diff --git a/src/libcore/alloc/mod.rs b/src/libcore/alloc/mod.rs
index e1892edb7c7f3..86a6fa7f8ba3c 100644
--- a/src/libcore/alloc/mod.rs
+++ b/src/libcore/alloc/mod.rs
@@ -18,7 +18,7 @@ use crate::ptr::{self, NonNull};
 /// something wrong when combining the given input arguments with this
 /// allocator.
 #[unstable(feature = "allocator_api", issue = "32838")]
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub struct AllocErr;
 
 // (we need this for downstream impl of trait Error)
diff --git a/src/libcore/marker.rs b/src/libcore/marker.rs
index 35bceaa25c36e..549933ceeb645 100644
--- a/src/libcore/marker.rs
+++ b/src/libcore/marker.rs
@@ -709,6 +709,7 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 /// So this, for example, can only be done on types implementing `Unpin`:
 ///
 /// ```rust
+/// # #![allow(unused_must_use)]
 /// use std::mem;
 /// use std::pin::Pin;
 ///
diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
index 07f7d28bb7546..3fa2b7a2d042c 100644
--- a/src/libcore/mem/mod.rs
+++ b/src/libcore/mem/mod.rs
@@ -808,6 +808,7 @@ pub fn take<T: Default>(dest: &mut T) -> T {
 /// [`Clone`]: ../../std/clone/trait.Clone.html
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[must_use = "if you don't need the old value, you can just assign the new value directly"]
 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
     swap(dest, &mut src);
     src
diff --git a/src/libproc_macro/bridge/client.rs b/src/libproc_macro/bridge/client.rs
index 088db92253acf..d2222d12623f9 100644
--- a/src/libproc_macro/bridge/client.rs
+++ b/src/libproc_macro/bridge/client.rs
@@ -290,6 +290,13 @@ impl BridgeState<'_> {
 }
 
 impl Bridge<'_> {
+    pub(crate) fn is_available() -> bool {
+        BridgeState::with(|state| match state {
+            BridgeState::Connected(_) | BridgeState::InUse => true,
+            BridgeState::NotConnected => false,
+        })
+    }
+
     fn enter<R>(self, f: impl FnOnce() -> R) -> R {
         // Hide the default panic output within `proc_macro` expansions.
         // NB. the server can't do this because it may use a different libstd.
diff --git a/src/libproc_macro/lib.rs b/src/libproc_macro/lib.rs
index a975ce93bb1a3..3cbe852de7b5a 100644
--- a/src/libproc_macro/lib.rs
+++ b/src/libproc_macro/lib.rs
@@ -45,6 +45,24 @@ use std::path::PathBuf;
 use std::str::FromStr;
 use std::{error, fmt, iter, mem};
 
+/// Determines whether proc_macro has been made accessible to the currently
+/// running program.
+///
+/// The proc_macro crate is only intended for use inside the implementation of
+/// procedural macros. All the functions in this crate panic if invoked from
+/// outside of a procedural macro, such as from a build script or unit test or
+/// ordinary Rust binary.
+///
+/// With consideration for Rust libraries that are designed to support both
+/// macro and non-macro use cases, `proc_macro::is_available()` provides a
+/// non-panicking way to detect whether the infrastructure required to use the
+/// API of proc_macro is presently available. Returns true if invoked from
+/// inside of a procedural macro, false if invoked from any other binary.
+#[unstable(feature = "proc_macro_is_available", issue = "71436")]
+pub fn is_available() -> bool {
+    bridge::Bridge::is_available()
+}
+
 /// The main type provided by this crate, representing an abstract stream of
 /// tokens, or, more specifically, a sequence of token trees.
 /// The type provide interfaces for iterating over those token trees and, conversely,
diff --git a/src/librustc_data_structures/profiling.rs b/src/librustc_data_structures/profiling.rs
index 23f3558cbdfa3..07d16c6483ec7 100644
--- a/src/librustc_data_structures/profiling.rs
+++ b/src/librustc_data_structures/profiling.rs
@@ -97,12 +97,17 @@ use std::time::{Duration, Instant};
 use measureme::{EventId, EventIdBuilder, SerializableString, StringId};
 use parking_lot::RwLock;
 
-/// MmapSerializatioSink is faster on macOS and Linux
-/// but FileSerializationSink is faster on Windows
-#[cfg(not(windows))]
-type SerializationSink = measureme::MmapSerializationSink;
-#[cfg(windows)]
-type SerializationSink = measureme::FileSerializationSink;
+cfg_if! {
+    if #[cfg(any(windows, target_os = "wasi"))] {
+        /// FileSerializationSink is faster on Windows
+        type SerializationSink = measureme::FileSerializationSink;
+    } else if #[cfg(target_arch = "wasm32")] {
+        type SerializationSink = measureme::ByteVecSink;
+    } else {
+        /// MmapSerializatioSink is faster on macOS and Linux
+        type SerializationSink = measureme::MmapSerializationSink;
+    }
+}
 
 type Profiler = measureme::Profiler<SerializationSink>;
 
@@ -602,31 +607,37 @@ pub fn duration_to_secs_str(dur: std::time::Duration) -> String {
 }
 
