aarch64-softfloat: forbid enabling the neon target feature

compiler/rustc_codegen_ssa/messages.ftl

@@ -1,5 +1,7 @@
 codegen_ssa_L4Bender_exporting_symbols_unimplemented = exporting symbols not implemented yet for L4Bender
 
+codegen_ssa_aarch64_softfloat_neon = enabling the `neon` target feature on the current target is unsound due to ABI issues
+
 codegen_ssa_add_native_library = failed to add native library {$library_path}: {$error}
 
 codegen_ssa_aix_strip_not_used = using host's `strip` binary to cross-compile to AIX which is not guaranteed to work

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -299,6 +299,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                 }
                 from_target_feature_attr(
                     tcx,
+                    did,
                     attr,
                     rust_target_features,
                     &mut codegen_fn_attrs.target_features,

compiler/rustc_codegen_ssa/src/errors.rs

@@ -1316,3 +1316,7 @@ pub(crate) struct XcrunSdkPathWarning {
     pub sdk_name: &'static str,
     pub stderr: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(codegen_ssa_aarch64_softfloat_neon)]
+pub(crate) struct Aarch64SoftfloatNeon;

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -8,6 +8,7 @@ use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
 use rustc_middle::middle::codegen_fn_attrs::TargetFeature;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
+use rustc_session::lint::builtin::AARCH64_SOFTFLOAT_NEON;
 use rustc_session::parse::feature_err;
 use rustc_span::{Span, Symbol, sym};
 use rustc_target::target_features::{self, Stability};
@@ -18,6 +19,7 @@ use crate::errors;
 /// Enabled target features are added to `target_features`.
 pub(crate) fn from_target_feature_attr(
     tcx: TyCtxt<'_>,
+    did: LocalDefId,
     attr: &hir::Attribute,
     rust_target_features: &UnordMap<String, target_features::Stability>,
     target_features: &mut Vec<TargetFeature>,
@@ -92,11 +94,22 @@ pub(crate) fn from_target_feature_attr(
                     // generating code so "it's fine".
                     if !tcx.sess.opts.actually_rustdoc {
                         if abi_feature_constraints.incompatible.contains(&name.as_str()) {
-                            tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
-                                span: item.span(),
-                                feature: name.as_str(),
-                                reason: "this feature is incompatible with the target ABI",
-                            });
+                            // For "neon" specifically, we emit an FCW instead of a hard error.
+                            // See <https://github.com/rust-lang/rust/issues/134375>.
+                            if name.as_str() == "neon" {
+                                tcx.emit_node_span_lint(
+                                    AARCH64_SOFTFLOAT_NEON,
+                                    tcx.local_def_id_to_hir_id(did),
+                                    item.span(),
+                                    errors::Aarch64SoftfloatNeon,
+                                );
+                            } else {
+                                tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
+                                    span: item.span(),
+                                    feature: name.as_str(),
+                                    reason: "this feature is incompatible with the target ABI",
+                                });
+                            }
                         }
                     }
                     target_features.push(TargetFeature { name, implied: name != feature_sym })

compiler/rustc_lint_defs/src/builtin.rs

@@ -16,6 +16,7 @@ declare_lint_pass! {
     /// that are used by other parts of the compiler.
     HardwiredLints => [
         // tidy-alphabetical-start
+        AARCH64_SOFTFLOAT_NEON,
         ABSOLUTE_PATHS_NOT_STARTING_WITH_CRATE,
         AMBIGUOUS_ASSOCIATED_ITEMS,
         AMBIGUOUS_GLOB_IMPORTS,
@@ -5069,14 +5070,14 @@ declare_lint! {
     ///
     /// ```text
     /// error: this function function definition is affected by the wasm ABI transition: it passes an argument of non-scalar type `MyType`
-    /// --> $DIR/wasm_c_abi_transition.rs:17:1
-    ///  |
-    ///  | pub extern "C" fn my_fun(_x: MyType) {}
-    ///  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-    ///  |
-    ///  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-    ///  = note: for more information, see issue #138762 <https://github.com/rust-lang/rust/issues/138762>
-    ///  = help: the "C" ABI Rust uses on wasm32-unknown-unknown will change to align with the standard "C" ABI for this target
+    ///   --> $DIR/wasm_c_abi_transition.rs:17:1
+    ///    |
+    ///    | pub extern "C" fn my_fun(_x: MyType) {}
+    ///    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    ///    |
+    ///    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    ///    = note: for more information, see issue #138762 <https://github.com/rust-lang/rust/issues/138762>
+    ///    = help: the "C" ABI Rust uses on wasm32-unknown-unknown will change to align with the standard "C" ABI for this target
     /// ```
     ///
     /// ### Explanation
@@ -5093,3 +5094,44 @@ declare_lint! {
         reference: "issue #138762 <https://github.com/rust-lang/rust/issues/138762>",
     };
 }
+
+declare_lint! {
+    /// The `aarch64_softfloat_neon` lint detects usage of `#[target_feature(enable = "neon")]` on
+    /// softfloat aarch64 targets. Enabling this target feature causes LLVM to alter the ABI of
+    /// function calls, making this attribute unsound to use.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (needs aarch64-unknown-none-softfloat)
+    /// #[target_feature(enable = "neon")]
+    /// fn with_neon() {}
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// error: enabling the `neon` target feature on the current target is unsound due to ABI issues
+    ///   --> $DIR/abi-incompatible-target-feature-attribute-fcw.rs:11:18
+    ///    |
+    ///    | #[target_feature(enable = "neon")]
+    ///    |                  ^^^^^^^^^^^^^^^
+    ///    |
+    ///    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    ///    = note: for more information, see issue #134375 <https://github.com/rust-lang/rust/issues/134375>
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// If a function like `with_neon` above ends up containing calls to LLVM builtins, those will
+    /// not use the correct ABI. This is caused by a lack of support in LLVM for mixing code with
+    /// and without the `neon` target feature. The target feature should never have been stabilized
+    /// on this target due to this issue, but the problem was not known at the time of
+    /// stabilization.
+    pub AARCH64_SOFTFLOAT_NEON,
+    Warn,
+    "detects code that could be affected by ABI issues on aarch64 softfloat targets",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
+        reference: "issue #134375 <https://github.com/rust-lang/rust/issues/134375>",
+    };
+}

