+        );
+        if llvm::LLVMRustHasMetadata(last_inst, md_ty) {
+            let md = llvm::LLVMRustDIGetInstMetadata(last_inst);
+            let md_todiff = llvm::LLVMMetadataAsValue(llcx, md);
+            llvm::LLVMSetMetadata(call, md_ty, md_todiff);
+        } else {
+            // We don't panic, since depending on whether we are in debug or release mode, we might
+            // have no debug info to copy, which would then be ok.
+            trace!("no dbg info");
+        }
+        // Now that we copied the metadata, get rid of dummy code.
+        llvm::LLVMRustEraseInstBefore(entry, last_inst);
+        llvm::LLVMRustEraseInstFromParent(last_inst);
+
+        let void_ty = llvm::LLVMVoidTypeInContext(llcx);
+        if llvm::LLVMTypeOf(call) != void_ty {
+            llvm::LLVMBuildRet(builder, call);
+        } else {
+            llvm::LLVMBuildRetVoid(builder);
+        };
+        llvm::LLVMDisposeBuilder(builder);
+
+        // Let's crash in case that we messed something up above and generated invalid IR.
+        llvm::LLVMVerifyFunction(outer_fn, llvm::LLVMVerifierFailureAction::LLVMAbortProcessAction);
+    }
+}
+
 // All Builders must have an llfn associated with them
 #[must_use]
 pub(crate) struct Builder<'a, 'll, 'tcx> {
diff --git a/compiler/rustc_codegen_llvm/src/errors.rs b/compiler/rustc_codegen_llvm/src/errors.rs
index 3cdb5b971d90..f340b06e876c 100644
--- a/compiler/rustc_codegen_llvm/src/errors.rs
+++ b/compiler/rustc_codegen_llvm/src/errors.rs
@@ -89,6 +89,11 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for ParseTargetMachineConfig<'_> {
     }
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_autodiff_without_lto)]
+#[note]
+pub(crate) struct AutoDiffWithoutLTO;
+
 #[derive(Diagnostic)]
 #[diag(codegen_llvm_lto_disallowed)]
 pub(crate) struct LtoDisallowed;
@@ -131,6 +136,8 @@ pub enum LlvmError<'a> {
     PrepareThinLtoModule,
     #[diag(codegen_llvm_parse_bitcode)]
     ParseBitcode,
+    #[diag(codegen_llvm_prepare_autodiff)]
+    PrepareAutoDiff { src: String, target: String, error: String },
 }
 
 pub(crate) struct WithLlvmError<'a>(pub LlvmError<'a>, pub String);
@@ -152,6 +159,7 @@ impl<G: EmissionGuarantee> Diagnostic<'_, G> for WithLlvmError<'_> {
             }
             PrepareThinLtoModule => fluent::codegen_llvm_prepare_thin_lto_module_with_llvm_err,
             ParseBitcode => fluent::codegen_llvm_parse_bitcode_with_llvm_err,
+            PrepareAutoDiff { .. } => fluent::codegen_llvm_prepare_autodiff_with_llvm_err,
         };
         self.0
             .into_diag(dcx, level)
diff --git a/compiler/rustc_codegen_llvm/src/lib.rs b/compiler/rustc_codegen_llvm/src/lib.rs
index 5235891a18df..4b2b50ccf2c2 100644
--- a/compiler/rustc_codegen_llvm/src/lib.rs
+++ b/compiler/rustc_codegen_llvm/src/lib.rs
@@ -26,9 +26,10 @@ use std::mem::ManuallyDrop;
 
 use back::owned_target_machine::OwnedTargetMachine;
 use back::write::{create_informational_target_machine, create_target_machine};
-use errors::ParseTargetMachineConfig;
+use errors::{AutoDiffWithoutLTO, ParseTargetMachineConfig};
 pub use llvm_util::target_features;
 use rustc_ast::expand::allocator::AllocatorKind;
+use rustc_ast::expand::autodiff_attrs::AutoDiffItem;
 use rustc_codegen_ssa::back::lto::{LtoModuleCodegen, SerializedModule, ThinModule};
 use rustc_codegen_ssa::back::write::{
     CodegenContext, FatLtoInput, ModuleConfig, TargetMachineFactoryConfig, TargetMachineFactoryFn,
@@ -42,7 +43,7 @@ use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_middle::ty::TyCtxt;
 use rustc_middle::util::Providers;
 use rustc_session::Session;
-use rustc_session::config::{OptLevel, OutputFilenames, PrintKind, PrintRequest};
+use rustc_session::config::{Lto, OptLevel, OutputFilenames, PrintKind, PrintRequest};
 use rustc_span::symbol::Symbol;
 
 mod back {
@@ -231,6 +232,19 @@ impl WriteBackendMethods for LlvmCodegenBackend {
     fn serialize_module(module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer) {
         (module.name, back::lto::ModuleBuffer::new(module.module_llvm.llmod()))
     }
+    /// Generate autodiff rules
+    fn autodiff(
+        cgcx: &CodegenContext<Self>,
+        module: &ModuleCodegen<Self::Module>,
+        diff_fncs: Vec<AutoDiffItem>,
+        config: &ModuleConfig,
+    ) -> Result<(), FatalError> {
+        if cgcx.lto != Lto::Fat {
+            let dcx = cgcx.create_dcx();
+            return Err(dcx.handle().emit_almost_fatal(AutoDiffWithoutLTO));
+        }
+        back::write::differentiate(module, cgcx, diff_fncs, config)
+    }
 }
 
 unsafe impl Send for LlvmCodegenBackend {} // Llvm is on a per-thread basis
diff --git a/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
new file mode 100644
index 000000000000..1cef0296fa7f
--- /dev/null
+++ b/compiler/rustc_codegen_llvm/src/llvm/enzyme_ffi.rs
@@ -0,0 +1,38 @@
+#![allow(non_camel_case_types)]
+
+use libc::{c_char, c_uint, size_t};
+
+use super::ffi::{Attribute, BasicBlock, Builder, Metadata, Module, Type, Value};
+extern "C" {
+    // Enzyme
+    pub fn LLVMRustHasMetadata(I: &Value, KindID: c_uint) -> bool;
+    pub fn LLVMRustEraseInstBefore(BB: &BasicBlock, I: &Value);
+    pub fn LLVMRustGetLastInstruction<'a>(BB: &BasicBlock) -> Option<&'a Value>;
+    pub fn LLVMRustDIGetInstMetadata(I: &Value) -> &Metadata;
+    pub fn LLVMRustEraseInstFromParent(V: &Value);
+    pub fn LLVMRustGetTerminator<'a>(B: &BasicBlock) -> &'a Value;
+
+    pub fn LLVMGetReturnType(T: &Type) -> &Type;
+    pub fn LLVMDumpModule(M: &Module);
+    pub fn LLVMCountStructElementTypes(T: &Type) -> c_uint;
+    pub fn LLVMVerifyFunction(V: &Value, action: LLVMVerifierFailureAction) -> bool;
+    pub fn LLVMGetParams(Fnc: &Value, parms: *mut &Value);
+    pub fn LLVMBuildCall2<'a>(
+        arg1: &Builder<'a>,
+        ty: &Type,
+        func: &Value,
+        args: *mut &Value,
+        num_args: size_t,
+        name: *const c_char,
+    ) -> &'a Value;
+    pub fn LLVMGetNamedFunction(M: &Module, Name: *const c_char) -> Option<&Value>;
+    pub fn LLVMIsEnumAttribute(A: &Attribute) -> bool;
+    pub fn LLVMIsStringAttribute(A: &Attribute) -> bool;
+}
+
+#[repr(C)]
+pub enum LLVMVerifierFailureAction {
+    LLVMAbortProcessAction,
+    LLVMPrintMessageAction,
+    LLVMReturnStatusAction,
+}
diff --git a/compiler/rustc_codegen_llvm/src/llvm/mod.rs b/compiler/rustc_codegen_llvm/src/llvm/mod.rs
index 909afe35a179..069f90c6878e 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/mod.rs
@@ -22,8 +22,11 @@ use crate::common::AsCCharPtr;
 
 pub mod archive_ro;
 pub mod diagnostic;
+pub mod enzyme_ffi;
 mod ffi;
 
+pub use self::enzyme_ffi::*;
+
 impl LLVMRustResult {
     pub fn into_result(self) -> Result<(), ()> {
         match self {
@@ -196,6 +199,10 @@ pub fn set_thread_local_mode(global: &Value, mode: ThreadLocalMode) {
     }
 }
 
+pub fn create_md_string<'a>(llcx: &'a Context, s: &str) -> &'a Metadata {
+    unsafe { LLVMMDStringInContext2(llcx, s.as_c_char_ptr(), s.len()) }
+}
+
 impl AttributeKind {
     /// Create an LLVM Attribute with no associated value.
     pub fn create_attr(self, llcx: &Context) -> &Attribute {
diff --git a/compiler/rustc_codegen_ssa/src/back/lto.rs b/compiler/rustc_codegen_ssa/src/back/lto.rs
index ab8b06a05fc7..9fd984b6419e 100644
--- a/compiler/rustc_codegen_ssa/src/back/lto.rs
+++ b/compiler/rustc_codegen_ssa/src/back/lto.rs
@@ -1,11 +1,13 @@
 use std::ffi::CString;
 use std::sync::Arc;
 
