Rollup merge of rust-lang#122018 - RalfJung:box-custom-alloc, r=oli-obk

only set noalias on Box with the global allocator

As discovered in rust-lang/miri#3341, `noalias` and custom allocators don't go well together.

rustc can now check whether a Box uses the global allocator. This replaces the previous ad-hoc and rather unprincipled check for a zero-sized allocator.

This is the rustc part of fixing that; Miri will also need a patch.

alloc/src/alloc.rs

@@ -50,6 +50,8 @@ extern "Rust" {
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 #[cfg(not(test))]
+// the compiler needs to know when a Box uses the global allocator vs a custom one
+#[cfg_attr(not(bootstrap), lang = "global_alloc_ty")]
 pub struct Global;
 
 #[cfg(test)]

alloc/src/boxed.rs

@@ -2062,6 +2062,9 @@ impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
 
+// It is quite crucial that we only allow the `Global` allocator here.
+// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
+// would need a lot of codegen and interpreter adjustments.
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}