Sized Hierarchy: Part I #137944

davidtwco · 2025-03-03T14:51:33Z

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 · 2025-03-03T15:21:57Z

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

compiler/rustc_type_ir/src/flags.rs

compiler/rustc_passes/src/check_attr.rs

tests/ui/target-feature/tied-features-no-implication-1.rs

traviscross · 2025-03-04T01:46:50Z

cc @rust-lang/lang

tmiasko · 2025-03-04T10:12:47Z

Does this perhaps fix #127336 by rejecting it?

davidtwco · 2025-03-06T10:22:24Z

Does this perhaps fix #127336 by rejecting it?

It doesn't currently.

davidtwco · 2025-03-06T15:59:23Z

Undrafting now that CI passes

As before, updating types using extern types to use `PointeeSized` bounds.

Unexpected Clippy lint triggering is fixed in upcoming commits but is necessary for `cfg(bootstrap)`.

Existing lints that had special-casing for `Sized` predicates ought to have these same special cases applied to `MetaSized` predicates.

One clippy test is `no_core` and needs to have `MetaSized` and `PointeeSized` added to it.

As in many previous commits, adding the new traits to minicore, but this time for cranelift and gcc.

It isn't clear why the `Deref` impl isn't found for this in a stage two build, but presumably relates to `rustc_middle::ty::RawList` containing an extern type and `Deref` not yet being relaxed to `PointeeSized` (this is technically a breaking change but unlikely to be one and will be tested in a follow-up).

These error messages include lines of the standard library which have changed and so need updated.

bors · 2025-05-14T13:09:01Z

☀️ Try build successful - checks-actions
Build commit: bde19de (bde19de91a25b5b6363890bcb8bc28b42263f16e)

rust-timer · 2025-05-14T14:38:18Z

Finished benchmarking commit (bde19de): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.1%, 9.1%]	149
Regressions ❌ (secondary)	0.7%	[0.0%, 2.7%]	89
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	3
All ❌✅ (primary)	0.9%	[0.1%, 9.1%]	149

Max RSS (memory usage)

Results (primary 2.6%, secondary 1.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.6%	[0.5%, 4.9%]	9
Regressions ❌ (secondary)	2.6%	[1.9%, 3.0%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-4.3%, -4.3%]	1
All ❌✅ (primary)	2.6%	[0.5%, 4.9%]	9

Cycles

Results (primary 3.1%, secondary -4.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.1%	[0.4%, 5.6%]	17
Regressions ❌ (secondary)	2.3%	[1.7%, 2.6%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-9.0%	[-14.3%, -2.4%]	9
All ❌✅ (primary)	3.1%	[0.4%, 5.6%]	17

Binary size

Results (primary 0.0%, secondary 0.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	28
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	28

Bootstrap: 773.561s -> 774.546s (0.13%)
Artifact size: 365.44 MiB -> 365.27 MiB (-0.05%)

davidtwco · 2025-05-14T14:39:59Z

Seems like addressing the review feedback above has regressed performance more :(

davidtwco · 2025-05-14T21:20:27Z

Submitted a bunch of perf runs to go overnight to try and isolate the change that caused the regression:

…<try> [PERF] Sized Hierarchy: Attempt 1 A perf run reverting changes to the unelaborated sizedness optimisation on rust-lang#137944 to determine if it caused the regression post-review feedback. r? `@ghost`

…<try> [PERF] Sized Hierarchy: Attempt 2 A perf run reverting changes to predicate elaboration on rust-lang#137944 to determine if it caused the regression post-review feedback. r? `@ghost`

…<try> [PERF] Sized Hierarchy: Attempt 3 A perf run reverting changes that made `PointeeSized`'s builtin impl unreachable on rust-lang#137944 to determine if it caused the regression post-review feedback. r? `@ghost`

…<try> [PERF] Sized Hierarchy: Attempt 4 A perf run reverting changes that used `as_lang_item` more over `is_lang_item` on rust-lang#137944 to determine if it caused the regression post-review feedback. r? `@ghost`