+use rustc_ast::expand::autodiff_attrs::AutoDiffItem;
 use rustc_data_structures::memmap::Mmap;
 use rustc_errors::FatalError;
 
 use super::write::CodegenContext;
 use crate::ModuleCodegen;
+use crate::back::write::ModuleConfig;
 use crate::traits::*;
 
 pub struct ThinModule<B: WriteBackendMethods> {
@@ -81,6 +83,23 @@ impl<B: WriteBackendMethods> LtoModuleCodegen<B> {
             LtoModuleCodegen::Thin(ref m) => m.cost(),
         }
     }
+
+    /// Run autodiff on Fat LTO module
+    pub unsafe fn autodiff(
+        self,
+        cgcx: &CodegenContext<B>,
+        diff_fncs: Vec<AutoDiffItem>,
+        config: &ModuleConfig,
+    ) -> Result<LtoModuleCodegen<B>, FatalError> {
+        match &self {
+            LtoModuleCodegen::Fat(module) => {
+                B::autodiff(cgcx, &module, diff_fncs, config)?;
+            }
+            _ => panic!("autodiff called with non-fat LTO module"),
+        }
+
+        Ok(self)
+    }
 }
 
 pub enum SerializedModule<M: ModuleBufferMethods> {
diff --git a/compiler/rustc_codegen_ssa/src/traits/write.rs b/compiler/rustc_codegen_ssa/src/traits/write.rs
index aabe9e33c4aa..97fe614aa10c 100644
--- a/compiler/rustc_codegen_ssa/src/traits/write.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/write.rs
@@ -1,3 +1,4 @@
+use rustc_ast::expand::autodiff_attrs::AutoDiffItem;
 use rustc_errors::{DiagCtxtHandle, FatalError};
 use rustc_middle::dep_graph::WorkProduct;
 
@@ -61,6 +62,12 @@ pub trait WriteBackendMethods: 'static + Sized + Clone {
         want_summary: bool,
     ) -> (String, Self::ThinBuffer);
     fn serialize_module(module: ModuleCodegen<Self::Module>) -> (String, Self::ModuleBuffer);
+    fn autodiff(
+        cgcx: &CodegenContext<Self>,
+        module: &ModuleCodegen<Self::Module>,
+        diff_fncs: Vec<AutoDiffItem>,
+        config: &ModuleConfig,
+    ) -> Result<(), FatalError>;
 }
 
 pub trait ThinBufferMethods: Send + Sync {
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index 6a6496f98270..690e080fbfce 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -723,7 +723,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
                                     item_inner.kind,
                                     ItemKind::Mod(
                                         _,
-                                        ModKind::Unloaded | ModKind::Loaded(_, Inline::No, _),
+                                        ModKind::Unloaded | ModKind::Loaded(_, Inline::No, _, _),
                                     )
                                 ) =>
                         {
@@ -889,7 +889,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             fn visit_item(&mut self, item: &'ast ast::Item) {
                 match &item.kind {
                     ItemKind::Mod(_, mod_kind)
-                        if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _)) =>
+                        if !matches!(mod_kind, ModKind::Loaded(_, Inline::Yes, _, _)) =>
                     {
                         feature_err(
                             self.sess,
@@ -1195,7 +1195,7 @@ impl InvocationCollectorNode for P<ast::Item> {
 
         let ecx = &mut collector.cx;
         let (file_path, dir_path, dir_ownership) = match mod_kind {
-            ModKind::Loaded(_, inline, _) => {
+            ModKind::Loaded(_, inline, _, _) => {
                 // Inline `mod foo { ... }`, but we still need to push directories.
                 let (dir_path, dir_ownership) = mod_dir_path(
                     ecx.sess,
@@ -1217,15 +1217,21 @@ impl InvocationCollectorNode for P<ast::Item> {
             ModKind::Unloaded => {
                 // We have an outline `mod foo;` so we need to parse the file.
                 let old_attrs_len = attrs.len();
-                let ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership } =
-                    parse_external_mod(
-                        ecx.sess,
-                        ident,
-                        span,
-                        &ecx.current_expansion.module,
-                        ecx.current_expansion.dir_ownership,
-                        &mut attrs,
-                    );
+                let ParsedExternalMod {
+                    items,
+                    spans,
+                    file_path,
+                    dir_path,
+                    dir_ownership,
+                    had_parse_error,
+                } = parse_external_mod(
+                    ecx.sess,
+                    ident,
+                    span,
+                    &ecx.current_expansion.module,
+                    ecx.current_expansion.dir_ownership,
+                    &mut attrs,
+                );
 
                 if let Some(lint_store) = ecx.lint_store {
                     lint_store.pre_expansion_lint(
@@ -1239,7 +1245,7 @@ impl InvocationCollectorNode for P<ast::Item> {
                     );
                 }
 
-                *mod_kind = ModKind::Loaded(items, Inline::No, spans);
+                *mod_kind = ModKind::Loaded(items, Inline::No, spans, had_parse_error);
                 node.attrs = attrs;
                 if node.attrs.len() > old_attrs_len {
                     // If we loaded an out-of-line module and added some inner attributes,
diff --git a/compiler/rustc_expand/src/module.rs b/compiler/rustc_expand/src/module.rs
index 614f52bbd285..85ea42e78ad4 100644
--- a/compiler/rustc_expand/src/module.rs
+++ b/compiler/rustc_expand/src/module.rs
@@ -37,6 +37,7 @@ pub(crate) struct ParsedExternalMod {
     pub file_path: PathBuf,
     pub dir_path: PathBuf,
     pub dir_ownership: DirOwnership,
+    pub had_parse_error: Result<(), ErrorGuaranteed>,
 }
 
 pub enum ModError<'a> {
@@ -74,14 +75,17 @@ pub(crate) fn parse_external_mod(
         attrs.extend(inner_attrs);
         (items, inner_span, mp.file_path)
     };
+
     // (1) ...instead, we return a dummy module.
-    let (items, spans, file_path) =
-        result.map_err(|err| err.report(sess, span)).unwrap_or_default();
+    let ((items, spans, file_path), had_parse_error) = match result {
+        Err(err) => (Default::default(), Err(err.report(sess, span))),
+        Ok(result) => (result, Ok(())),
+    };
 
     // Extract the directory path for submodules of the module.
     let dir_path = file_path.parent().unwrap_or(&file_path).to_owned();
 
-    ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership }
+    ParsedExternalMod { items, spans, file_path, dir_path, dir_ownership, had_parse_error }
 }
 
 pub(crate) fn mod_dir_path(
diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index 093cc16fb4c1..3543784bc728 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -3038,7 +3038,7 @@ impl EarlyLintPass for SpecialModuleName {
         for item in &krate.items {
             if let ast::ItemKind::Mod(
                 _,
-                ast::ModKind::Unloaded | ast::ModKind::Loaded(_, ast::Inline::No, _),
+                ast::ModKind::Unloaded | ast::ModKind::Loaded(_, ast::Inline::No, _, _),
             ) = item.kind
             {
                 if item.attrs.iter().any(|a| a.has_name(sym::path)) {
diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index b79205ff946d..439f664de4b4 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
@@ -388,6 +388,17 @@ extern "C" void LLVMRustAddCallSiteAttributes(LLVMValueRef Instr,
   AddAttributes(Call, Index, Attrs, AttrsLen);
 }
 
+extern "C" LLVMValueRef LLVMRustGetTerminator(LLVMBasicBlockRef BB) {
+  Instruction *ret = unwrap(BB)->getTerminator();
+  return wrap(ret);
+}
+
+extern "C" void LLVMRustEraseInstFromParent(LLVMValueRef Instr) {
+  if (auto I = dyn_cast<Instruction>(unwrap<Value>(Instr))) {
+    I->eraseFromParent();
+  }
+}
+
 extern "C" LLVMAttributeRef
 LLVMRustCreateAttrNoValue(LLVMContextRef C, LLVMRustAttributeKind RustAttr) {
   return wrap(Attribute::get(*unwrap(C), fromRust(RustAttr)));
@@ -954,6 +965,47 @@ extern "C" void LLVMRustAddModuleFlagString(
       MDString::get(unwrap(M)->getContext(), StringRef(Value, ValueLen)));
 }
 
