Auto merge of rust-lang#71445 - Dylan-DPC:rollup-31givp1, r=Dylan-DPC

Rollup of 5 pull requests

Successful merges:

 - rust-lang#71256 (Lint must_use on mem::replace)
 - rust-lang#71350 (Error code explanation extra check)
 - rust-lang#71369 (allow wasm32 compilation of librustc_data_structures/profiling.rs)
 - rust-lang#71400 (proc_macro::is_available())
 - rust-lang#71440 (Implement `Copy` for `AllocErr`)

Failed merges:

r? @ghost

Author: bors

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)

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;
 ///