`apply_member_constraints`: fix placeholder check #142071

lcnr · 2025-06-05T10:27:39Z

Checking whether the member region is an existential region from a higher universe is just wrong and I am pretty sure we've added that check by accident as the naming was just horribly confusing before #140466.

I've encountered this issue separately while working on #139587, but feel like it's probably easier to separately FCP this change. This allows the following code to compile

trait Proj<'a> {
    type Assoc;
}
impl<'a, 'b, F: FnOnce() -> &'b ()> Proj<'a> for F {
    type Assoc = ();
}

fn is_proj<F: for<'a> Proj<'a>>(f: F) {}
fn define<'a>() -> impl Sized + use<'a> {
    // This adds a use of `opaque::<'a>` with hidden type `&'unconstrained_b ()`.
    // 'unconstrained_b is an inference variable from a higher universe as it gets
    // created inside of the binder of `F: for<'a> Proj<'a>`. This previously
    // caused us to not apply member constraints. We now do, constraining
    // it to `'a`.
    is_proj(define::<'a>);
    &()
}

fn main() {}

This should not be breaking change, even in theory. Applying member constraints is incomplete in rare circumstances which means that applying them in more cases can cause spurious errors, cc #140569/#142073. However, as we always skipped these member regions in apply_member_constraints the skipped region is guaranteed to cause an error in check_member_constraints later on.

rustbot · 2025-06-05T10:27:43Z

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lcnr · 2025-06-05T10:40:52Z

@rfcbot fcp merge

rfcbot · 2025-06-05T10:40:54Z

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

jackh726 · 2025-07-28T13:49:10Z

tests/ui/nll/member-constraints/non-root-universe-existential-1.rs

+    // This was unnecessary. It is totally acceptable for member regions
+    // to be able to name placeholders from higher universes, as long as
+    // they don't actually do so.


This comment is weird: "It is totally acceptable ... as long as they don't actually do so"?

"to be able to" vs "to do"

As in: it's fine for the member region to be an existential region created in a higher universe, as long as it doesn't actually name any placeholder from that higher universe.

How would you restructure this comment? Is the following clearer?

It is totally acceptable for member regions to be able to name placeholders from higher universes, as long as they don't actually refer to a placeholder.

it's fine for the member region to be an existential region created in a higher universe, as long as it doesn't actually name any placeholder from that higher universe.

This is the most clear

rfcbot · 2025-07-28T13:52:41Z

🔔 This is now entering its final comment period, as per the review above. 🔔

nikomatsakis · 2025-07-28T20:42:59Z

@rfcbot reviewed

That example is pretty mind-binding -- the key thing that took me a while to understand was that the universal variable created by proving for<'a> Proj<'a> is not the thing you are talking about which gets constrained by 'a -- it is rather the extra 'b that appears in the self type. OK.

tests/ui/nll/member-constraints/non-root-universe-existential-2.rs

rfcbot · 2025-08-07T13:52:45Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

lcnr · 2025-08-08T12:32:49Z

@rustbot ready

compiler-errors · 2025-08-14T17:03:56Z

@bors r+ rollup=never

bors · 2025-08-14T17:03:59Z

📌 Commit cde14e6 has been approved by compiler-errors

It is now in the queue for this repository.

bors · 2025-08-14T19:21:26Z

⌛ Testing commit cde14e6 with merge a43ff36...

`apply_member_constraints`: fix placeholder check Checking whether the member region is *an existential region from a higher universe* is just wrong and I am pretty sure we've added that check by accident as the naming was just horribly confusing before #140466. I've encountered this issue separately while working on #139587, but feel like it's probably easier to separately FCP this change. This allows the following code to compile ```rust trait Proj<'a> { type Assoc; } impl<'a, 'b, F: FnOnce() -> &'b ()> Proj<'a> for F { type Assoc = (); } fn is_proj<F: for<'a> Proj<'a>>(f: F) {} fn define<'a>() -> impl Sized + use<'a> { // This adds a use of `opaque::<'a>` with hidden type `&'unconstrained_b ()`. // 'unconstrained_b is an inference variable from a higher universe as it gets // created inside of the binder of `F: for<'a> Proj<'a>`. This previously // caused us to not apply member constraints. We now do, constraining // it to `'a`. is_proj(define::<'a>); &() } fn main() {} ``` This should not be breaking change, even in theory. Applying member constraints is incomplete in rare circumstances which means that applying them in more cases can cause spurious errors, cc #140569/#142073. However, as we always skipped these member regions in `apply_member_constraints` the skipped region is guaranteed to cause an error in `check_member_constraints` later on.

rust-log-analyzer · 2025-08-14T19:36:47Z

The job x86_64-mingw-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

[RUSTC-TIMING] windows_targets test:false 0.187
error: could not compile `windows-targets` (lib)

Caused by:
  process didn't exit successfully: `C:\a\rust\rust\build\bootstrap\debug\rustc C:\a\rust\rust\build\bootstrap\debug\rustc --crate-name windows_targets --edition=2021 C:\Users\runneradmin\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\windows-targets-0.52.6\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no --warn=rust_2018_idioms --warn=unexpected_cfgs --warn=missing_docs --check-cfg "cfg(windows_raw_dylib, windows_debugger_visualizer, windows_slim_errors)" --check-cfg cfg(docsrs,test) --check-cfg "cfg(feature, values())" -C metadata=34ce080c45117a20 -C extra-filename=-28bb00e79ea7a2cb --out-dir C:\a\rust\rust\build\bootstrap\x86_64-pc-windows-gnu\debug\deps --target x86_64-pc-windows-gnu -C strip=debuginfo -L dependency=C:\a\rust\rust\build\bootstrap\x86_64-pc-windows-gnu\debug\deps -L dependency=C:\a\rust\rust\build\bootstrap\debug\deps --cap-lints allow --cfg=bootstrap --cfg=windows_raw_dylib -Csymbol-mangling-version=v0 -Zunstable-options -Zmacro-backtrace -Csplit-debuginfo=packed -Alinker-messages -Zunstable-options -Cdebuginfo=2 -Z binary-dep-depinfo` (exit code: 0xc00000fd, STATUS_STACK_OVERFLOW)
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] version_check test:false 0.361
[RUSTC-TIMING] build_script_build test:false 0.430
[RUSTC-TIMING] windows_sys test:false 3.371
Build completed unsuccessfully in 0:03:04