 // Memory reporting
-#[cfg(unix)]
-fn get_resident() -> Option<usize> {
-    let field = 1;
-    let contents = fs::read("/proc/self/statm").ok()?;
-    let contents = String::from_utf8(contents).ok()?;
-    let s = contents.split_whitespace().nth(field)?;
-    let npages = s.parse::<usize>().ok()?;
-    Some(npages * 4096)
-}
-
-#[cfg(windows)]
-fn get_resident() -> Option<usize> {
-    use std::mem::{self, MaybeUninit};
-    use winapi::shared::minwindef::DWORD;
-    use winapi::um::processthreadsapi::GetCurrentProcess;
-    use winapi::um::psapi::{GetProcessMemoryInfo, PROCESS_MEMORY_COUNTERS};
-
-    let mut pmc = MaybeUninit::<PROCESS_MEMORY_COUNTERS>::uninit();
-    match unsafe {
-        GetProcessMemoryInfo(GetCurrentProcess(), pmc.as_mut_ptr(), mem::size_of_val(&pmc) as DWORD)
-    } {
-        0 => None,
-        _ => {
-            let pmc = unsafe { pmc.assume_init() };
-            Some(pmc.WorkingSetSize as usize)
+cfg_if! {
+    if #[cfg(windows)] {
+        fn get_resident() -> Option<usize> {
+            use std::mem::{self, MaybeUninit};
+            use winapi::shared::minwindef::DWORD;
+            use winapi::um::processthreadsapi::GetCurrentProcess;
+            use winapi::um::psapi::{GetProcessMemoryInfo, PROCESS_MEMORY_COUNTERS};
+
+            let mut pmc = MaybeUninit::<PROCESS_MEMORY_COUNTERS>::uninit();
+            match unsafe {
+                GetProcessMemoryInfo(GetCurrentProcess(), pmc.as_mut_ptr(), mem::size_of_val(&pmc) as DWORD)
+            } {
+                0 => None,
+                _ => {
+                    let pmc = unsafe { pmc.assume_init() };
+                    Some(pmc.WorkingSetSize as usize)
+                }
+            }
+        }
+    } else if #[cfg(unix)] {
+        fn get_resident() -> Option<usize> {
+            let field = 1;
+            let contents = fs::read("/proc/self/statm").ok()?;
+            let contents = String::from_utf8(contents).ok()?;
+            let s = contents.split_whitespace().nth(field)?;
+            let npages = s.parse::<usize>().ok()?;
+            Some(npages * 4096)
+        }
+    } else {
+        fn get_resident() -> Option<usize> {
+            None
         }
     }
 }
diff --git a/src/librustc_error_codes/error_codes/E0060.md b/src/librustc_error_codes/error_codes/E0060.md
index 7a07b8e7ed669..e6906d72367d8 100644
--- a/src/librustc_error_codes/error_codes/E0060.md
+++ b/src/librustc_error_codes/error_codes/E0060.md
@@ -2,12 +2,14 @@ External C functions are allowed to be variadic. However, a variadic function
 takes a minimum number of arguments. For example, consider C's variadic `printf`
 function:
 
-```
+```compile_fail,E0060
 use std::os::raw::{c_char, c_int};
 
 extern "C" {
     fn printf(_: *const c_char, ...) -> c_int;
 }
+
+unsafe { printf(); } // error!
 ```
 
 Using this declaration, it must be called with at least one argument, so
diff --git a/src/librustc_error_codes/error_codes/E0130.md b/src/librustc_error_codes/error_codes/E0130.md
index 539049edb33b7..a270feaf58c17 100644
--- a/src/librustc_error_codes/error_codes/E0130.md
+++ b/src/librustc_error_codes/error_codes/E0130.md
@@ -2,7 +2,7 @@ A pattern was declared as an argument in a foreign function declaration.
 
 Erroneous code example:
 
-```compile_fail
+```compile_fail,E0130
 extern {
     fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
                                 //        function declarations
diff --git a/src/librustc_error_codes/error_codes/E0198.md b/src/librustc_error_codes/error_codes/E0198.md
index 687214a205096..90f1e54287496 100644
--- a/src/librustc_error_codes/error_codes/E0198.md
+++ b/src/librustc_error_codes/error_codes/E0198.md
@@ -2,7 +2,7 @@ A negative implementation was marked as unsafe.
 
 Erroneous code example:
 
-```compile_fail
+```compile_fail,E0198
 struct Foo;
 
 unsafe impl !Clone for Foo { } // error!
diff --git a/src/librustc_error_codes/error_codes/E0202.md b/src/librustc_error_codes/error_codes/E0202.md
index b20d338c5fd99..afc61ec2e48ff 100644
--- a/src/librustc_error_codes/error_codes/E0202.md
+++ b/src/librustc_error_codes/error_codes/E0202.md
@@ -1,5 +1,15 @@
 Inherent associated types were part of [RFC 195] but are not yet implemented.
 See [the tracking issue][iss8995] for the status of this implementation.
 
+Erroneous code example:
+
+```compile_fail,E0202
+struct Foo;
+
+impl Foo {
+    type Bar = isize; // error!
+}
+```
+
 [RFC 195]: https://github.com/rust-lang/rfcs/blob/master/text/0195-associated-items.md
 [iss8995]: https://github.com/rust-lang/rust/issues/8995
diff --git a/src/librustc_error_codes/error_codes/E0230.md b/src/librustc_error_codes/error_codes/E0230.md
index 9dbcb8e010b1c..cfb72e74319c1 100644
--- a/src/librustc_error_codes/error_codes/E0230.md
+++ b/src/librustc_error_codes/error_codes/E0230.md
@@ -3,15 +3,11 @@ message for when a particular trait isn't implemented on a type placed in a
 position that needs that trait. For example, when the following code is
 compiled:
 
-```compile_fail
+```compile_fail,E0230
 #![feature(rustc_attrs)]
 
-fn foo<T: Index<u8>>(x: T){}
-
-#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
-trait Index<Idx> { /* ... */ }
-
-foo(true); // `bool` does not implement `Index<u8>`
+#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{B}>`"] // error
+trait BadAnnotation<A> {}
 ```
 
