Auto merge of #137944 - davidtwco:sized-hierarchy, r=oli-obk

Sized Hierarchy: Part I

This patch implements the non-const parts of rust-lang/rfcs#3729. It introduces two new traits to the standard library, `MetaSized` and `PointeeSized`. See the RFC for the rationale behind these traits and to discuss whether this change makes sense in the abstract.

These traits are unstable (as is their constness), so users cannot refer to them without opting-in to `feature(sized_hierarchy)`. These traits are not behind `cfg`s as this would make implementation unfeasible, there would simply be too many `cfg`s required to add the necessary bounds everywhere. So, like `Sized`, these traits are automatically implemented by the compiler.

RFC 3729 describes changes which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

- `?Sized` is rewritten as `MetaSized`
- `MetaSized` is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.

There are no edition migrations implemented in this,  as these are primarily required for the constness parts of the RFC and prior to stabilisation of this (and so will come in follow-up PRs alongside the const parts). All diagnostic output should remain the same (showing `?Sized` even if the compiler sees `MetaSized`) unless the `sized_hierarchy` feature is enabled.

Due to the use of unstable extern types in the standard library and rustc, some bounds in both projects have had to be relaxed already - this is unfortunate but unavoidable so that these extern types can continue to be used where they were before. Performing these relaxations in the standard library and rustc are desirable longer-term anyway, but some bounds are not as relaxed as they ideally would be due to the inability to relax `Deref::Target` (this will be investigated separately).

It is hoped that this is implemented such that it could be merged and these traits could exist "under the hood" without that being observable to the user (other than in any performance impact this has on the compiler, etc). Some details might leak through due to the standard library relaxations, but this has not been observed in test output.

**Notes:**

- Any commits starting with "upstream:" can be ignored, as these correspond to other upstream PRs that this is based on which have yet to be merged.
- This best reviewed commit-by-commit. I've attempted to make the implementation easy to follow and keep similar changes and test output updates together.
  - Each commit has a short description describing its purpose.
  - This patch is large but it's primarily in the test suite.
- I've worked on the performance of this patch and a few optimisations are implemented so that the performance impact is neutral-to-minor.
- `PointeeSized` is a different name from the RFC just to make it more obvious that it is different from `std::ptr::Pointee` but all the names are yet to be bikeshed anyway.
- `@nikomatsakis` has confirmed [that this can proceed as an experiment from the t-lang side](https://rust-lang.zulipchat.com/#narrow/channel/435869-project-goals/topic/SVE.20and.20SME.20on.20AArch64.20.28goals.23270.29/near/506196491)
- FCP in rust-lang/rust#137944 (comment)

Fixes rust-lang/rust#79409.

r? `@ghost` (I'll discuss this with relevant teams to find a reviewer)

Author: bors

example/mini_core.rs

@@ -14,8 +14,14 @@
 #![no_core]
 #![allow(dead_code, internal_features, ambiguous_wide_pointer_comparisons)]
 
+#[lang = "pointee_sized"]
+pub trait PointeeSized {}
+
+#[lang = "meta_sized"]
+pub trait MetaSized: PointeeSized {}
+
 #[lang = "sized"]
-pub trait Sized {}
+pub trait Sized: MetaSized {}
 
 #[lang = "destruct"]
 pub trait Destruct {}
@@ -24,35 +30,35 @@ pub trait Destruct {}
 pub trait Tuple {}
 
 #[lang = "unsize"]
-pub trait Unsize<T: ?Sized> {}
+pub trait Unsize<T: PointeeSized>: PointeeSized {}
 
 #[lang = "coerce_unsized"]
 pub trait CoerceUnsized<T> {}
 
-impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
-impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {}
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}
-impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {}
+impl<'a, 'b: 'a, T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<&'a U> for &'b T {}
+impl<'a, T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<&'a mut U> for &'a mut T {}
+impl<T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<*const U> for *const T {}
+impl<T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<*mut U> for *mut T {}
 
