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
• FCP in Sized Hierarchy: Part I #137944 (comment)

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

[bors]

📌 Commit a31e1f1 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit a31e1f1 with merge 86d0aef...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 86d0aef to master...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 55d4364 (parent) -> 86d0aef (this PR)

Test differences

Show 433 test diffs

Stage 1

• [ui] tests/ui/extern/extern-types-size_of_val.rs: [missing] -> pass (J0)
• [ui] tests/ui/extern/extern-types-size_of_val.rs#align: pass -> [missing] (J0)
• [ui] tests/ui/extern/extern-types-size_of_val.rs#size: pass -> [missing] (J0)
• [ui] tests/ui/feature-gates/feature-gate-sized-hierarchy.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/alias-bounds.rs#next: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/alias-bounds.rs#old: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/default-bound.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/default-supertrait.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions-1.rs#next: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions-1.rs#old: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions.rs#next: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions.rs#old: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/extern-type-behind-ptr.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/impls.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/overflow.rs#current: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/overflow.rs#next: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/pointee-supertrait.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/pretty-print-no-feat-dep-has-feat.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/pretty-print-no-feat.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/pretty-print-opaque-no-feat.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/pretty-print-opaque.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/pretty-print.rs: [missing] -> pass (J0)
• [ui] tests/ui/sized-hierarchy/trait-aliases.rs: [missing] -> pass (J0)
• [ui] tests/ui/target-feature/forbidden-hardfloat-target-feature-cfg.rs: [missing] -> pass (J0)

Stage 2

• [ui] tests/ui/extern/extern-types-size_of_val.rs: [missing] -> pass (J1)
• [ui] tests/ui/extern/extern-types-size_of_val.rs#align: pass -> [missing] (J1)
• [ui] tests/ui/extern/extern-types-size_of_val.rs#size: pass -> [missing] (J1)
• [ui] tests/ui/feature-gates/feature-gate-sized-hierarchy.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/alias-bounds.rs#next: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/alias-bounds.rs#old: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/default-bound.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/default-supertrait.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions-1.rs#next: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions-1.rs#old: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions.rs#next: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/elaboration-opt-regions.rs#old: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/extern-type-behind-ptr.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/impls.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/overflow.rs#current: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/overflow.rs#next: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/pointee-supertrait.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/pretty-print-no-feat-dep-has-feat.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/pretty-print-no-feat.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/pretty-print-opaque-no-feat.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/pretty-print-opaque.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/pretty-print.rs: [missing] -> pass (J1)
• [ui] tests/ui/sized-hierarchy/trait-aliases.rs: [missing] -> pass (J1)
• [ui] tests/ui/target-feature/forbidden-hardfloat-target-feature-cfg.rs: [missing] -> pass (J1)
• [run-make] tests/run-make/compressed-debuginfo-zstd: ignore (ignored if LLVM wasn't build with zstd for ELF section compression (we want LLVM/LLD to be built with zstd support)) -> pass (J2)

Additionally, 384 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: x86_64-gnu-llvm-19-3, x86_64-gnu-llvm-20-3
• J1: aarch64-apple, aarch64-gnu, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, test-various, x86_64-apple-2, x86_64-gnu, x86_64-gnu-llvm-19-2, x86_64-gnu-llvm-20-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1
• J2: x86_64-gnu-nopt

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 86d0aef80403f095d8bbabf44d9fdecfcd45f076 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. dist-aarch64-linux: 5492.3s -> 8351.1s (52.0%)
2. x86_64-apple-2: 5524.1s -> 3592.7s (-35.0%)
3. x86_64-rust-for-linux: 3532.9s -> 2538.0s (-28.2%)
4. mingw-check-1: 2314.9s -> 1663.7s (-28.1%)
5. mingw-check-tidy: 52.7s -> 66.8s (26.7%)
6. dist-aarch64-apple: 6185.5s -> 4603.6s (-25.6%)
7. mingw-check-2: 2510.8s -> 1944.8s (-22.5%)
8. test-various: 5666.1s -> 4696.3s (-17.1%)
9. arm-android: 6541.0s -> 5461.2s (-16.5%)
10. x86_64-gnu-tools: 6949.8s -> 5831.7s (-16.1%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[davidtwco]

Thanks @oli-obk, @lcnr, @fee1-dead, @fmease and @workingjubilee for reviews!

[rust-timer]

Finished benchmarking commit (86d0aef): comparison URL.

Overall result: ❌ regressions - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	1.3%	[0.1%, 9.9%]	111
Regressions ❌ (secondary)	0.9%	[0.2%, 2.9%]	101
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.7%	[-0.7%, -0.7%]	1
All ❌✅ (primary)	1.3%	[0.1%, 9.9%]	111

Max RSS (memory usage)

Results (primary 2.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.6%	[1.1%, 4.9%]	6
Regressions ❌ (secondary)	2.6%	[1.0%, 3.9%]	4
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-5.2%	[-8.3%, -2.1%]	2
All ❌✅ (primary)	2.0%	[-1.3%, 4.9%]	7

Cycles

Results (primary 2.6%, secondary 2.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.6%	[0.7%, 5.7%]	27
Regressions ❌ (secondary)	2.0%	[1.3%, 3.1%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[0.7%, 5.7%]	27

Binary size

Results (primary 0.0%, secondary 0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	28
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	16
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	28

Bootstrap: 756.784s -> 693.274s (-8.39%)
Artifact size: 372.10 MiB -> 372.01 MiB (-0.02%)

[Kobzol]

(Note: the bootstrap change is unrelated to this PR, we used to unnecessarily compile too much stuff in the bootstrap benchmark).

[rylev]

As noted in this summary comment, the performance regression is expected and will be addressed in follow ups.

@rustbot label: +perf-regression-triaged