 There will be an error about `bool` not implementing `Index<u8>`, followed by a
diff --git a/src/librustc_error_codes/error_codes/E0231.md b/src/librustc_error_codes/error_codes/E0231.md
index 4f80da54540b1..23a0a88ecdd9b 100644
--- a/src/librustc_error_codes/error_codes/E0231.md
+++ b/src/librustc_error_codes/error_codes/E0231.md
@@ -3,15 +3,11 @@ message for when a particular trait isn't implemented on a type placed in a
 position that needs that trait. For example, when the following code is
 compiled:
 
-```compile_fail
+```compile_fail,E0231
 #![feature(rustc_attrs)]
 
-fn foo<T: Index<u8>>(x: T){}
-
-#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
-trait Index<Idx> { /* ... */ }
-
-foo(true); // `bool` does not implement `Index<u8>`
+#[rustc_on_unimplemented = "error on `{Self}` with params `<{A},{}>`"] // error!
+trait BadAnnotation<A> {}
 ```
 
 there will be an error about `bool` not implementing `Index<u8>`, followed by a
diff --git a/src/librustc_error_codes/error_codes/E0232.md b/src/librustc_error_codes/error_codes/E0232.md
index 07a031488d091..b310caefa6e31 100644
--- a/src/librustc_error_codes/error_codes/E0232.md
+++ b/src/librustc_error_codes/error_codes/E0232.md
@@ -3,15 +3,11 @@ message for when a particular trait isn't implemented on a type placed in a
 position that needs that trait. For example, when the following code is
 compiled:
 
-```compile_fail
+```compile_fail,E0232
 #![feature(rustc_attrs)]
 
-fn foo<T: Index<u8>>(x: T){}
-
-#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
-trait Index<Idx> { /* ... */ }
-
-foo(true); // `bool` does not implement `Index<u8>`
+#[rustc_on_unimplemented(lorem="")] // error!
+trait BadAnnotation {}
 ```
 
 there will be an error about `bool` not implementing `Index<u8>`, followed by a
diff --git a/src/librustc_error_codes/error_codes/E0281.md b/src/librustc_error_codes/error_codes/E0281.md
index 1a9796d2790ea..1d7904b67ddb4 100644
--- a/src/librustc_error_codes/error_codes/E0281.md
+++ b/src/librustc_error_codes/error_codes/E0281.md
@@ -4,7 +4,7 @@ You tried to supply a type which doesn't implement some trait in a location
 which expected that trait. This error typically occurs when working with
 `Fn`-based types. Erroneous code example:
 
-```compile-fail
+```compile_fail
 fn foo<F: Fn(usize)>(x: F) { }
 
 fn main() {
diff --git a/src/librustc_error_codes/error_codes/E0364.md b/src/librustc_error_codes/error_codes/E0364.md
index ec1fb911f3cc2..d01fb0c9c4229 100644
--- a/src/librustc_error_codes/error_codes/E0364.md
+++ b/src/librustc_error_codes/error_codes/E0364.md
@@ -3,27 +3,27 @@ attempted to `pub use` a type or value that was not itself public.
 
 Erroneous code example:
 
-```compile_fail
-mod foo {
-    const X: u32 = 1;
-}
-
-pub use foo::X;
+```compile_fail,E0364
+mod a {
+    fn foo() {}
 
-fn main() {}
+    mod a {
+        pub use super::foo; // error!
+    }
+}
 ```
 
 The solution to this problem is to ensure that the items that you are
 re-exporting are themselves marked with `pub`:
 
 ```
-mod foo {
-    pub const X: u32 = 1;
-}
-
-pub use foo::X;
+mod a {
+    pub fn foo() {} // ok!
 
