Reject relaxed bounds inside associated type bounds (ATB) #135331

fmease · 2025-01-10T12:21:32Z

Reject relaxed bounds — most notably ?Sized — inside associated type bounds TraitRef<AssocTy: …>.

This was previously accepted without warning despite being incorrect: ATBs are not a place where we perform sized elaboration, meaning TraitRef<AssocTy: …> does not elaborate to TraitRef<AssocTy: Sized + …> if … doesn't contain ?Sized. Therefore ?Sized is meaningless. In no other (stable) place do we (intentionally) allow relaxed bounds where we don't also perform sized elab, this is highly inconsistent and confusing! Another point of comparison: For the desugared $SelfTy: TraitRef, $SelfTy::AssocTy: … we don't do sized elab either (and thus also don't allow relaxed bounds).

Moreover — as I've alluded to back in #135841 (review) — some later validation steps only happen during sized elaboration during HIR ty lowering¹. Namely, rejecting duplicates (e.g., ?Trait + ?Trait) and ensuring that Trait in ?Trait is equal to Sized². As you can probably guess, on stable/master we don't run these checks for ATBs (so we allow even more nonsensical bounds like Iterator<Item: ?Copy> despite T-types's ruling established in the FCP'ed #135841).

This PR rectifies all of this. I cratered this back in 2025-01-10 with (allegedly) no regressions found (report, its analysis). However a contributor manually found two occurrences of TraitRef<AssocTy: ?Sized> in small hobby projects (presumably via GH code search). I immediately sent downstream PRs: Gui-Yom/turbo-metrics#14, ireina7/summon#1 (however, the owners have showed no reaction so far).

I'm leaning towards banning these forms without a FCW because a FCW isn't worth the maintenance cost³. Note that associated type bounds were stabilized in 1.79.0 (released 2024-06-13 which is 13 months ago), so the proliferation of ATBs shouldn't be that high yet. If you think we should do another crater run since the last one was 6 months ago, I'm fine with that.

Fixes #135229.

I consider this a flaw in the implementation and I've already added a huge FIXME. ↩
To be more precise, if the internal flag -Zexperimental-default-bounds is provided other "default traits" (needs internal feature lang_items) are permitted as well (cc closely related internal feature: more_maybe_bounds). ↩
Having to track this and adding an entire lint whose remnants would remain in the code base forever (we never fully remove lints). ↩

fmease · 2025-01-10T12:21:43Z

@bors try

bors · 2025-01-10T12:22:59Z

⌛ Trying commit 5c245de with merge de39175...

[crater-only] Ban assoc ty unbounds cc rust-lang#135229 r? ghost

bors · 2025-01-10T14:10:39Z

☀️ Try build successful - checks-actions
Build commit: de39175 (de39175d9e0f63ff8967b81f9639ba28d11ae6ef)

fmease · 2025-01-10T14:13:32Z

@craterbot check

craterbot · 2025-01-10T14:13:53Z

👌 Experiment pr-135331 created and queued.
🤖 Automatically detected try build de39175
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-01-10T14:14:59Z

🚧 Experiment pr-135331 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2025-01-11T16:46:36Z

🎉 Experiment pr-135331 is completed!
📊 117 regressed and 36 fixed (566362 total)
📰 Open the full report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Skgland · 2025-01-11T17:52:03Z

Based on running Crater Analysis on this crater report
110 out of 117 build failed (unknown) regressions appear spurious caused by running out of disk space, i.e. their logs contain the string no space left on device.
The remaining seven appear to be spurious as well, based on manual review of the logs.