…<try> [PERF] Sized Hierarchy: Attempt 1 A perf run reverting changes to the unelaborated sizedness optimisation on rust-lang#137944 to determine if it caused the regression post-review feedback. r? `@ghost`

…trait-ref, r=<try> trait_sel: deep reject `match_normalize_trait_ref` Spotted during an in-person review of rust-lang#137944 at RustWeek: `match_normalize_trait_ref` could be using `DeepRejectCtxt` to exit early as an optimisation for projection candidates, like is done with param candidates. r? `@lcnr` cc `@oli-obk`

davidtwco · 2025-05-15T07:02:04Z

None of those partial reverts explain the 5% regression over the patch's previous regression. Some were mildly better or mildly worse, but not sufficient. I didn't revert each change to exactly what it was proor, but it should have been close enough. It could be a combination of things? Only thing I didn't check was the addition of the DeepRejectCtxt, but it seems unlikely to be that?

davidtwco · 2025-05-15T08:41:41Z

It appears that the difference is just changes in the benchmark suite.

…trait-ref, r=lcnr trait_sel: deep reject `match_normalize_trait_ref` Spotted during an in-person review of rust-lang#137944 at RustWeek: `match_normalize_trait_ref` could be using `DeepRejectCtxt` to exit early as an optimisation for projection candidates, like is done with param candidates. r? `@lcnr` cc `@oli-obk`

This was referenced Mar 3, 2025

Hierarchy of Sized traits rust-lang/rfcs#3729

Open

SVE and SME on AArch64 rust-lang/rust-project-goals#270

Open

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davidtwco force-pushed the sized-hierarchy branch from 73f1b4f to 7f509ab Compare March 3, 2025 16:29

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davidtwco force-pushed the sized-hierarchy branch from 2537bfb to b31fd85 Compare March 3, 2025 18:04

workingjubilee reviewed Mar 3, 2025

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davidtwco force-pushed the sized-hierarchy branch from b31fd85 to 2beed43 Compare March 6, 2025 10:21

davidtwco force-pushed the sized-hierarchy branch from 2beed43 to 839b844 Compare March 6, 2025 14:22

davidtwco added 7 commits May 14, 2025 12:07

rustdoc: PointeeSized bounds with extern types

f870f0e

As before, updating types using extern types to use `PointeeSized` bounds.

bootstrap: address lint failures

c4e4570

Unexpected Clippy lint triggering is fixed in upcoming commits but is necessary for `cfg(bootstrap)`.

clippy: add MetaSized conditions

d651774

Existing lints that had special-casing for `Sized` predicates ought to have these same special cases applied to `MetaSized` predicates.

clippy: {Meta,Pointee,}Sized in non-minicore

9d586d1

One clippy test is `no_core` and needs to have `MetaSized` and `PointeeSized` added to it.

cranelift/gcc: {Meta,Pointee,}Sized in minicore

d2c8491

As in many previous commits, adding the new traits to minicore, but this time for cranelift and gcc.

miri: bless tests

20f1ece

These error messages include lines of the standard library which have changed and so need updated.

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davidtwco force-pushed the sized-hierarchy branch from 84e7a29 to e1ad8d3 Compare May 14, 2025 20:46

This was referenced May 14, 2025

[PERF] Sized Hierarchy: Attempt 1 #141015

Closed

[PERF] Sized Hierarchy: Attempt 2 #141016

Closed

[PERF] Sized Hierarchy: Attempt 3 #141017

Closed

[PERF] Sized Hierarchy: Attempt 4 #141018

Closed

davidtwco force-pushed the sized-hierarchy branch from e1ad8d3 to 473fba9 Compare May 15, 2025 13:07

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Sized Hierarchy: Part I #137944

Sized Hierarchy: Part I #137944

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Sized Hierarchy: Part I #137944

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Sized Hierarchy: Part I #137944

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rust-timer commented May 14, 2025

Overall result: ❌ regressions - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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