-fn main() {}
+    mod a {
+        pub use super::foo;
+    }
+}
 ```
 
 See the [Use Declarations][use-declarations] section of the reference for
diff --git a/src/librustc_error_codes/error_codes/E0378.md b/src/librustc_error_codes/error_codes/E0378.md
index 7f4374738de28..c6fe997f3dcfc 100644
--- a/src/librustc_error_codes/error_codes/E0378.md
+++ b/src/librustc_error_codes/error_codes/E0378.md
@@ -3,7 +3,7 @@ or a newtype wrapper around a pointer.
 
 Erroneous code example:
 
-```compile-fail,E0378
+```compile_fail,E0378
 #![feature(dispatch_from_dyn)]
 use std::ops::DispatchFromDyn;
 
diff --git a/src/librustc_error_codes/error_codes/E0590.md b/src/librustc_error_codes/error_codes/E0590.md
index 8032e45903ce2..df7aa4f0a1e8d 100644
--- a/src/librustc_error_codes/error_codes/E0590.md
+++ b/src/librustc_error_codes/error_codes/E0590.md
@@ -3,7 +3,7 @@
 
 Example of erroneous code:
 
-```compile_fail
+```compile_fail,E0590
 while break {}
 ```
 
diff --git a/src/librustc_error_codes/error_codes/E0639.md b/src/librustc_error_codes/error_codes/E0639.md
index c2d9662337fa0..4646e37fb7526 100644
--- a/src/librustc_error_codes/error_codes/E0639.md
+++ b/src/librustc_error_codes/error_codes/E0639.md
@@ -3,5 +3,17 @@ instantiated from outside of the defining crate as it has been marked
 as `non_exhaustive` and as such more fields/variants may be added in
 future that could cause adverse side effects for this code.
 
+Erroneous code example:
+
+```ignore (it only works cross-crate)
+#[non_exhaustive]
+pub struct NormalStruct {
+    pub first_field: u16,
+    pub second_field: u16,
+}
+
+let ns = NormalStruct { first_field: 640, second_field: 480 }; // error!
+```
+
 It is recommended that you look for a `new` function or equivalent in the
 crate's documentation.
diff --git a/src/librustc_error_codes/error_codes/E0644.md b/src/librustc_error_codes/error_codes/E0644.md
index 7a653bd2264fd..277643dfb1ab7 100644
--- a/src/librustc_error_codes/error_codes/E0644.md
+++ b/src/librustc_error_codes/error_codes/E0644.md
@@ -2,17 +2,17 @@ A closure or generator was constructed that references its own type.
 
 Erroneous example:
 
-```compile-fail,E0644
+```compile_fail,E0644
 fn fix<F>(f: &F)
   where F: Fn(&F)
 {
-  f(&f);
+    f(&f);
 }
 