+extern "C" LLVMValueRef LLVMRustGetLastInstruction(LLVMBasicBlockRef BB) {
+  auto Point = unwrap(BB)->rbegin();
+  if (Point != unwrap(BB)->rend())
+    return wrap(&*Point);
+  return nullptr;
+}
+
+extern "C" void LLVMRustEraseInstBefore(LLVMBasicBlockRef bb, LLVMValueRef I) {
+  auto &BB = *unwrap(bb);
+  auto &Inst = *unwrap<Instruction>(I);
+  auto It = BB.begin();
+  while (&*It != &Inst)
+    ++It;
+  // Make sure we found the Instruction.
+  assert(It != BB.end());
+  // We don't want to erase the instruction itself.
+  It--;
+  // Delete in rev order to ensure no dangling references.
+  while (It != BB.begin()) {
+    auto Prev = std::prev(It);
+    It->eraseFromParent();
+    It = Prev;
+  }
+  It->eraseFromParent();
+}
+
+extern "C" bool LLVMRustHasMetadata(LLVMValueRef inst, unsigned kindID) {
+  if (auto *I = dyn_cast<Instruction>(unwrap<Value>(inst))) {
+    return I->hasMetadata(kindID);
+  }
+  return false;
+}
+
+extern "C" LLVMMetadataRef LLVMRustDIGetInstMetadata(LLVMValueRef x) {
+  if (auto *I = dyn_cast<Instruction>(unwrap<Value>(x))) {
+    auto *MD = I->getDebugLoc().getAsMDNode();
+    return wrap(MD);
+  }
+  return nullptr;
+}
+
 extern "C" void LLVMRustGlobalAddMetadata(LLVMValueRef Global, unsigned Kind,
                                           LLVMMetadataRef MD) {
   unwrap<GlobalObject>(Global)->addMetadata(Kind, *unwrap<MDNode>(MD));
diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index 2426eb81678e..443ddfc94ec2 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -69,24 +69,30 @@ pub(crate) fn lex_token_trees<'psess, 'src>(
         token: Token::dummy(),
         diag_info: TokenTreeDiagInfo::default(),
     };
-    let (_open_spacing, stream, res) = lexer.lex_token_trees(/* is_delimited */ false);
-    let unmatched_delims = lexer.diag_info.unmatched_delims;
-
-    if res.is_ok() && unmatched_delims.is_empty() {
-        Ok(stream)
-    } else {
-        // Return error if there are unmatched delimiters or unclosed delimiters.
-        // We emit delimiter mismatch errors first, then emit the unclosing delimiter mismatch
-        // because the delimiter mismatch is more likely to be the root cause of error
-        let mut buffer: Vec<_> = unmatched_delims
-            .into_iter()
-            .filter_map(|unmatched_delim| make_unclosed_delims_error(unmatched_delim, psess))
-            .collect();
-        if let Err(errs) = res {
-            // Add unclosing delimiter or diff marker errors
-            buffer.extend(errs);
+    let res = lexer.lex_token_trees(/* is_delimited */ false);
+
+    let mut unmatched_delims: Vec<_> = lexer
+        .diag_info
+        .unmatched_delims
+        .into_iter()
+        .filter_map(|unmatched_delim| make_unclosed_delims_error(unmatched_delim, psess))
+        .collect();
+
+    match res {
+        Ok((_open_spacing, stream)) => {
+            if unmatched_delims.is_empty() {
+                Ok(stream)
+            } else {
+                // Return error if there are unmatched delimiters or unclosed delimiters.
+                Err(unmatched_delims)
+            }
+        }
+        Err(errs) => {
+            // We emit delimiter mismatch errors first, then emit the unclosing delimiter mismatch
+            // because the delimiter mismatch is more likely to be the root cause of error
+            unmatched_delims.extend(errs);
+            Err(unmatched_delims)
         }
-        Err(buffer)
     }
 }
 
diff --git a/compiler/rustc_parse/src/lexer/tokentrees.rs b/compiler/rustc_parse/src/lexer/tokentrees.rs
index ee38f16d4ecd..b3f83a320241 100644
--- a/compiler/rustc_parse/src/lexer/tokentrees.rs
+++ b/compiler/rustc_parse/src/lexer/tokentrees.rs
@@ -1,12 +1,10 @@
 use rustc_ast::token::{self, Delimiter, Token};
 use rustc_ast::tokenstream::{DelimSpacing, DelimSpan, Spacing, TokenStream, TokenTree};
 use rustc_ast_pretty::pprust::token_to_string;
-use rustc_errors::{Applicability, Diag};
-use rustc_span::symbol::kw;
+use rustc_errors::Diag;
 
 use super::diagnostics::{report_suspicious_mismatch_block, same_indentation_level};
 use super::{Lexer, UnmatchedDelim};
-use crate::Parser;
 
 impl<'psess, 'src> Lexer<'psess, 'src> {
     // Lex into a token stream. The `Spacing` in the result is that of the
@@ -14,7 +12,7 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
     pub(super) fn lex_token_trees(
         &mut self,
         is_delimited: bool,
-    ) -> (Spacing, TokenStream, Result<(), Vec<Diag<'psess>>>) {
+    ) -> Result<(Spacing, TokenStream), Vec<Diag<'psess>>> {
         // Move past the opening delimiter.
         let open_spacing = self.bump_minimal();
 
@@ -27,25 +25,25 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
                     debug_assert!(!matches!(delim, Delimiter::Invisible(_)));
                     buf.push(match self.lex_token_tree_open_delim(delim) {
                         Ok(val) => val,
-                        Err(errs) => return (open_spacing, TokenStream::new(buf), Err(errs)),
+                        Err(errs) => return Err(errs),
                     })
                 }
                 token::CloseDelim(delim) => {
                     // Invisible delimiters cannot occur here because `TokenTreesReader` parses
                     // code directly from strings, with no macro expansion involved.
                     debug_assert!(!matches!(delim, Delimiter::Invisible(_)));
-                    return (
-                        open_spacing,
-                        TokenStream::new(buf),
-                        if is_delimited { Ok(()) } else { Err(vec![self.close_delim_err(delim)]) },
-                    );
+                    return if is_delimited {
+                        Ok((open_spacing, TokenStream::new(buf)))
+                    } else {
+                        Err(vec![self.close_delim_err(delim)])
+                    };
                 }
                 token::Eof => {
-                    return (
-                        open_spacing,
-                        TokenStream::new(buf),
-                        if is_delimited { Err(vec![self.eof_err()]) } else { Ok(()) },
-                    );
+                    return if is_delimited {
+                        Err(vec![self.eof_err()])
+                    } else {
+                        Ok((open_spacing, TokenStream::new(buf)))
+                    };
                 }
                 _ => {
                     // Get the next normal token.
@@ -107,10 +105,7 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
         // Lex the token trees within the delimiters.
         // We stop at any delimiter so we can try to recover if the user
         // uses an incorrect delimiter.
-        let (open_spacing, tts, res) = self.lex_token_trees(/* is_delimited */ true);
-        if let Err(errs) = res {
-            return Err(self.unclosed_delim_err(tts, errs));
-        }
+        let (open_spacing, tts) = self.lex_token_trees(/* is_delimited */ true)?;
 
         // Expand to cover the entire delimited token tree.
         let delim_span = DelimSpan::from_pair(pre_span, self.token.span);
@@ -247,67 +242,6 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
         this_spacing
     }
 
-    fn unclosed_delim_err(
-        &mut self,
-        tts: TokenStream,
-        mut errs: Vec<Diag<'psess>>,
-    ) -> Vec<Diag<'psess>> {
-        // If there are unclosed delims, see if there are diff markers and if so, point them
-        // out instead of complaining about the unclosed delims.
-        let mut parser = Parser::new(self.psess, tts, None);
-        let mut diff_errs = vec![];
-        // Suggest removing a `{` we think appears in an `if`/`while` condition.
-        // We want to suggest removing a `{` only if we think we're in an `if`/`while` condition,
-        // but we have no way of tracking this in the lexer itself, so we piggyback on the parser.
-        let mut in_cond = false;
-        while parser.token != token::Eof {
-            if let Err(diff_err) = parser.err_vcs_conflict_marker() {
-                diff_errs.push(diff_err);
-            } else if parser.is_keyword_ahead(0, &[kw::If, kw::While]) {
-                in_cond = true;
-            } else if matches!(
-                parser.token.kind,
-                token::CloseDelim(Delimiter::Brace) | token::FatArrow
-            ) {
-                // End of the `if`/`while` body, or the end of a `match` guard.
-                in_cond = false;
-            } else if in_cond && parser.token == token::OpenDelim(Delimiter::Brace) {
-                // Store the `&&` and `let` to use their spans later when creating the diagnostic
-                let maybe_andand = parser.look_ahead(1, |t| t.clone());
-                let maybe_let = parser.look_ahead(2, |t| t.clone());
-                if maybe_andand == token::OpenDelim(Delimiter::Brace) {
-                    // This might be the beginning of the `if`/`while` body (i.e., the end of the
-                    // condition).
-                    in_cond = false;
-                } else if maybe_andand == token::AndAnd && maybe_let.is_keyword(kw::Let) {
-                    let mut err = parser.dcx().struct_span_err(
-                        parser.token.span,
-                        "found a `{` in the middle of a let-chain",
-                    );
-                    err.span_suggestion(
-                        parser.token.span,
-                        "consider removing this brace to parse the `let` as part of the same chain",
-                        "",
-                        Applicability::MachineApplicable,
-                    );
-                    err.span_label(
-                        maybe_andand.span.to(maybe_let.span),
-                        "you might have meant to continue the let-chain here",
-                    );
-                    errs.push(err);
-                }
-            }
-            parser.bump();
-        }
-        if !diff_errs.is_empty() {
-            for err in errs {
-                err.cancel();
-            }
-            return diff_errs;
-        }
-        errs
-    }
-
     fn close_delim_err(&mut self, delim: Delimiter) -> Diag<'psess> {
         // An unexpected closing delimiter (i.e., there is no matching opening delimiter).
         let token_str = token_to_string(&self.token);
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 58b4bf8980b1..e27fc963eb90 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -45,7 +45,7 @@ impl<'a> Parser<'a> {
             let (inner_attrs, items, inner_span) =
                 self.parse_mod(&token::CloseDelim(Delimiter::Brace))?;
             attrs.extend(inner_attrs);
-            ModKind::Loaded(items, Inline::Yes, inner_span)
+            ModKind::Loaded(items, Inline::Yes, inner_span, Ok(()))
         };
         Ok((id, ItemKind::Mod(safety, mod_kind)))
     }
diff --git a/compiler/rustc_passes/src/stability.rs b/compiler/rustc_passes/src/stability.rs
index 2809ad453ff4..fcd5afe55c12 100644
--- a/compiler/rustc_passes/src/stability.rs
+++ b/compiler/rustc_passes/src/stability.rs
@@ -411,6 +411,9 @@ impl<'a, 'tcx> Visitor<'tcx> for Annotator<'a, 'tcx> {
                 kind = AnnotationKind::DeprecationProhibited;
                 const_stab_inherit = InheritConstStability::Yes;
             }
+            hir::ItemKind::Use(_, _) => {
+                kind = AnnotationKind::DeprecationProhibited;
+            }
             hir::ItemKind::Struct(ref sd, _) => {
                 if let Some(ctor_def_id) = sd.ctor_def_id() {
                     self.annotate(
diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 293cee500bbd..924b8afa329f 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -770,7 +770,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                 );
             }
 
