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 a performance optimization, skip elaborating the supertraits of `Sized`, and if a `MetaSized` obligation is being checked, then look for a `Sized` predicate in the parameter environment. This makes the `ParamEnv` smaller which should improve compiler performance as it avoids all the iteration over the larger `ParamEnv`.

`nominal_obligations` calls `predicates_of` on a `Sized` obligation, effectively elaborating the trait and making the well-formedness checking machinery do a bunch of extra work checking a `MetaSized` obligation is well-formed, but given that both `Sized` and `MetaSized` are built-ins, if `Sized` is otherwise well-formed, so `MetaSized` will be.

These should never be shown to users at the moment.

Some rustdoc tests are `no_core` and need to have `MetaSized` and `PointeeSized` added to them.

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's unclear why this change in miri is necessary.

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

davidtwco · 2025-05-01T17:25:54Z

Rebased, addressed most of @lcnr's feedback, fixed the crater regressions and added tests, still to do:

red-lightning123.hwc regression with the new overflow errors
#![rustc_no_implicit_bounds] per @lcnr's request
re-run perf

davidtwco · 2025-05-01T17:27:46Z

@bors try @rust-timer queue

davidtwco · 2025-05-01T17:33:42Z

Oh, apparently GitHub is still "processing the push", never seen that before

rustbot · 2025-05-01T19:01:33Z

This PR modifies tests/auxiliary/minicore.rs.

cc @jieyouxu

davidtwco · 2025-05-01T19:02:15Z

I had to git commit --amend, make no changes, and then force push - but it's updated now.

davidtwco · 2025-05-01T19:02:22Z

@bors try @rust-timer queue

bors · 2025-05-01T19:03:34Z

⌛ Trying commit 04d7895 with merge 18f4fd1...

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) Fixes rust-lang#79409. r? `@ghost` (I'll discuss this with relevant teams to find a reviewer)

bors · 2025-05-01T21:11:59Z

☀️ Try build successful - checks-actions
Build commit: 18f4fd1 (18f4fd1d643f4d3e2d2edf23b46129871523de68)

rust-timer · 2025-05-01T22:56:37Z

Finished benchmarking commit (18f4fd1): 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%, 4.9%]	137
Regressions ❌ (secondary)	0.9%	[0.1%, 2.6%]	69
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	0.9%	[0.1%, 4.9%]	137

Max RSS (memory usage)

Results (primary 0.8%, secondary -0.4%)

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)	1.0%	[0.4%, 4.6%]	48
Regressions ❌ (secondary)	2.4%	[2.3%, 2.6%]	4
Improvements ✅ (primary)	-0.8%	[-1.7%, -0.5%]	7
Improvements ✅ (secondary)	-2.6%	[-3.6%, -1.8%]	5
All ❌✅ (primary)	0.8%	[-1.7%, 4.6%]	55

Cycles

Results (primary 1.2%, secondary 2.7%)

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)	1.3%	[0.4%, 3.4%]	48
Regressions ❌ (secondary)	2.7%	[2.0%, 4.8%]	10
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.2%	[-0.7%, 3.4%]	49

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%]	20
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	20

Bootstrap: 768.358s -> 770.38s (0.26%)
Artifact size: 365.48 MiB -> 365.58 MiB (0.03%)

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 12 commits May 1, 2025 11:01

rustdoc: skip MetaSized bounds

e30abf9

These should never be shown to users at the moment.

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

e825fd1

Some rustdoc tests are `no_core` and need to have `MetaSized` and `PointeeSized` added to them.

rustdoc: PointeeSized bounds with extern types

a139f6a

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

bootstrap: address lint failures

66ea82b

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

clippy: add MetaSized conditions

824d7d2

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

514ab1d

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

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

f30425d

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

miri: fix build

c9a1396

It's unclear why this change in miri is necessary.

miri: bless tests

be60558

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

tests: add ice test for generic-const-exprs

04d7895

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davidtwco force-pushed the sized-hierarchy branch from 74279fe to 04d7895 Compare May 1, 2025 19:01

rustbot added the A-test-infra-minicore Area: `minicore` test auxiliary and `//@ add-core-stubs` label May 1, 2025

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Sized Hierarchy: Part I #137944

Sized Hierarchy: Part I #137944

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Sized Hierarchy: Part I #137944

Are you sure you want to change the base?

Sized Hierarchy: Part I #137944

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Overall result: ❌ regressions - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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