full automated report

Regressed: 117
build failed(unknown): 117
----------------------------------
Results:
no-space: 110
----------------------------------
sum: 110
others: 7
----------------------------------
[
    (
        "ceronman.writing-a-c-compiler.ce987db61026febea0e12806e99932b735808d61",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/gh/ceronman.writing-a-c-compiler",
    ),
    (
        "benrussell.simple_open-rs.feb28e8321d6c3db2cd5b20a6c009c70db767118",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/gh/benrussell.simple_open-rs",
    ),
    (
        "dennisoelkers.brghtnss.43a8295abe3b93dfc257f7908710ef7bb4d2675e",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/gh/dennisoelkers.brghtnss",
    ),
    (
        "gauteh.hidefix.5569aa88689b7dd1867354103e0485894b0d06ec",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/gh/gauteh.hidefix",
    ),
    (
        "smasher164.judgefmt.945d1d88d4083018bcd997df019a03656bc24865",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/gh/smasher164.judgefmt",
    ),
    (
        "pillow-routing-0.4.2",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/reg/pillow-routing-0.4.2",
    ),
    (
        "tree-sitter-po-0.0.1",
        "try%23de39175d9e0f63ff8967b81f9639ba28d11ae6ef/reg/tree-sitter-po-0.0.1",
    ),
]

More robustly deal with relaxed bounds and improve their diagnostics Scaffolding for #135229 (CC #135331) Fixes #136944. Fixes #142718.

craterbot · 2025-07-31T10:49:17Z

🎉 Experiment pr-135331-1 is completed!
📊 14 regressed and 9 fixed (673600 total)
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

fmease · 2025-07-31T14:51:15Z

Update: Crater found two new root regressions.

One (le-stream-3.6.3) doesn't have any dependents and the other one (KSXGitHub.parse-arch-pkg-desc) has a dependent by the same org / author (KSXGitHub.arch-pkg-db). All other ones are spurious.

I've already sent out new "incoming breakage" PRs:

rfcbot · 2025-08-09T15:08:06Z

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 · 2025-08-11T07:45:00Z

@bors r+ rollup

bors · 2025-08-11T07:45:03Z

📌 Commit 788fb08 has been approved by lcnr

It is now in the queue for this repository.

Rollup of 5 pull requests Successful merges: - #135331 (Reject relaxed bounds inside associated type bounds (ATB)) - #144156 (Check coroutine upvars in dtorck constraint) - #145091 (`NllRegionVariableOrigin` remove `from_forall`) - #145194 (Ignore coroutine witness type region args in auto trait confirmation) - #145225 (Fix macro infinite recursion test to not trigger warning about semicolon in expr) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #135331 - fmease:ban-assoc-ty-unbounds, r=lcnr Reject relaxed bounds inside associated type bounds (ATB) **Reject** relaxed bounds — most notably `?Sized` — inside associated type bounds `TraitRef<AssocTy: …>`. This was previously accepted without warning despite being incorrect: ATBs are *not* a place where we perform *sized elaboration*, meaning `TraitRef<AssocTy: …>` does *not* elaborate to `TraitRef<AssocTy: Sized + …>` if `…` doesn't contain `?Sized`. Therefore `?Sized` is meaningless. In no other (stable) place do we (intentionally) allow relaxed bounds where we don't also perform sized elab, this is highly inconsistent and confusing! Another point of comparison: For the desugared `$SelfTy: TraitRef, $SelfTy::AssocTy: …` we don't do sized elab either (and thus also don't allow relaxed bounds). Moreover — as I've alluded to back in #135841 (review) — some later validation steps only happen during sized elaboration during HIR ty lowering[^1]. Namely, rejecting duplicates (e.g., `?Trait + ?Trait`) and ensuring that `Trait` in `?Trait` is equal to `Sized`[^2]. As you can probably guess, on stable/master we don't run these checks for ATBs (so we allow even more nonsensical bounds like `Iterator<Item: ?Copy>` despite T-types's ruling established in the FCP'ed #135841). This PR rectifies all of this. I cratered this back in 2025-01-10 with (allegedly) no regressions found ([report](#135331 (comment)), [its analysis](#135331 (comment))). [However a contributor manually found two occurrences](#135229 (comment)) of `TraitRef<AssocTy: ?Sized>` in small hobby projects (presumably via GH code search). I immediately sent downstream PRs: Gui-Yom/turbo-metrics#14, ireina7/summon#1 (however, the owners have showed no reaction so far). I'm leaning towards banning these forms **without a FCW** because a FCW isn't worth the maintenance cost[^3]. Note that associated type bounds were stabilized in 1.79.0 (released 2024-06-13 which is 13 months ago), so the proliferation of ATBs shouldn't be that high yet. If you think we should do another crater run since the last one was 6 months ago, I'm fine with that. Fixes #135229. [^1]: I consider this a flaw in the implementation and [I've already added a huge FIXME](https://github.com/rust-lang/rust/blob/82a02aefe07092c737c852daccebf49ca25507e3/compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs#L195-L207). [^2]: To be more precise, if the internal flag `-Zexperimental-default-bounds` is provided other "default traits" (needs internal feature `lang_items`) are permitted as well (cc closely related internal feature: `more_maybe_bounds`). [^3]: Having to track this and adding an entire lint whose remnants would remain in the code base forever (we never *fully* remove lints).