-            ItemKind::Mod(..) => {
+            ItemKind::Mod(.., ref mod_kind) => {
                 let module = self.r.new_module(
                     Some(parent),
                     ModuleKind::Def(def_kind, def_id, ident.name),
@@ -781,6 +781,10 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                 );
                 self.r.define(parent, ident, TypeNS, (module, vis, sp, expansion));
 
+                if let ast::ModKind::Loaded(_, _, _, Err(_)) = mod_kind {
+                    self.r.mods_with_parse_errors.insert(def_id);
+                }
+
                 // Descend into the module.
                 self.parent_scope.module = module;
             }
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 4c76617a3918..368eb3c26c7b 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -3056,7 +3056,7 @@ impl<'tcx> visit::Visitor<'tcx> for UsePlacementFinder {
 
     fn visit_item(&mut self, item: &'tcx ast::Item) {
         if self.target_module == item.id {
-            if let ItemKind::Mod(_, ModKind::Loaded(items, _inline, mod_spans)) = &item.kind {
+            if let ItemKind::Mod(_, ModKind::Loaded(items, _inline, mod_spans, _)) = &item.kind {
                 let inject = mod_spans.inject_use_span;
                 if is_span_suitable_for_use_injection(inject) {
                     self.first_legal_span = Some(inject);
diff --git a/compiler/rustc_resolve/src/ident.rs b/compiler/rustc_resolve/src/ident.rs
index 466e190028ac..5906a682f3e1 100644
--- a/compiler/rustc_resolve/src/ident.rs
+++ b/compiler/rustc_resolve/src/ident.rs
@@ -1428,6 +1428,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ignore_import: Option<Import<'ra>>,
     ) -> PathResult<'ra> {
         let mut module = None;
+        let mut module_had_parse_errors = false;
         let mut allow_super = true;
         let mut second_binding = None;
 
@@ -1471,9 +1472,14 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             continue;
                         }
                     }
-                    return PathResult::failed(ident, false, finalize.is_some(), module, || {
-                        ("there are too many leading `super` keywords".to_string(), None)
-                    });
+                    return PathResult::failed(
+                        ident,
+                        false,
+                        finalize.is_some(),
+                        module_had_parse_errors,
+                        module,
+                        || ("there are too many leading `super` keywords".to_string(), None),
+                    );
                 }
                 if segment_idx == 0 {
                     if name == kw::SelfLower {
@@ -1511,19 +1517,26 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
             // Report special messages for path segment keywords in wrong positions.
             if ident.is_path_segment_keyword() && segment_idx != 0 {
-                return PathResult::failed(ident, false, finalize.is_some(), module, || {
-                    let name_str = if name == kw::PathRoot {
-                        "crate root".to_string()
-                    } else {
-                        format!("`{name}`")
-                    };
-                    let label = if segment_idx == 1 && path[0].ident.name == kw::PathRoot {
-                        format!("global paths cannot start with {name_str}")
-                    } else {
-                        format!("{name_str} in paths can only be used in start position")
-                    };
-                    (label, None)
-                });
+                return PathResult::failed(
+                    ident,
+                    false,
+                    finalize.is_some(),
+                    module_had_parse_errors,
+                    module,
+                    || {
+                        let name_str = if name == kw::PathRoot {
+                            "crate root".to_string()
+                        } else {
+                            format!("`{name}`")
+                        };
+                        let label = if segment_idx == 1 && path[0].ident.name == kw::PathRoot {
+                            format!("global paths cannot start with {name_str}")
+                        } else {
+                            format!("{name_str} in paths can only be used in start position")
+                        };
+                        (label, None)
+                    },
+                );
             }
 
             let binding = if let Some(module) = module {
@@ -1589,6 +1602,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
                     let maybe_assoc = opt_ns != Some(MacroNS) && PathSource::Type.is_expected(res);
                     if let Some(next_module) = binding.module() {
+                        if self.mods_with_parse_errors.contains(&next_module.def_id()) {
+                            module_had_parse_errors = true;
+                        }
                         module = Some(ModuleOrUniformRoot::Module(next_module));
                         record_segment_res(self, res);
                     } else if res == Res::ToolMod && !is_last && opt_ns.is_some() {
@@ -1614,6 +1630,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             ident,
                             is_last,
                             finalize.is_some(),
+                            module_had_parse_errors,
                             module,
                             || {
                                 let label = format!(
@@ -1637,19 +1654,26 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         }
                     }
 
-                    return PathResult::failed(ident, is_last, finalize.is_some(), module, || {
-                        self.report_path_resolution_error(
-                            path,
-                            opt_ns,
-                            parent_scope,
-                            ribs,
-                            ignore_binding,
-                            ignore_import,
-                            module,
-                            segment_idx,
-                            ident,
-                        )
-                    });
+                    return PathResult::failed(
+                        ident,
+                        is_last,
+                        finalize.is_some(),
+                        module_had_parse_errors,
+                        module,
+                        || {
+                            self.report_path_resolution_error(
+                                path,
+                                opt_ns,
+                                parent_scope,
+                                ribs,
+                                ignore_binding,
+                                ignore_import,
+                                module,
+                                segment_idx,
+                                ident,
+                            )
+                        },
+                    );
                 }
             }
         }
diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index 51fbcb8ebb80..2ed3f4d2c4fb 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -670,9 +670,14 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
     fn throw_unresolved_import_error(
         &mut self,
-        errors: Vec<(Import<'_>, UnresolvedImportError)>,
+        mut errors: Vec<(Import<'_>, UnresolvedImportError)>,
         glob_error: bool,
     ) {
+        errors.retain(|(_import, err)| match err.module {
+            // Skip `use` errors for `use foo::Bar;` if `foo.rs` has unrecovered parse errors.
+            Some(def_id) if self.mods_with_parse_errors.contains(&def_id) => false,
+            _ => true,
+        });
         if errors.is_empty() {
             return;
         }
@@ -898,6 +903,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 label,
                 suggestion,
                 module,
+                error_implied_by_parse_error: _,
             } => {
                 if no_ambiguity {
                     assert!(import.imported_module.get().is_none());
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 789d74876f72..5cb7bb9b461c 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -4395,6 +4395,12 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             PathResult::Module(ModuleOrUniformRoot::Module(module)) if !module.is_normal() => {
                 PartialRes::new(module.res().unwrap())
             }
+            // A part of this path references a `mod` that had a parse error. To avoid resolution
+            // errors for each reference to that module, we don't emit an error for them until the
+            // `mod` is fixed. this can have a significant cascade effect.
+            PathResult::Failed { error_implied_by_parse_error: true, .. } => {
+                PartialRes::new(Res::Err)
+            }
             // In `a(::assoc_item)*` `a` cannot be a module. If `a` does resolve to a module we
             // don't report an error right away, but try to fallback to a primitive type.
             // So, we are still able to successfully resolve something like
@@ -4443,6 +4449,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 suggestion,
                 module,
                 segment_name,
+                error_implied_by_parse_error: _,
             } => {
                 return Err(respan(span, ResolutionError::FailedToResolve {
                     segment: Some(segment_name),
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index ca4adce37ce0..94adfcd3b915 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -450,6 +450,7 @@ enum PathResult<'ra> {
         module: Option<ModuleOrUniformRoot<'ra>>,
         /// The segment name of target
         segment_name: Symbol,
+        error_implied_by_parse_error: bool,
     },
 }
 