bors · 2025-08-14T19:37:26Z

💔 Test failed - checks-actions

lcnr · 2025-08-15T13:54:14Z

unrelated

@bors retry

bors · 2025-08-15T18:52:16Z

⌛ Testing commit cde14e6 with merge cd7cbe8...

bors · 2025-08-15T23:09:23Z

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing cd7cbe8 to master...

github-actions · 2025-08-15T23:12:55Z

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 8b1889c (parent) -> cd7cbe8 (this PR)

Test differences

Show 9 test diffs

Stage 1

[ui] tests/ui/nll/member-constraints/non-root-universe-existential-1.rs#current: [missing] -> pass (J0)
[ui] tests/ui/nll/member-constraints/non-root-universe-existential-1.rs#next: [missing] -> pass (J0)
[ui] tests/ui/nll/member-constraints/non-root-universe-existential-2.rs: [missing] -> pass (J0)

Stage 2

[ui] tests/ui/nll/member-constraints/non-root-universe-existential-1.rs#current: [missing] -> pass (J1)
[ui] tests/ui/nll/member-constraints/non-root-universe-existential-1.rs#next: [missing] -> pass (J1)
[ui] tests/ui/nll/member-constraints/non-root-universe-existential-2.rs: [missing] -> pass (J1)

Additionally, 3 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard cd7cbe818e4a66d46fe2df993d1b8518eba8a5cd --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

dist-x86_64-apple: 9726.3s -> 14491.2s (49.0%)
dist-apple-various: 5628.7s -> 7271.1s (29.2%)
dist-aarch64-apple: 7486.2s -> 5803.3s (-22.5%)
pr-check-2: 2718.4s -> 2265.6s (-16.7%)
arm-android: 7025.4s -> 5871.7s (-16.4%)
aarch64-gnu-debug: 5222.7s -> 4410.4s (-15.6%)
dist-x86_64-netbsd: 5598.7s -> 4805.8s (-14.2%)
x86_64-gnu-llvm-20-1: 3777.4s -> 3303.1s (-12.6%)
x86_64-gnu-llvm-19-1: 3774.5s -> 3312.4s (-12.2%)
pr-check-1: 1628.8s -> 1444.1s (-11.3%)

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.

rust-timer · 2025-08-16T00:22:24Z

Finished benchmarking commit (cd7cbe8): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	12.3%	[12.3%, 12.3%]	1
Regressions ❌ (secondary)	0.6%	[0.2%, 1.8%]	4
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	4.7%	[-3.0%, 12.3%]	2

Max RSS (memory usage)

Results (secondary 3.6%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.6%	[3.6%, 3.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.6%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

Binary size

Results (primary -0.6%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	4
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Bootstrap: 470.263s -> 469.562s (-0.15%)
Artifact size: 377.48 MiB -> 377.55 MiB (0.02%)

Kobzol · 2025-08-16T05:56:44Z

clap_derive and tt_muncher are noise.

@rustbot label: +perf-regression-triaged

rustbot assigned oli-obk Jun 5, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 5, 2025

lcnr added T-types Relevant to the types team, which will review and decide on the PR/issue. and removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 5, 2025

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Jun 5, 2025

lcnr mentioned this pull request Jun 5, 2025

nll-relate: improve hr opaque types support #142079

Open

compiler-errors approved these changes Jun 5, 2025

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compiler-errors assigned compiler-errors and unassigned oli-obk Jun 5, 2025

compiler-errors added S-waiting-on-fcp Status: PR is in FCP and is awaiting for FCP to complete. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 11, 2025

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rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Jul 28, 2025

lcnr commented Jul 28, 2025

View reviewed changes

tests/ui/nll/member-constraints/non-root-universe-existential-2.rs Outdated Show resolved Hide resolved

rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. to-announce Announce this issue on triage meeting and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Aug 7, 2025

rustbot mentioned this pull request Aug 7, 2025

Tracking issue for release notes of #142071: apply_member_constraints: fix placeholder check #145054

Closed

apply_member_constraints: fix placeholder check

cde14e6

lcnr force-pushed the revealing-use branch from 6fb9b63 to cde14e6 Compare August 8, 2025 12:32

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 8, 2025

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 14, 2025

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 14, 2025

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 15, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 15, 2025

bors merged commit cd7cbe8 into rust-lang:master Aug 15, 2025
11 checks passed

rustbot added this to the 1.91.0 milestone Aug 15, 2025

rustbot added the perf-regression Performance regression. label Aug 16, 2025

bors mentioned this pull request Aug 16, 2025

support non-defining uses of opaques in borrowck #145244

Open

rustbot added the perf-regression-triaged The performance regression has been triaged. label Aug 16, 2025

apply_member_constraints: fix placeholder check #142071

apply_member_constraints: fix placeholder check #142071

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Aug 16, 2025

Overall result: ❌✅ regressions and improvements - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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`apply_member_constraints`: fix placeholder check #142071

`apply_member_constraints`: fix placeholder check #142071

lcnr commented Jun 5, 2025 •

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edited by jackh726

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