Auto merge of rust-lang#137944 - davidtwco:sized-hierarchy, r=<try>

Sized Hierarchy

This patch implements rust-lang/rfcs#3729. It introduces two new traits to the standard library, `MetaSized` and `PointeeSized`, and makes `MetaSized` and `Sized` into const traits (relying on unstable `feature(const_trait_impl)`). 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 migrations which are necessary to preserve backwards compatibility given the introduction of these traits, which are implemented and as follows:

- On the current edition, `Sized` is rewritten as `const Sized`
  - If the `sized_hierarchy` feature is enabled, then an edition migration lint to rewrite the bound to `const Sized` will be emitted.
  - On the next edition, non-const `Sized` will resume being the default bound.
- On the current edition, `?Sized` is rewritten as `const MetaSized`
  - If the `sized_hierarchy` feature is enabled, then an edition migration lint to rewrite the bound to `const MetaSized` will be emitted.
  - On the next edition, writing `?Sized` will be prohibited.
- On the current edition, `const MetaSized` is added as a default supertrait for all traits w/out an explicit sizedness supertrait already.
  - If the `sized_hierarchy` feature is enabled, then an edition migration lint to add an explicit `const MetaSized` supertrait will be emitted.
  - On the next edition, there is no default `const MetaSized` supertrait.

Each of these migrations is not conditional on whether the item being migrated *needs* the migration to the stricter bound - this would be preferable but is not yet implemented (if it is possible to implement). All diagnostic output should remain the same (showing `?Sized` even if the compiler sees `const 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). Only once `sized_hierarchy` is stabilised would edition migration lints start to be emitted and diagnostic output show the "real" sizedness traits behind-the-scenes, rather than `?Sized`. 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 (library: +573/-184, compiler: +1268/-310, tests: +3720/-452).
- It is expected that this will have performance regressions initially and I'll aim to resolve those prior to merging if possible.
  - I'd appreciate feedback on how best to go about this from those familiar with the type system.
- On my local machine, this passes all of the test suites, a stage two build and a tidy check.
- `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)

Fixes rust-lang#79409.

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

Author: bors

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -254,10 +254,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             let place = &self.move_data.move_paths[mpi].place;
             let ty = place.ty(self.body, self.infcx.tcx).ty;
 
-            if self.infcx.param_env.caller_bounds().iter().any(|c| {
-                c.as_trait_clause().is_some_and(|pred| {
-                    pred.skip_binder().self_ty() == ty && self.infcx.tcx.is_fn_trait(pred.def_id())
-                })
+            if self.infcx.param_env.trait_clauses().any(|pred| {
+                pred.skip_binder().self_ty() == ty && self.infcx.tcx.is_fn_trait(pred.def_id())
             }) {
                 // Suppress the next suggestion since we don't want to put more bounds onto
                 // something that already has `Fn`-like bounds (or is a closure), so we can't

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -226,15 +226,13 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
         // Normalize the assumptions we use to borrowck the program.
         let mut constraints = vec![];
         let mut known_type_outlives_obligations = vec![];
-        for bound in param_env.caller_bounds() {
-            if let Some(outlives) = bound.as_type_outlives_clause() {
-                self.normalize_and_push_type_outlives_obligation(
-                    outlives,
-                    span,
-                    &mut known_type_outlives_obligations,
-                    &mut constraints,
-                );
-            };
+        for outlives in param_env.type_outlives_clauses() {
+            self.normalize_and_push_type_outlives_obligation(
+                outlives,
+                span,
+                &mut known_type_outlives_obligations,
+                &mut constraints,
+            );
         }
 
         let unnormalized_input_output_tys = self