@@ -458,6 +459,7 @@ impl<'ra> PathResult<'ra> {
         ident: Ident,
         is_error_from_last_segment: bool,
         finalize: bool,
+        error_implied_by_parse_error: bool,
         module: Option<ModuleOrUniformRoot<'ra>>,
         label_and_suggestion: impl FnOnce() -> (String, Option<Suggestion>),
     ) -> PathResult<'ra> {
@@ -470,6 +472,7 @@ impl<'ra> PathResult<'ra> {
             suggestion,
             is_error_from_last_segment,
             module,
+            error_implied_by_parse_error,
         }
     }
 }
@@ -1198,6 +1201,8 @@ pub struct Resolver<'ra, 'tcx> {
     /// This is the `Span` where an `extern crate foo;` suggestion would be inserted, if `foo`
     /// could be a crate that wasn't imported. For diagnostics use only.
     current_crate_outer_attr_insert_span: Span,
+
+    mods_with_parse_errors: FxHashSet<DefId>,
 }
 
 /// This provides memory for the rest of the crate. The `'ra` lifetime that is
@@ -1543,6 +1548,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             impl_unexpanded_invocations: Default::default(),
             impl_binding_keys: Default::default(),
             current_crate_outer_attr_insert_span,
+            mods_with_parse_errors: Default::default(),
         };
 
         let root_parent_scope = ParentScope::module(graph_root, &resolver);
diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
index 0b4d0e04c295..669a9c2428f8 100644
--- a/compiler/rustc_resolve/src/macros.rs
+++ b/compiler/rustc_resolve/src/macros.rs
@@ -166,7 +166,7 @@ fn soft_custom_inner_attributes_gate(path: &ast::Path, invoc: &Invocation) -> bo
         [seg1, seg2] if seg1.ident.name == sym::rustfmt && seg2.ident.name == sym::skip => {
             if let InvocationKind::Attr { item, .. } = &invoc.kind {
                 if let Annotatable::Item(item) = item {
-                    if let ItemKind::Mod(_, ModKind::Loaded(_, Inline::No, _)) = item.kind {
+                    if let ItemKind::Mod(_, ModKind::Loaded(_, Inline::No, _, _)) = item.kind {
                         return true;
                     }
                 }
diff --git a/compiler/rustc_symbol_mangling/src/legacy.rs b/compiler/rustc_symbol_mangling/src/legacy.rs
index 59ccd6dff858..0d6d8488a23c 100644
--- a/compiler/rustc_symbol_mangling/src/legacy.rs
+++ b/compiler/rustc_symbol_mangling/src/legacy.rs
@@ -2,7 +2,7 @@ use std::fmt::{self, Write};
 use std::mem::{self, discriminant};
 
 use rustc_data_structures::stable_hasher::{Hash64, HashStable, StableHasher};
-use rustc_hir::def_id::CrateNum;
+use rustc_hir::def_id::{CrateNum, DefId};
 use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
 use rustc_middle::bug;
 use rustc_middle::ty::print::{PrettyPrinter, Print, PrintError, Printer};
@@ -378,6 +378,33 @@ impl<'tcx> Printer<'tcx> for SymbolPrinter<'tcx> {
             Ok(())
         }
     }
+
+    fn print_impl_path(
+        &mut self,
+        impl_def_id: DefId,
+        args: &'tcx [GenericArg<'tcx>],
+        mut self_ty: Ty<'tcx>,
+        mut impl_trait_ref: Option<ty::TraitRef<'tcx>>,
+    ) -> Result<(), PrintError> {
+        let mut typing_env = ty::TypingEnv::post_analysis(self.tcx, impl_def_id);
+        if !args.is_empty() {
+            typing_env.param_env =
+                ty::EarlyBinder::bind(typing_env.param_env).instantiate(self.tcx, args);
+        }
+
+        match &mut impl_trait_ref {
+            Some(impl_trait_ref) => {
+                assert_eq!(impl_trait_ref.self_ty(), self_ty);
+                *impl_trait_ref = self.tcx.normalize_erasing_regions(typing_env, *impl_trait_ref);
+                self_ty = impl_trait_ref.self_ty();
+            }
+            None => {
+                self_ty = self.tcx.normalize_erasing_regions(typing_env, self_ty);
+            }
+        }
+
+        self.default_print_impl_path(impl_def_id, args, self_ty, impl_trait_ref)
+    }
 }
 
 impl<'tcx> PrettyPrinter<'tcx> for SymbolPrinter<'tcx> {
diff --git a/library/core/src/alloc/mod.rs b/library/core/src/alloc/mod.rs
index aa841db045ce..f17687ed6e41 100644
--- a/library/core/src/alloc/mod.rs
+++ b/library/core/src/alloc/mod.rs
@@ -10,11 +10,6 @@ pub use self::global::GlobalAlloc;
 #[stable(feature = "alloc_layout", since = "1.28.0")]
 pub use self::layout::Layout;
 #[stable(feature = "alloc_layout", since = "1.28.0")]
-#[deprecated(
-    since = "1.52.0",
-    note = "Name does not follow std convention, use LayoutError",
-    suggestion = "LayoutError"
-)]
 #[allow(deprecated, deprecated_in_future)]
 pub use self::layout::LayoutErr;
 #[stable(feature = "alloc_layout_error", since = "1.50.0")]
diff --git a/library/core/src/hint.rs b/library/core/src/hint.rs
index c59e4414d372..9c054b99a27a 100644
--- a/library/core/src/hint.rs
+++ b/library/core/src/hint.rs
@@ -310,6 +310,8 @@ pub fn spin_loop() {
 /// behavior in the calling code. This property makes `black_box` useful for writing code in which
 /// certain optimizations are not desired, such as benchmarks.
 ///
+/// <div class="warning">
+///
 /// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
 /// extent to which it can block optimisations may vary depending upon the platform and code-gen
 /// backend used. Programs cannot rely on `black_box` for *correctness*, beyond it behaving as the
@@ -317,6 +319,8 @@ pub fn spin_loop() {
 /// This also means that this function does not offer any guarantees for cryptographic or security
 /// purposes.
 ///
+/// </div>
+///
 /// [`std::convert::identity`]: crate::convert::identity
 ///
 /// # When is this useful?
@@ -357,7 +361,7 @@ pub fn spin_loop() {
 /// ```
 /// use std::hint::black_box;
 ///
-/// // Same `contains` function
+/// // Same `contains` function.
 /// fn contains(haystack: &[&str], needle: &str) -> bool {
 ///     haystack.iter().any(|x| x == &needle)
 /// }
@@ -366,8 +370,13 @@ pub fn spin_loop() {
 ///     let haystack = vec!["abc", "def", "ghi", "jkl", "mno"];
 ///     let needle = "ghi";
 ///     for _ in 0..10 {
-///         // Adjust our benchmark loop contents
-///         black_box(contains(black_box(&haystack), black_box(needle)));
+///         // Force the compiler to run `contains`, even though it is a pure function whose
+///         // results are unused.
+///         black_box(contains(
+///             // Prevent the compiler from making assumptions about the input.
+///             black_box(&haystack),
+///             black_box(needle),
+///         ));
 ///     }
 /// }
 /// ```
@@ -382,6 +391,83 @@ pub fn spin_loop() {
 ///
 /// This makes our benchmark much more realistic to how the function would actually be used, where
 /// arguments are usually not known at compile time and the result is used in some way.
+///
+/// # How to use this
+///
+/// In practice, `black_box` serves two purposes:
+///
+/// 1. It prevents the compiler from making optimizations related to the value returned by `black_box`
+/// 2. It forces the value passed to `black_box` to be calculated, even if the return value of `black_box` is unused
+///
+/// ```
+/// use std::hint::black_box;
+///
+/// let zero = 0;
+/// let five = 5;
+///
+/// // The compiler will see this and remove the `* five` call, because it knows that multiplying
+/// // any integer by 0 will result in 0.
+/// let c = zero * five;
+///
+/// // Adding `black_box` here disables the compiler's ability to reason about the first operand in the multiplication.
+/// // It is forced to assume that it can be any possible number, so it cannot remove the `* five`
+/// // operation.
+/// let c = black_box(zero) * five;
+/// ```
+///
+/// While most cases will not be as clear-cut as the above example, it still illustrates how
+/// `black_box` can be used. When benchmarking a function, you usually want to wrap its inputs in
+/// `black_box` so the compiler cannot make optimizations that would be unrealistic in real-life
+/// use.
+///
+/// ```
+/// use std::hint::black_box;
+///
+/// // This is a simple function that increments its input by 1. Note that it is pure, meaning it
+/// // has no side-effects. This function has no effect if its result is unused. (An example of a
+/// // function *with* side-effects is `println!()`.)
+/// fn increment(x: u8) -> u8 {
+///     x + 1
+/// }
+///
+/// // Here, we call `increment` but discard its result. The compiler, seeing this and knowing that
+/// // `increment` is pure, will eliminate this function call entirely. This may not be desired,
+/// // though, especially if we're trying to track how much time `increment` takes to execute.
+/// let _ = increment(black_box(5));
+///
+/// // Here, we force `increment` to be executed. This is because the compiler treats `black_box`
+/// // as if it has side-effects, and thus must compute its input.
+/// let _ = black_box(increment(black_box(5)));
+/// ```
+///
+/// There may be additional situations where you want to wrap the result of a function in
+/// `black_box` to force its execution. This is situational though, and may not have any effect
+/// (such as when the function returns a zero-sized type such as [`()` unit][unit]).
+///
+/// Note that `black_box` has no effect on how its input is treated, only its output. As such,
+/// expressions passed to `black_box` may still be optimized:
+///
+/// ```
+/// use std::hint::black_box;
+///
+/// // The compiler sees this...
+/// let y = black_box(5 * 10);
+///
+/// // ...as this. As such, it will likely simplify `5 * 10` to just `50`.
+/// let _0 = 5 * 10;
+/// let y = black_box(_0);
+/// ```
+///
+/// In the above example, the `5 * 10` expression is considered distinct from the `black_box` call,
+/// and thus is still optimized by the compiler. You can prevent this by moving the multiplication
+/// operation outside of `black_box`:
+///
+/// ```
+/// use std::hint::black_box;
+///
+/// // No assumptions can be made about either operand, so the multiplication is not optimized out.
+/// let y = black_box(5) * black_box(10);
+/// ```
 #[inline]
 #[stable(feature = "bench_black_box", since = "1.66.0")]
 #[rustc_const_unstable(feature = "const_black_box", issue = "none")]
diff --git a/library/std/src/collections/mod.rs b/library/std/src/collections/mod.rs
index 889ed3c53803..ba57c5eb3530 100644
--- a/library/std/src/collections/mod.rs
+++ b/library/std/src/collections/mod.rs
@@ -433,8 +433,7 @@ pub use self::hash_map::HashMap;
 #[doc(inline)]
 pub use self::hash_set::HashSet;
 #[stable(feature = "rust1", since = "1.0.0")]
-// FIXME(#82080) The deprecation here is only theoretical, and does not actually produce a warning.
-#[deprecated(note = "moved to `std::ops::Bound`", since = "1.26.0")]
+// FIXME(#82080) This has moved but #[deprecated] on `use` is unsupported.
 #[doc(hidden)]
 pub use crate::ops::Bound;
 