 fn main() {
-  fix(&|y| {
-    // Here, when `x` is called, the parameter `y` is equal to `x`.
-  });
+    fix(&|y| {
+        // Here, when `x` is called, the parameter `y` is equal to `x`.
+    });
 }
 ```
 
diff --git a/src/librustc_error_codes/error_codes/E0658.md b/src/librustc_error_codes/error_codes/E0658.md
index 44a38cee23dfd..d821b9027f136 100644
--- a/src/librustc_error_codes/error_codes/E0658.md
+++ b/src/librustc_error_codes/error_codes/E0658.md
@@ -2,7 +2,7 @@ An unstable feature was used.
 
 Erroneous code example:
 
-```compile_fail,E658
+```compile_fail,E0658
 #[repr(u128)] // error: use of unstable library feature 'repr128'
 enum Foo {
     Bar(u64),
diff --git a/src/librustc_error_codes/error_codes/E0669.md b/src/librustc_error_codes/error_codes/E0669.md
index 39de01eebe825..f078c441b3421 100644
--- a/src/librustc_error_codes/error_codes/E0669.md
+++ b/src/librustc_error_codes/error_codes/E0669.md
@@ -1,5 +1,17 @@
 Cannot convert inline assembly operand to a single LLVM value.
 
+Erroneous code example:
+
+```compile_fail,E0669
+#![feature(llvm_asm)]
+
+fn main() {
+    unsafe {
+        llvm_asm!("" :: "r"("")); // error!
+    }
+}
+```
+
 This error usually happens when trying to pass in a value to an input inline
 assembly operand that is actually a pair of values. In particular, this can
 happen when trying to pass in a slice, for instance a `&str`. In Rust, these
diff --git a/src/librustc_error_codes/error_codes/E0698.md b/src/librustc_error_codes/error_codes/E0698.md
index d0fcec3990241..3ba992a8476ed 100644
--- a/src/librustc_error_codes/error_codes/E0698.md
+++ b/src/librustc_error_codes/error_codes/E0698.md
@@ -3,7 +3,7 @@ generator can be constructed.
 
 Erroneous code example:
 
-```edition2018,compile-fail,E0698
+```edition2018,compile_fail,E0698
 async fn bar<T>() -> () {}
 
 async fn foo() {
diff --git a/src/librustc_error_codes/error_codes/E0700.md b/src/librustc_error_codes/error_codes/E0700.md
index 41ac4d948c22c..b1eb8b66ad682 100644
--- a/src/librustc_error_codes/error_codes/E0700.md
+++ b/src/librustc_error_codes/error_codes/E0700.md
@@ -3,7 +3,7 @@ appear within the `impl Trait` itself.
 
 Erroneous code example:
 
-```compile-fail,E0700
+```compile_fail,E0700
 use std::cell::Cell;
 
 trait Trait<'a> { }
diff --git a/src/librustc_error_codes/error_codes/E0708.md b/src/librustc_error_codes/error_codes/E0708.md
index a0f53e38b5325..9287fc803d1de 100644
--- a/src/librustc_error_codes/error_codes/E0708.md
+++ b/src/librustc_error_codes/error_codes/E0708.md
@@ -2,7 +2,7 @@
 
 Erroneous code example:
 
-```compile_fail,edition2018
+```compile_fail,edition2018,E0708
 #![feature(async_closure)]
 
 fn main() {
diff --git a/src/librustc_error_codes/error_codes/E0714.md b/src/librustc_error_codes/error_codes/E0714.md
index e8707f3e39039..45d1cafa69062 100644
--- a/src/librustc_error_codes/error_codes/E0714.md
+++ b/src/librustc_error_codes/error_codes/E0714.md
@@ -1,5 +1,19 @@
 A `#[marker]` trait contained an associated item.
 
+Erroneous code example:
+
+```compile_fail,E0714
+#![feature(marker_trait_attr)]
+#![feature(associated_type_defaults)]
+
+#[marker]
+trait MarkerConst {
+    const N: usize; // error!
+}
+
+fn main() {}
+```
+
 The items of marker traits cannot be overridden, so there's no need to have them
 when they cannot be changed per-type anyway.  If you wanted them for ergonomic
 reasons, consider making an extension trait instead.
diff --git a/src/librustc_error_codes/error_codes/E0715.md b/src/librustc_error_codes/error_codes/E0715.md
index 9e20e81368338..8f0022d942547 100644
--- a/src/librustc_error_codes/error_codes/E0715.md
+++ b/src/librustc_error_codes/error_codes/E0715.md
@@ -1,5 +1,24 @@
 An `impl` for a `#[marker]` trait tried to override an associated item.
 
+Erroneous code example:
+
+```compile_fail,E0715
+#![feature(marker_trait_attr)]
+
+#[marker]
+trait Marker {
+    const N: usize = 0;
+    fn do_something() {}
+}
+
+struct OverrideConst;
+impl Marker for OverrideConst { // error!
+    const N: usize = 1;
+}
+
+fn main() {}
+```
+
 Because marker traits are allowed to have multiple implementations for the same
 type, it's not allowed to override anything in those implementations, as it
 would be ambiguous which override should actually be used.
diff --git a/src/librustc_error_codes/error_codes/E0727.md b/src/librustc_error_codes/error_codes/E0727.md
index 528807ee9afe2..be1b68e645d01 100644
--- a/src/librustc_error_codes/error_codes/E0727.md
+++ b/src/librustc_error_codes/error_codes/E0727.md
@@ -2,14 +2,16 @@ A `yield` clause was used in an `async` context.
 
 Example of erroneous code:
 
-```compile_fail
+```compile_fail,E0727,edition2018
 #![feature(generators)]
 
-let generator = || {
-    async {
-        yield;
-    }
-};
+fn main() {
+    let generator = || {
+        async {
+            yield;
+        }
+    };
+}
 ```
 
 Here, the `yield` keyword is used in an `async` block,
@@ -17,10 +19,12 @@ which is not yet supported.
 
 To fix this error, you have to move `yield` out of the `async` block:
 
-```
+```edition2018
 #![feature(generators)]
 
-let generator = || {
-    yield;
-};
+fn main() {
+    let generator = || {
+        yield;
+    };
+}
 ```
diff --git a/src/librustc_error_codes/error_codes/E0732.md b/src/librustc_error_codes/error_codes/E0732.md
index 03137590e883f..7347e6654c5b3 100644
--- a/src/librustc_error_codes/error_codes/E0732.md
+++ b/src/librustc_error_codes/error_codes/E0732.md
@@ -1,5 +1,18 @@
 An `enum` with a discriminant must specify a `#[repr(inttype)]`.
 
+Erroneous code example:
+
+```compile_fail,E0732
+#![feature(arbitrary_enum_discriminant)]
+
+enum Enum { // error!
+    Unit = 1,
+    Tuple() = 2,
+    Struct{} = 3,
+}
+# fn main() {}
+```
+
 A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
 variant with a discriminant, or where there are both unit variants with
 discriminants and non-unit variants. This restriction ensures that there
@@ -23,7 +36,9 @@ fn discriminant(v : &Enum) -> u8 {
     unsafe { *(v as *const Enum as *const u8) }
 }
 
-assert_eq!(3, discriminant(&Enum::Unit));
-assert_eq!(2, discriminant(&Enum::Tuple(5)));
-assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
+fn main() {
+    assert_eq!(3, discriminant(&Enum::Unit));
+    assert_eq!(2, discriminant(&Enum::Tuple(5)));
+    assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
+}
 ```
diff --git a/src/librustc_error_codes/error_codes/E0740.md b/src/librustc_error_codes/error_codes/E0740.md
index a9eb5f2790d47..3777678518964 100644
--- a/src/librustc_error_codes/error_codes/E0740.md
+++ b/src/librustc_error_codes/error_codes/E0740.md
@@ -1 +1,16 @@
 A `union` cannot have fields with destructors.
+
+Erroneous code example:
+
+```compile_fail,E0740
+union Test {
+    a: A, // error!
+}
+
+#[derive(Debug)]
+struct A(i32);
+
+impl Drop for A {
+    fn drop(&mut self) { println!("A"); }
+}
+```
diff --git a/src/librustc_error_codes/error_codes/E0744.md b/src/librustc_error_codes/error_codes/E0744.md
index 602fbc50a7153..56b947a8282ed 100644
--- a/src/librustc_error_codes/error_codes/E0744.md
+++ b/src/librustc_error_codes/error_codes/E0744.md
@@ -3,16 +3,13 @@ Control-flow expressions are not allowed inside a const context.
 At the moment, `if` and `match`, as well as the looping constructs `for`,
 `while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.
 