 #[lang = "dispatch_from_dyn"]
 pub trait DispatchFromDyn<T> {}
 
 // &T -> &U
-impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<&'a U> for &'a T {}
+impl<'a, T: PointeeSized + Unsize<U>, U: PointeeSized> DispatchFromDyn<&'a U> for &'a T {}
 // &mut T -> &mut U
-impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<&'a mut U> for &'a mut T {}
+impl<'a, T: PointeeSized + Unsize<U>, U: PointeeSized> DispatchFromDyn<&'a mut U> for &'a mut T {}
 // *const T -> *const U
-impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<*const U> for *const T {}
+impl<T: PointeeSized + Unsize<U>, U: PointeeSized> DispatchFromDyn<*const U> for *const T {}
 // *mut T -> *mut U
-impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<*mut U> for *mut T {}
-impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
+impl<T: PointeeSized + Unsize<U>, U: PointeeSized> DispatchFromDyn<*mut U> for *mut T {}
+impl<T: MetaSized + Unsize<U>, U: MetaSized> DispatchFromDyn<Box<U>> for Box<T> {}
 
 #[lang = "legacy_receiver"]
 pub trait LegacyReceiver {}
 
-impl<T: ?Sized> LegacyReceiver for &T {}
-impl<T: ?Sized> LegacyReceiver for &mut T {}
-impl<T: ?Sized> LegacyReceiver for Box<T> {}
+impl<T: PointeeSized> LegacyReceiver for &T {}
+impl<T: PointeeSized> LegacyReceiver for &mut T {}
+impl<T: MetaSized> LegacyReceiver for Box<T> {}
 
 #[lang = "copy"]
 pub trait Copy {}
@@ -74,9 +80,9 @@ impl Copy for isize {}
 impl Copy for f32 {}
 impl Copy for f64 {}
 impl Copy for char {}
-impl<'a, T: ?Sized> Copy for &'a T {}
-impl<T: ?Sized> Copy for *const T {}
-impl<T: ?Sized> Copy for *mut T {}
+impl<'a, T: PointeeSized> Copy for &'a T {}
+impl<T: PointeeSized> Copy for *const T {}
+impl<T: PointeeSized> Copy for *mut T {}
 impl<T: Copy> Copy for Option<T> {}
 
 #[lang = "sync"]
@@ -94,17 +100,17 @@ unsafe impl Sync for i32 {}
 unsafe impl Sync for isize {}
 unsafe impl Sync for char {}
 unsafe impl Sync for f32 {}
-unsafe impl<'a, T: ?Sized> Sync for &'a T {}
+unsafe impl<'a, T: PointeeSized> Sync for &'a T {}
 unsafe impl<T: Sync, const N: usize> Sync for [T; N] {}
 
 #[lang = "freeze"]
 unsafe auto trait Freeze {}
 
-unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
-unsafe impl<T: ?Sized> Freeze for *const T {}
-unsafe impl<T: ?Sized> Freeze for *mut T {}
-unsafe impl<T: ?Sized> Freeze for &T {}
-unsafe impl<T: ?Sized> Freeze for &mut T {}
+unsafe impl<T: PointeeSized> Freeze for PhantomData<T> {}
+unsafe impl<T: PointeeSized> Freeze for *const T {}
+unsafe impl<T: PointeeSized> Freeze for *mut T {}
+unsafe impl<T: PointeeSized> Freeze for &T {}
+unsafe impl<T: PointeeSized> Freeze for &mut T {}
 
 #[lang = "structural_peq"]
 pub trait StructuralPartialEq {}
@@ -443,7 +449,7 @@ pub enum Option<T> {
 pub use Option::*;
 
 #[lang = "phantom_data"]
-pub struct PhantomData<T: ?Sized>;
+pub struct PhantomData<T: PointeeSized>;
 
 #[lang = "fn_once"]
 #[rustc_paren_sugar]
@@ -564,18 +570,18 @@ pub trait Deref {
 #[repr(transparent)]
 #[rustc_layout_scalar_valid_range_start(1)]
 #[rustc_nonnull_optimization_guaranteed]
-pub struct NonNull<T: ?Sized>(pub *const T);
+pub struct NonNull<T: PointeeSized>(pub *const T);
 
-impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
-impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
+impl<T: PointeeSized, U: PointeeSized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
+impl<T: PointeeSized, U: PointeeSized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 
-pub struct Unique<T: ?Sized> {
+pub struct Unique<T: PointeeSized> {
     pub pointer: NonNull<T>,
     pub _marker: PhantomData<T>,
 }
 
-impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
-impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
+impl<T: PointeeSized, U: PointeeSized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
+impl<T: PointeeSized, U: PointeeSized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
 
 #[lang = "global_alloc_ty"]
 pub struct Global;