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index c6800aedca97..3b3595f0966d 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -1320,7 +1320,33 @@ impl Config {
 
         // Set flags.
         config.paths = std::mem::take(&mut flags.paths);
-        config.skip = flags.skip.into_iter().chain(flags.exclude).collect();
+        config.skip = flags
+            .skip
+            .into_iter()
+            .chain(flags.exclude)
+            .map(|p| {
+                let p = if cfg!(windows) {
+                    PathBuf::from(p.to_str().unwrap().replace('/', "\\"))
+                } else {
+                    p
+                };
+
+                // Jump to top-level project path to support passing paths
+                // from sub directories.
+                let top_level_path = config.src.join(&p);
+                assert!(
+                    config.src.join(&top_level_path).exists(),
+                    "{} does not exist.",
+                    top_level_path.display()
+                );
+
+                // Never return top-level path here as it would break `--skip`
+                // logic on rustc's internal test framework which is utilized
+                // by compiletest.
+                p
+            })
+            .collect();
+
         config.include_default_paths = flags.include_default_paths;
         config.rustc_error_format = flags.rustc_error_format;
         config.json_output = flags.json_output;
diff --git a/src/tools/clippy/clippy_lints/src/duplicate_mod.rs b/src/tools/clippy/clippy_lints/src/duplicate_mod.rs
index ed27e38ef2d1..1dac7b971f95 100644
--- a/src/tools/clippy/clippy_lints/src/duplicate_mod.rs
+++ b/src/tools/clippy/clippy_lints/src/duplicate_mod.rs
@@ -63,7 +63,7 @@ impl_lint_pass!(DuplicateMod => [DUPLICATE_MOD]);
 
 impl EarlyLintPass for DuplicateMod {
     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
-        if let ItemKind::Mod(_, ModKind::Loaded(_, Inline::No, mod_spans)) = &item.kind
+        if let ItemKind::Mod(_, ModKind::Loaded(_, Inline::No, mod_spans, _)) = &item.kind
             && let FileName::Real(real) = cx.sess().source_map().span_to_filename(mod_spans.inner_span)
             && let Some(local_path) = real.into_local_path()
             && let Ok(absolute_path) = local_path.canonicalize()
diff --git a/src/tools/clippy/clippy_lints/src/excessive_nesting.rs b/src/tools/clippy/clippy_lints/src/excessive_nesting.rs
index ffc76366983a..dfea40db182f 100644
--- a/src/tools/clippy/clippy_lints/src/excessive_nesting.rs
+++ b/src/tools/clippy/clippy_lints/src/excessive_nesting.rs
@@ -163,7 +163,7 @@ impl Visitor<'_> for NestingVisitor<'_, '_> {
         }
 
         match &item.kind {
-            ItemKind::Trait(_) | ItemKind::Impl(_) | ItemKind::Mod(.., ModKind::Loaded(_, Inline::Yes, _)) => {
+            ItemKind::Trait(_) | ItemKind::Impl(_) | ItemKind::Mod(.., ModKind::Loaded(_, Inline::Yes, _, _)) => {
                 self.nest_level += 1;
 
                 if !self.check_indent(item.span, item.id) {
diff --git a/src/tools/clippy/clippy_utils/src/ast_utils.rs b/src/tools/clippy/clippy_utils/src/ast_utils.rs
index 620a9b56eb55..12074dd16e60 100644
--- a/src/tools/clippy/clippy_utils/src/ast_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/ast_utils.rs
@@ -379,7 +379,7 @@ pub fn eq_item_kind(l: &ItemKind, r: &ItemKind) -> bool {
         (Mod(lu, lmk), Mod(ru, rmk)) => {
             lu == ru
                 && match (lmk, rmk) {
-                    (ModKind::Loaded(litems, linline, _), ModKind::Loaded(ritems, rinline, _)) => {
+                    (ModKind::Loaded(litems, linline, _, _), ModKind::Loaded(ritems, rinline, _, _)) => {
                         linline == rinline && over(litems, ritems, |l, r| eq_item(l, r, eq_item_kind))
                     },
                     (ModKind::Unloaded, ModKind::Unloaded) => true,
diff --git a/src/tools/rustfmt/src/items.rs b/src/tools/rustfmt/src/items.rs
index 18932587f1fb..c3debc2f4f04 100644
--- a/src/tools/rustfmt/src/items.rs
+++ b/src/tools/rustfmt/src/items.rs
@@ -3597,7 +3597,7 @@ pub(crate) fn rewrite_extern_crate(
 pub(crate) fn is_mod_decl(item: &ast::Item) -> bool {
     !matches!(
         item.kind,
-        ast::ItemKind::Mod(_, ast::ModKind::Loaded(_, ast::Inline::Yes, _))
+        ast::ItemKind::Mod(_, ast::ModKind::Loaded(_, ast::Inline::Yes, _, _))
     )
 }
 
diff --git a/src/tools/rustfmt/src/modules.rs b/src/tools/rustfmt/src/modules.rs
index 493b04f16c6f..a40ee7f66a97 100644
--- a/src/tools/rustfmt/src/modules.rs
+++ b/src/tools/rustfmt/src/modules.rs
@@ -316,12 +316,11 @@ impl<'ast, 'psess, 'c> ModResolver<'ast, 'psess> {
             self.directory = directory;
         }
         match (sub_mod.ast_mod_kind, sub_mod.items) {
-            (Some(Cow::Borrowed(ast::ModKind::Loaded(items, _, _))), _) => {
+            (Some(Cow::Borrowed(ast::ModKind::Loaded(items, _, _, _))), _) => {
                 self.visit_mod_from_ast(items)
             }
-            (Some(Cow::Owned(ast::ModKind::Loaded(items, _, _))), _) | (_, Cow::Owned(items)) => {
-                self.visit_mod_outside_ast(items)
-            }
+            (Some(Cow::Owned(ast::ModKind::Loaded(items, _, _, _))), _)
+            | (_, Cow::Owned(items)) => self.visit_mod_outside_ast(items),
             (_, _) => Ok(()),
         }
     }
diff --git a/src/tools/rustfmt/src/visitor.rs b/src/tools/rustfmt/src/visitor.rs
index 9b116b620b79..805e13b78036 100644
--- a/src/tools/rustfmt/src/visitor.rs
+++ b/src/tools/rustfmt/src/visitor.rs
@@ -927,7 +927,7 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
         let ident_str = rewrite_ident(&self.get_context(), ident).to_owned();
         self.push_str(&ident_str);
 
-        if let ast::ModKind::Loaded(ref items, ast::Inline::Yes, ref spans) = mod_kind {
+        if let ast::ModKind::Loaded(ref items, ast::Inline::Yes, ref spans, _) = mod_kind {
             let ast::ModSpans {
                 inner_span,
                 inject_use_span: _,
diff --git a/tests/ui/deprecation/deprecation-sanity.rs b/tests/ui/deprecation/deprecation-sanity.rs
index 9ea75b68f81c..27a8fc3580ae 100644
--- a/tests/ui/deprecation/deprecation-sanity.rs
+++ b/tests/ui/deprecation/deprecation-sanity.rs
@@ -39,4 +39,11 @@ impl Default for X {
     }
 }
 