-```compile_fail,E0658
+```compile_fail,E0744
 const _: i32 = {
     let mut x = 0;
-    loop {
-        x += 1;
-        if x == 4 {
-            break;
-        }
+
+    for i in 0..4 { // error!
+        x += i;
     }
-    x
 };
 ```
 
diff --git a/src/librustc_parse/parser/expr.rs b/src/librustc_parse/parser/expr.rs
index 986f5410e26c0..3f08fb79790fb 100644
--- a/src/librustc_parse/parser/expr.rs
+++ b/src/librustc_parse/parser/expr.rs
@@ -547,8 +547,7 @@ impl<'a> Parser<'a> {
                 // Rewind to before attempting to parse the type with generics, to recover
                 // from situations like `x as usize < y` in which we first tried to parse
                 // `usize < y` as a type with generic arguments.
-                let parser_snapshot_after_type = self.clone();
-                mem::replace(self, parser_snapshot_before_type);
+                let parser_snapshot_after_type = mem::replace(self, parser_snapshot_before_type);
 
                 match self.parse_path(PathStyle::Expr) {
                     Ok(path) => {
@@ -560,7 +559,7 @@ impl<'a> Parser<'a> {
                                 // example because `parse_ty_no_plus` returns `Err` on keywords,
                                 // but `parse_path` returns `Ok` on them due to error recovery.
                                 // Return original error and parser state.
-                                mem::replace(self, parser_snapshot_after_type);
+                                *self = parser_snapshot_after_type;
                                 return Err(type_err);
                             }
                         };
@@ -601,7 +600,7 @@ impl<'a> Parser<'a> {
                     Err(mut path_err) => {
                         // Couldn't parse as a path, return original error and parser state.
                         path_err.cancel();
-                        mem::replace(self, parser_snapshot_after_type);
+                        *self = parser_snapshot_after_type;
                         return Err(type_err);
                     }
                 }
diff --git a/src/librustc_parse/parser/generics.rs b/src/librustc_parse/parser/generics.rs
index f4729e306f806..8e8f864728ce6 100644
--- a/src/librustc_parse/parser/generics.rs
+++ b/src/librustc_parse/parser/generics.rs
@@ -105,7 +105,7 @@ impl<'a> Parser<'a> {
                     }
                     Err(mut err) => {
                         err.cancel();
-                        std::mem::replace(self, snapshot);
+                        *self = snapshot;
                         break;
                     }
                 }
diff --git a/src/librustc_parse/parser/item.rs b/src/librustc_parse/parser/item.rs
index ae8a20f209b99..e9f5f2c0deafc 100644
--- a/src/librustc_parse/parser/item.rs
+++ b/src/librustc_parse/parser/item.rs
@@ -1650,7 +1650,7 @@ impl<'a> Parser<'a> {
                 // Recover from attempting to parse the argument as a type without pattern.
                 Err(mut err) => {
                     err.cancel();
-                    mem::replace(self, parser_snapshot_before_ty);
+                    *self = parser_snapshot_before_ty;
                     self.recover_arg_parse()?
                 }
             }
diff --git a/src/librustc_parse/parser/stmt.rs b/src/librustc_parse/parser/stmt.rs
index b3764d2d47be1..e5d0ab247aa46 100644
--- a/src/librustc_parse/parser/stmt.rs
+++ b/src/librustc_parse/parser/stmt.rs
@@ -163,8 +163,8 @@ impl<'a> Parser<'a> {
                 Ok(ty) => (None, Some(ty)),
                 Err(mut err) => {
                     // Rewind to before attempting to parse the type and continue parsing.
-                    let parser_snapshot_after_type = self.clone();
-                    mem::replace(self, parser_snapshot_before_type);
+                    let parser_snapshot_after_type =
+                        mem::replace(self, parser_snapshot_before_type);
                     if let Ok(snip) = self.span_to_snippet(pat.span) {
                         err.span_label(pat.span, format!("while parsing the type for `{}`", snip));
                     }
@@ -201,7 +201,7 @@ impl<'a> Parser<'a> {
                 // Couldn't parse the type nor the initializer, only raise the type error and
                 // return to the parser state before parsing the type as the initializer.
                 // let x: <parse_error>;
-                mem::replace(self, snapshot);
+                *self = snapshot;
                 return Err(ty_err);
             }
             (Err(err), None) => {
diff --git a/src/librustc_resolve/late/lifetimes.rs b/src/librustc_resolve/late/lifetimes.rs
index defb2c06b32ca..0d12bf08747fa 100644
--- a/src/librustc_resolve/late/lifetimes.rs
