Sized Hierarchy: Part I

[davidtwco]

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 cfgs as this would make implementation unfeasible, there would simply be too many cfgs 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

Fixes #79409.

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

[davidtwco]

I can reproduce this locally but I have no idea why it would be related to this patch. Clippy needed adjusting.

[traviscross]

cc @rust-lang/lang

[tmiasko]

Does this perhaps fix #127336 by rejecting it?

[davidtwco]

Does this perhaps fix #127336 by rejecting it?

It doesn't currently.

[davidtwco]

Undrafting now that CI passes

[davidtwco]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 9356650 with merge ea49398...

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] build_helper test:false 0.241
error: the borrowed expression implements the required traits
    --> src/bootstrap/src/core/build_steps/test.rs:2560:40
     |
2560 |         dylib_paths.push(PathBuf::from(&builder.sysroot_target_libdir(compiler, target)));
     |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `builder.sysroot_target_libdir(compiler, target)`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
     = note: `-D clippy::needless-borrows-for-generic-args` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(clippy::needless_borrows_for_generic_args)]`