+mod inner {
+    pub struct Y;
+}
+
+#[deprecated] //~ ERROR this `#[deprecated]` annotation has no effect
+pub use inner::Y;
+
 fn main() { }
diff --git a/tests/ui/deprecation/deprecation-sanity.stderr b/tests/ui/deprecation/deprecation-sanity.stderr
index 383212ad9b4b..e05fbc0aa14c 100644
--- a/tests/ui/deprecation/deprecation-sanity.stderr
+++ b/tests/ui/deprecation/deprecation-sanity.stderr
@@ -68,7 +68,13 @@ LL | #[deprecated = "hello"]
    |
    = note: `#[deny(useless_deprecated)]` on by default
 
-error: aborting due to 10 previous errors
+error: this `#[deprecated]` annotation has no effect
+  --> $DIR/deprecation-sanity.rs:46:1
+   |
+LL | #[deprecated]
+   | ^^^^^^^^^^^^^ help: remove the unnecessary deprecation attribute
+
+error: aborting due to 11 previous errors
 
 Some errors have detailed explanations: E0538, E0539, E0541, E0565.
 For more information about an error, try `rustc --explain E0538`.
diff --git a/tests/ui/imports/unused-import-issue-87973.fixed b/tests/ui/imports/unused-import-issue-87973.fixed
index d1167d7d4861..402655e4fbc8 100644
--- a/tests/ui/imports/unused-import-issue-87973.fixed
+++ b/tests/ui/imports/unused-import-issue-87973.fixed
@@ -1,5 +1,6 @@
 //@ run-rustfix
 #![deny(unused_imports)]
+#![allow(useless_deprecated)]
 
 // Check that attributes get removed too. See #87973.
 //~^ ERROR unused import
diff --git a/tests/ui/imports/unused-import-issue-87973.rs b/tests/ui/imports/unused-import-issue-87973.rs
index 1b016ff814c6..0186ddd5ec75 100644
--- a/tests/ui/imports/unused-import-issue-87973.rs
+++ b/tests/ui/imports/unused-import-issue-87973.rs
@@ -1,5 +1,6 @@
 //@ run-rustfix
 #![deny(unused_imports)]
+#![allow(useless_deprecated)]
 
 // Check that attributes get removed too. See #87973.
 #[deprecated]
diff --git a/tests/ui/imports/unused-import-issue-87973.stderr b/tests/ui/imports/unused-import-issue-87973.stderr
index a43e92b14580..3628f319411c 100644
--- a/tests/ui/imports/unused-import-issue-87973.stderr
+++ b/tests/ui/imports/unused-import-issue-87973.stderr
@@ -1,5 +1,5 @@
 error: unused import: `std::fs`
-  --> $DIR/unused-import-issue-87973.rs:8:5
+  --> $DIR/unused-import-issue-87973.rs:9:5
    |
 LL | use std::fs;
    |     ^^^^^^^
diff --git a/tests/ui/parser/brace-in-let-chain.rs b/tests/ui/parser/brace-in-let-chain.rs
index 1f34c73a2c3b..2009bc88d9e8 100644
--- a/tests/ui/parser/brace-in-let-chain.rs
+++ b/tests/ui/parser/brace-in-let-chain.rs
@@ -3,7 +3,7 @@
 #![feature(let_chains)]
 fn main() {
     if let () = ()
-        && let () = () { //~ERROR: found a `{` in the middle of a let-chain
+        && let () = () {
         && let () = ()
     {
     }
@@ -11,7 +11,7 @@ fn main() {
 
 fn quux() {
     while let () = ()
-        && let () = () { //~ERROR: found a `{` in the middle of a let-chain
+        && let () = () {
         && let () = ()
     {
     }
diff --git a/tests/ui/parser/brace-in-let-chain.stderr b/tests/ui/parser/brace-in-let-chain.stderr
index 913a34700dfc..12af95c27868 100644
--- a/tests/ui/parser/brace-in-let-chain.stderr
+++ b/tests/ui/parser/brace-in-let-chain.stderr
@@ -27,33 +27,5 @@ LL |     }
 LL | }
    |                                                     ^
 