+++ b/src/librustc_resolve/late/lifetimes.rs
@@ -26,7 +26,7 @@ use rustc_span::symbol::{kw, sym};
 use rustc_span::Span;
 use std::borrow::Cow;
 use std::cell::Cell;
-use std::mem::{replace, take};
+use std::mem::take;
 
 use log::debug;
 
@@ -371,7 +371,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
         self.with(Scope::Body { id: body.id(), s: self.scope }, |_, this| {
             this.visit_body(body);
         });
-        replace(&mut self.labels_in_fn, saved);
+        self.labels_in_fn = saved;
     }
 
     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
diff --git a/src/libstd/thread/local.rs b/src/libstd/thread/local.rs
index 29e99c0afd27f..094c468a6770e 100644
--- a/src/libstd/thread/local.rs
+++ b/src/libstd/thread/local.rs
@@ -301,7 +301,7 @@ mod lazy {
             // value (an aliasing violation). To avoid setting the "I'm running a
             // destructor" flag we just use `mem::replace` which should sequence the
             // operations a little differently and make this safe to call.
-            mem::replace(&mut *ptr, Some(value));
+            let _ = mem::replace(&mut *ptr, Some(value));
 
             // After storing `Some` we want to get a reference to the contents of
             // what we just stored. While we could use `unwrap` here and it should
diff --git a/src/test/ui/imports/import-in-block.rs b/src/test/ui/imports/import-in-block.rs
index c0ba6220b5443..19703904ece91 100644
--- a/src/test/ui/imports/import-in-block.rs
+++ b/src/test/ui/imports/import-in-block.rs
@@ -4,7 +4,7 @@
 pub fn main() {
     use std::mem::replace;
     let mut x = 5;
-    replace(&mut x, 6);
+    let _ = replace(&mut x, 6);
     {
         use std::mem::*;
         let mut y = 6;
diff --git a/src/test/ui/issues/issue-23611-enum-swap-in-drop.rs b/src/test/ui/issues/issue-23611-enum-swap-in-drop.rs
index 6ef7fd42ec6d0..403cf970bcb0a 100644
--- a/src/test/ui/issues/issue-23611-enum-swap-in-drop.rs
+++ b/src/test/ui/issues/issue-23611-enum-swap-in-drop.rs
@@ -153,6 +153,7 @@ impl<'a> Drop for E<'a> {
             }
         };
 
+        #[allow(unused_must_use)]
         if do_drop {
             mem::replace(self, E::A(GaspA(f_a, 0xA3A0, log, D::new("drop", 6, log)), true));
         }
diff --git a/src/test/ui/proc-macro/auxiliary/is-available.rs b/src/test/ui/proc-macro/auxiliary/is-available.rs
new file mode 100644
index 0000000000000..0caf186db1d5f
--- /dev/null
+++ b/src/test/ui/proc-macro/auxiliary/is-available.rs
@@ -0,0 +1,14 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![feature(proc_macro_is_available)]
+
+extern crate proc_macro;
+
+use proc_macro::{Literal, TokenStream, TokenTree};
+
+#[proc_macro]
+pub fn from_inside_proc_macro(_input: TokenStream) -> TokenStream {
+    proc_macro::is_available().to_string().parse().unwrap()
+}
diff --git a/src/test/ui/proc-macro/is-available.rs b/src/test/ui/proc-macro/is-available.rs
new file mode 100644
index 0000000000000..943d9fe797a6e
--- /dev/null
+++ b/src/test/ui/proc-macro/is-available.rs
@@ -0,0 +1,17 @@
+// run-pass
+
+#![feature(proc_macro_hygiene, proc_macro_is_available)]
+
+extern crate proc_macro;
+
+// aux-build:is-available.rs
+extern crate is_available;
+
+fn main() {
+    let a = proc_macro::is_available();
+    let b = is_available::from_inside_proc_macro!();
+    let c = proc_macro::is_available();
+    assert!(!a);
+    assert!(b);
+    assert!(!c);
+}
diff --git a/src/tools/tidy/src/error_codes_check.rs b/src/tools/tidy/src/error_codes_check.rs
index 428c57d3ee822..243d41598f86c 100644
--- a/src/tools/tidy/src/error_codes_check.rs
+++ b/src/tools/tidy/src/error_codes_check.rs
@@ -15,21 +15,57 @@ const WHITELIST: &[&str] = &[
     "E0727", "E0729",
 ];
 
+// Some error codes don't have any tests apparently...
+const IGNORE_EXPLANATION_CHECK: &[&str] =
+    &["E0570", "E0601", "E0602", "E0639", "E0729", "E0749", "E0750", "E0751"];
+
 fn check_error_code_explanation(
     f: &str,
     error_codes: &mut HashMap<String, bool>,
     err_code: String,
-) {
+) -> bool {
+    let mut invalid_compile_fail_format = false;
+    let mut found_error_code = false;
+
     for line in f.lines() {
         let s = line.trim();
-        if s.starts_with("```") && s.contains("compile_fail") && s.contains('E') {
-            error_codes.insert(err_code, true);
-            return;
+        if s.starts_with("```") {
+            if s.contains("compile_fail") && s.contains('E') {
+                if !found_error_code {
+                    error_codes.insert(err_code.clone(), true);
+                    found_error_code = true;
+                }
+            } else if s.contains("compile-fail") {
+                invalid_compile_fail_format = true;