compiler/rustc_target/src/target_features.rs

@@ -946,14 +946,16 @@ impl Target {
                 // the use of soft-float, so all we can do here is some crude hacks.
                 match &*self.abi {
                     "softfloat" => {
-                        // This is not fully correct, LLVM actually doesn't let us enforce the softfloat
-                        // ABI properly... see <https://github.com/rust-lang/rust/issues/134375>.
-                        // FIXME: should we forbid "neon" here? But that would be a breaking change.
-                        NOTHING
+                        // LLVM will use float registers when `fp-armv8` is available, e.g. for
+                        // calls to built-ins. The only way to ensure a consistent softfloat ABI
+                        // on aarch64 is to never enable `fp-armv8`, so we enforce that.
+                        // In Rust we tie `neon` and `fp-armv8` together, therefore `neon` is the
+                        // feature we have to mark as incompatible.
+                        FeatureConstraints { required: &[], incompatible: &["neon"] }
                     }
                     _ => {
                         // Everything else is assumed to use a hardfloat ABI. neon and fp-armv8 must be enabled.
-                        // These are Rust feature names and we use "neon" to control both of them.
+                        // `FeatureConstraints` uses Rust feature names, hence only "neon" shows up.
                         FeatureConstraints { required: &["neon"], incompatible: &[] }
                     }
                 }

tests/ui/target-feature/abi-incompatible-target-feature-attribute-fcw.rs

@@ -0,0 +1,14 @@
+//@ compile-flags: --crate-type=lib
+//@ compile-flags: --target=aarch64-unknown-none-softfloat
+//@ needs-llvm-components: aarch64
+#![feature(no_core, lang_items)]
+#![no_core]
+#![deny(aarch64_softfloat_neon)]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+#[target_feature(enable = "neon")]
+//~^ERROR: enabling the `neon` target feature on the current target is unsound
+//~|WARN: previously accepted
+pub unsafe fn my_fun() {}

tests/ui/target-feature/abi-incompatible-target-feature-attribute-fcw.stderr

@@ -0,0 +1,31 @@
+error: enabling the `neon` target feature on the current target is unsound due to ABI issues
+  --> $DIR/abi-incompatible-target-feature-attribute-fcw.rs:11:18
+   |
+LL | #[target_feature(enable = "neon")]
+   |                  ^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #134375 <https://github.com/rust-lang/rust/issues/134375>
+note: the lint level is defined here
+  --> $DIR/abi-incompatible-target-feature-attribute-fcw.rs:6:9
+   |
+LL | #![deny(aarch64_softfloat_neon)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+Future incompatibility report: Future breakage diagnostic:
+error: enabling the `neon` target feature on the current target is unsound due to ABI issues
+  --> $DIR/abi-incompatible-target-feature-attribute-fcw.rs:11:18
+   |
+LL | #[target_feature(enable = "neon")]
+   |                  ^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #134375 <https://github.com/rust-lang/rust/issues/134375>
+note: the lint level is defined here
+  --> $DIR/abi-incompatible-target-feature-attribute-fcw.rs:6:9
+   |
+LL | #![deny(aarch64_softfloat_neon)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+