-error: found a `{` in the middle of a let-chain
-  --> $DIR/brace-in-let-chain.rs:14:24
-   |
-LL |         && let () = () {
-   |                        ^
-LL |         && let () = ()
-   |         ------ you might have meant to continue the let-chain here
-   |
-help: consider removing this brace to parse the `let` as part of the same chain
-   |
-LL -         && let () = () {
-LL +         && let () = ()
-   |
-
-error: found a `{` in the middle of a let-chain
-  --> $DIR/brace-in-let-chain.rs:6:24
-   |
-LL |         && let () = () {
-   |                        ^
-LL |         && let () = ()
-   |         ------ you might have meant to continue the let-chain here
-   |
-help: consider removing this brace to parse the `let` as part of the same chain
-   |
-LL -         && let () = () {
-LL +         && let () = ()
-   |
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
diff --git a/tests/ui/parser/circular_modules_main.stderr b/tests/ui/parser/circular_modules_main.stderr
index 2de70789358b..e1780fe9fd9d 100644
--- a/tests/ui/parser/circular_modules_main.stderr
+++ b/tests/ui/parser/circular_modules_main.stderr
@@ -4,22 +4,5 @@ error: circular modules: $DIR/circular_modules_main.rs -> $DIR/circular_modules_
 LL | mod circular_modules_main;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0425]: cannot find function `hi_str` in module `circular_modules_main`
-  --> $DIR/circular_modules_hello.rs:7:43
-   |
-LL |     println!("{}", circular_modules_main::hi_str());
-   |                                           ^^^^^^ not found in `circular_modules_main`
-   |
-help: consider importing this function
-   |
-LL + use hi_str;
-   |
-help: if you import `hi_str`, refer to it directly
-   |
-LL -     println!("{}", circular_modules_main::hi_str());
-LL +     println!("{}", hi_str());
-   |
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0425`.
diff --git a/tests/ui/parser/diff-markers/unclosed-delims-in-macro.rs b/tests/ui/parser/diff-markers/unclosed-delims-in-macro.rs
index da1774acea54..41a7de03d4b7 100644
--- a/tests/ui/parser/diff-markers/unclosed-delims-in-macro.rs
+++ b/tests/ui/parser/diff-markers/unclosed-delims-in-macro.rs
@@ -1,9 +1,11 @@
+// The diff marker detection was removed for this example, because it relied on
+// the lexer having a dependency on the parser, which was horrible.
+
 macro_rules! foo {
 <<<<<<< HEAD
-    //~^ ERROR encountered diff marker
     () {
 =======
     () { //
 >>>>>>> 7a4f13c blah blah blah
     }
-}
+} //~ this file contains an unclosed delimiter
diff --git a/tests/ui/parser/diff-markers/unclosed-delims-in-macro.stderr b/tests/ui/parser/diff-markers/unclosed-delims-in-macro.stderr
index 927821ddfaed..b33f2b5d1b8b 100644
--- a/tests/ui/parser/diff-markers/unclosed-delims-in-macro.stderr
+++ b/tests/ui/parser/diff-markers/unclosed-delims-in-macro.stderr
@@ -1,23 +1,16 @@
-error: encountered diff marker
-  --> $DIR/unclosed-delims-in-macro.rs:2:1
+error: this file contains an unclosed delimiter
+  --> $DIR/unclosed-delims-in-macro.rs:11:48
    |
+LL | macro_rules! foo {
+   |                  - unclosed delimiter
 LL | <<<<<<< HEAD
-   | ^^^^^^^ between this marker and `=======` is the code that we're merging into
+LL |     () {
+   |        - this delimiter might not be properly closed...
 ...
-LL | =======
-   | ------- between this marker and `>>>>>>>` is the incoming code
-LL |     () { //
-LL | >>>>>>> 7a4f13c blah blah blah
-   | ^^^^^^^ this marker concludes the conflict region
-   |
-   = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
-           to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
-   = help: if you're having merge conflicts after pulling new code:
-           the top section is the code you already had and the bottom section is the remote code
-           if you're in the middle of a rebase:
-           the top section is the code being rebased onto and the bottom section is the code coming from the current commit being rebased
-   = note: for an explanation on these markers from the `git` documentation:
-           visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>
+LL | }
+   | -                                             ^
+   | |
+   | ...as it matches this but it has different indentation
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/parser/diff-markers/unclosed-delims.rs b/tests/ui/parser/diff-markers/unclosed-delims.rs
index 7d400c3827bb..827c1eebb9d5 100644
--- a/tests/ui/parser/diff-markers/unclosed-delims.rs
+++ b/tests/ui/parser/diff-markers/unclosed-delims.rs
@@ -1,18 +1,12 @@
+// The diff marker detection was removed for this example, because it relied on
+// the lexer having a dependency on the parser, which was horrible.
+
 mod tests {
     #[test]
 <<<<<<< HEAD
-//~^ ERROR encountered diff marker
-//~| NOTE between this marker and `=======`
-
-//~| NOTE conflict markers indicate that
-//~| HELP if you're having merge conflicts
-//~| NOTE for an explanation on these markers
-
     fn test1() {
 =======
-//~^ NOTE between this marker and `>>>>>>>`
     fn test2() {
 >>>>>>> 7a4f13c blah blah blah
-//~^ NOTE this marker concludes the conflict region
     }
-}
+} //~ this file contains an unclosed delimiter
diff --git a/tests/ui/parser/diff-markers/unclosed-delims.stderr b/tests/ui/parser/diff-markers/unclosed-delims.stderr
index 1eab96442b4f..b2541aa47bae 100644
--- a/tests/ui/parser/diff-markers/unclosed-delims.stderr
+++ b/tests/ui/parser/diff-markers/unclosed-delims.stderr
@@ -1,23 +1,16 @@
-error: encountered diff marker
-  --> $DIR/unclosed-delims.rs:3:1
+error: this file contains an unclosed delimiter
+  --> $DIR/unclosed-delims.rs:12:48
    |
-LL | <<<<<<< HEAD
-   | ^^^^^^^ between this marker and `=======` is the code that we're merging into
+LL | mod tests {
+   |           - unclosed delimiter
 ...
-LL | =======
-   | ------- between this marker and `>>>>>>>` is the incoming code
+LL |     fn test1() {
+   |                - this delimiter might not be properly closed...
 ...
-LL | >>>>>>> 7a4f13c blah blah blah
-   | ^^^^^^^ this marker concludes the conflict region
-   |
-   = note: conflict markers indicate that a merge was started but could not be completed due to merge conflicts
-           to resolve a conflict, keep only the code you want and then delete the lines containing conflict markers
-   = help: if you're having merge conflicts after pulling new code:
-           the top section is the code you already had and the bottom section is the remote code
-           if you're in the middle of a rebase:
-           the top section is the code being rebased onto and the bottom section is the code coming from the current commit being rebased
-   = note: for an explanation on these markers from the `git` documentation:
-           visit <https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging#_checking_out_conflicts>
+LL | }
+   | -                                             ^
+   | |
+   | ...as it matches this but it has different indentation
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/resolve/parse-error-resolve.rs b/tests/ui/resolve/parse-error-resolve.rs
new file mode 100644
index 000000000000..1e0772648afe
--- /dev/null
+++ b/tests/ui/resolve/parse-error-resolve.rs
@@ -0,0 +1,7 @@
+mod parse_error;
+use parse_error::Canonical; // ok, `parse_error.rs` had parse errors
+
+fn main() {
+    let _ = "" + 1; //~ ERROR E0369
+    parse_error::Canonical.foo(); // ok, `parse_error.rs` had parse errors
+}
diff --git a/tests/ui/resolve/parse-error-resolve.stderr b/tests/ui/resolve/parse-error-resolve.stderr
new file mode 100644
index 000000000000..30475aa0ee63
--- /dev/null
+++ b/tests/ui/resolve/parse-error-resolve.stderr
@@ -0,0 +1,22 @@
+error: expected one of `+`, `,`, `::`, `=`, or `>`, found `From`
+  --> $DIR/parse_error.rs:2:46
+   |
+LL | impl<S: Into<std::borrow::Cow<'static, str>> From<S> for Canonical {
+   |                                              ^^^^ expected one of `+`, `,`, `::`, `=`, or `>`
+   |
+help: you might have meant to end the type parameters here
+   |
+LL | impl<S: Into<std::borrow::Cow<'static, str>>> From<S> for Canonical {
+   |                                             +
+
+error[E0369]: cannot add `{integer}` to `&str`
+  --> $DIR/parse-error-resolve.rs:5:16
+   |
+LL |     let _ = "" + 1;
+   |             -- ^ - {integer}
+   |             |
+   |             &str
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0369`.
diff --git a/tests/ui/resolve/parse_error.rs b/tests/ui/resolve/parse_error.rs
new file mode 100644
index 000000000000..4cd025e1edf5
--- /dev/null
+++ b/tests/ui/resolve/parse_error.rs
@@ -0,0 +1,5 @@
+struct Canonical;
+impl<S: Into<std::borrow::Cow<'static, str>> From<S> for Canonical { //~ ERROR expected
+    fn foo() {}
+}
+fn bar() {}
diff --git a/tests/ui/resolve/parse_error.stderr b/tests/ui/resolve/parse_error.stderr
new file mode 100644
index 000000000000..f764f08875ff
--- /dev/null
+++ b/tests/ui/resolve/parse_error.stderr
@@ -0,0 +1,13 @@
+error: expected one of `+`, `,`, `::`, `=`, or `>`, found `From`
+  --> $DIR/parse_error.rs:2:46
+   |
+LL | impl<S: Into<std::borrow::Cow<'static, str>> From<S> for Canonical {
+   |                                              ^^^^ expected one of `+`, `,`, `::`, `=`, or `>`
+   |
+help: you might have meant to end the type parameters here
+   |
+LL | impl<S: Into<std::borrow::Cow<'static, str>>> From<S> for Canonical {
+   |                                             +
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/symbol-names/types.legacy.stderr b/tests/ui/symbol-names/types.legacy.stderr
index 87c3acae0bd6..c368b3186098 100644
--- a/tests/ui/symbol-names/types.legacy.stderr
+++ b/tests/ui/symbol-names/types.legacy.stderr
@@ -502,5 +502,23 @@ error: demangling-alt(a::b::Type<[T; N]>)
 LL |     #[rustc_symbol_name]
    |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 84 previous errors
+error: symbol-name(_ZN1a1b35Type$LT$$u5b$u8$u3b$$u20$0$u5d$$GT$17h[HASH]E)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(a::b::Type<[u8; 0]>::h[HASH])
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(a::b::Type<[u8; 0]>)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 87 previous errors
 
diff --git a/tests/ui/symbol-names/types.rs b/tests/ui/symbol-names/types.rs
index 7ed19e0e5a82..38735e1aa509 100644
--- a/tests/ui/symbol-names/types.rs
+++ b/tests/ui/symbol-names/types.rs
@@ -266,6 +266,17 @@ pub fn b() {
     //[v0]~| ERROR ::b::Type<[_; _]>>)
     //[v0]~| ERROR demangling-alt(<a::b::Type<[_; _]>>)
     impl<const N: usize, T> Type<[T; N]> {}
+
+    const ZERO: usize = 0;
+
+    #[rustc_symbol_name]
+    //[legacy,verbose-legacy]~^ ERROR symbol-name(_ZN1a1b35Type$LT$$u5b$u8$u3b$$u20$0$u5d$$GT$
+    //[legacy,verbose-legacy]~| ERROR demangling(a::b::Type<[u8; 0]>::
+    //[legacy,verbose-legacy]~| ERROR demangling-alt(a::b::Type<[u8; 0]>)
+    //[v0]~^^^^ ERROR symbol-name(_RMsq_NvCsCRATE_HASH_1a1bINtB<REF>_4TypeAhj0_E)
+    //[v0]~| ERROR ::b::Type<[u8; 0usize]>>)
+    //[v0]~| ERROR demangling-alt(<a::b::Type<[u8; 0]>>)
+    impl Type<[u8; ZERO]> {}
 }
 
 fn main() {}
diff --git a/tests/ui/symbol-names/types.v0.stderr b/tests/ui/symbol-names/types.v0.stderr
index 58680e002022..90012a2dcf72 100644
--- a/tests/ui/symbol-names/types.v0.stderr
+++ b/tests/ui/symbol-names/types.v0.stderr
@@ -502,5 +502,23 @@ error: demangling-alt(<a::b::Type<[_; _]>>)
 LL |     #[rustc_symbol_name]
    |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 84 previous errors
+error: symbol-name(_RMsq_NvCsCRATE_HASH_1a1bINtB<REF>_4TypeAhj0_E)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(<a[HASH]::b::Type<[u8; 0usize]>>)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(<a::b::Type<[u8; 0]>>)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 87 previous errors
 
diff --git a/tests/ui/symbol-names/types.verbose-legacy.stderr b/tests/ui/symbol-names/types.verbose-legacy.stderr
index 87c3acae0bd6..c368b3186098 100644
--- a/tests/ui/symbol-names/types.verbose-legacy.stderr
+++ b/tests/ui/symbol-names/types.verbose-legacy.stderr
@@ -502,5 +502,23 @@ error: demangling-alt(a::b::Type<[T; N]>)
 LL |     #[rustc_symbol_name]
    |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 84 previous errors
+error: symbol-name(_ZN1a1b35Type$LT$$u5b$u8$u3b$$u20$0$u5d$$GT$17h[HASH]E)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling(a::b::Type<[u8; 0]>::h[HASH])
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: demangling-alt(a::b::Type<[u8; 0]>)
+  --> $DIR/types.rs:272:5
+   |
+LL |     #[rustc_symbol_name]
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 87 previous errors