+            }
         } else if s.starts_with("#### Note: this error code is no longer emitted by the compiler") {
-            error_codes.get_mut(&err_code).map(|x| *x = true);
-            return;
+            if !found_error_code {
+                error_codes.get_mut(&err_code).map(|x| *x = true);
+                found_error_code = true;
+            }
         }
     }
+    invalid_compile_fail_format
+}
+
+fn check_if_error_code_is_test_in_explanation(f: &str, err_code: &String) -> bool {
+    let mut can_be_ignored = false;
+
+    for line in f.lines() {
+        let s = line.trim();
+        if s.starts_with("#### Note: this error code is no longer emitted by the compiler") {
+            return true;
+        }
+        if s.starts_with("```") {
+            if s.contains("compile_fail") && s.contains(err_code) {
+                return true;
+            } else if s.contains("(") {
+                // It's very likely that we can't actually make it fail compilation...
+                can_be_ignored = true;
+            }
+        }
+    }
+    can_be_ignored
 }
 
 macro_rules! some_or_continue {
@@ -41,7 +77,12 @@ macro_rules! some_or_continue {
     };
 }
 
-fn extract_error_codes(f: &str, error_codes: &mut HashMap<String, bool>, path: &Path) {
+fn extract_error_codes(
+    f: &str,
+    error_codes: &mut HashMap<String, bool>,
+    path: &Path,
+    errors: &mut Vec<String>,
+) {
     let mut reached_no_explanation = false;
 
     for line in f.lines() {
@@ -55,10 +96,26 @@ fn extract_error_codes(f: &str, error_codes: &mut HashMap<String, bool>, path: &
                 // Now we extract the tests from the markdown file!
                 let md = some_or_continue!(s.splitn(2, "include_str!(\"").nth(1));
                 let md_file_name = some_or_continue!(md.splitn(2, "\")").next());
-                let path = some_or_continue!(path.parent()).join(md_file_name);
+                let path = some_or_continue!(path.parent())
+                    .join(md_file_name)
+                    .canonicalize()
+                    .expect("failed to canonicalize error explanation file path");
                 match read_to_string(&path) {
                     Ok(content) => {
-                        check_error_code_explanation(&content, error_codes, err_code);
+                        if !IGNORE_EXPLANATION_CHECK.contains(&err_code.as_str())
+                            && !check_if_error_code_is_test_in_explanation(&content, &err_code)
+                        {
+                            errors.push(format!(
+                                "`{}` doesn't use its own error code in compile_fail example",
+                                path.display(),
+                            ));
+                        }
+                        if check_error_code_explanation(&content, error_codes, err_code) {
+                            errors.push(format!(
+                                "`{}` uses invalid tag `compile-fail` instead of `compile_fail`",
+                                path.display(),
+                            ));
+                        }
                     }
                     Err(e) => {
                         eprintln!("Couldn't read `{}`: {}", path.display(), e);
@@ -94,22 +151,24 @@ fn extract_error_codes_from_tests(f: &str, error_codes: &mut HashMap<String, boo
 }
 
 pub fn check(path: &Path, bad: &mut bool) {
+    let mut errors = Vec::new();
     println!("Checking which error codes lack tests...");
     let mut error_codes: HashMap<String, bool> = HashMap::new();
     super::walk(path, &mut |path| super::filter_dirs(path), &mut |entry, contents| {
         let file_name = entry.file_name();
         if file_name == "error_codes.rs" {
-            extract_error_codes(contents, &mut error_codes, entry.path());
+            extract_error_codes(contents, &mut error_codes, entry.path(), &mut errors);
         } else if entry.path().extension() == Some(OsStr::new("stderr")) {
             extract_error_codes_from_tests(contents, &mut error_codes);
         }
     });
-    println!("Found {} error codes", error_codes.len());
+    if errors.is_empty() {
+        println!("Found {} error codes", error_codes.len());
 
-    let mut errors = Vec::new();
-    for (err_code, nb) in &error_codes {
-        if !*nb && !WHITELIST.contains(&err_code.as_str()) {
-            errors.push(format!("Error code {} needs to have at least one UI test!", err_code));
+        for (err_code, nb) in &error_codes {
+            if !*nb && !WHITELIST.contains(&err_code.as_str()) {
+                errors.push(format!("Error code {} needs to have at least one UI test!", err_code));
+            }
         }
     }
     errors.sort();