Further tighten up relaxed bounds Follow-up to #142693, #135331 and #135841. Fixes #143122. * Reject relaxed bounds `?Trait` in the bounds of trait aliases. Just like `trait Trait {}` doesn't mean `trait Trait: Sized {}` and we therefore reject `trait Trait: ?Sized {}`, `trait Trait =;` (sic!) doesn't mean `trait Trait = Sized;` (never did!) and as a logical consequence `trait Trait = ?Sized;` is meaningless and should be forbidden. * Don't permit `?Sized` in more places (e.g., supertrait bounds, trait object types) if feature `more_maybe_bounds` is enabled. That internal feature is only meant to allow the user to define & use *new* default traits (that have fewer rules to follow for now to ease experimentation). * Unconditionally check that the `Trait` in `?Trait` is a default trait. Previously, we would only perform this check in selected places which was very brittle and led to bugs slipping through. * Slightly improve diagnostics.

Further tighten up relaxed bounds Follow-up to rust-lang#142693, rust-lang#135331 and rust-lang#135841. Fixes rust-lang#143122. * Reject relaxed bounds `?Trait` in the bounds of trait aliases. Just like `trait Trait {}` doesn't mean `trait Trait: Sized {}` and we therefore reject `trait Trait: ?Sized {}`, `trait Trait =;` (sic!) doesn't mean `trait Trait = Sized;` (never did!) and as a logical consequence `trait Trait = ?Sized;` is meaningless and should be forbidden. * Don't permit `?Sized` in more places (e.g., supertrait bounds, trait object types) if feature `more_maybe_bounds` is enabled. That internal feature is only meant to allow the user to define & use *new* default traits (that have fewer rules to follow for now to ease experimentation). * Unconditionally check that the `Trait` in `?Trait` is a default trait. Previously, we would only perform this check in selected places which was very brittle and led to bugs slipping through. * Slightly improve diagnostics.

Rollup merge of #147734 - fmease:tighten-relaxed, r=lcnr Further tighten up relaxed bounds Follow-up to #142693, #135331 and #135841. Fixes #143122. * Reject relaxed bounds `?Trait` in the bounds of trait aliases. Just like `trait Trait {}` doesn't mean `trait Trait: Sized {}` and we therefore reject `trait Trait: ?Sized {}`, `trait Trait =;` (sic!) doesn't mean `trait Trait = Sized;` (never did!) and as a logical consequence `trait Trait = ?Sized;` is meaningless and should be forbidden. * Don't permit `?Sized` in more places (e.g., supertrait bounds, trait object types) if feature `more_maybe_bounds` is enabled. That internal feature is only meant to allow the user to define & use *new* default traits (that have fewer rules to follow for now to ease experimentation). * Unconditionally check that the `Trait` in `?Trait` is a default trait. Previously, we would only perform this check in selected places which was very brittle and led to bugs slipping through. * Slightly improve diagnostics.

Further tighten up relaxed bounds Follow-up to rust-lang/rust#142693, rust-lang/rust#135331 and rust-lang/rust#135841. Fixes rust-lang/rust#143122. * Reject relaxed bounds `?Trait` in the bounds of trait aliases. Just like `trait Trait {}` doesn't mean `trait Trait: Sized {}` and we therefore reject `trait Trait: ?Sized {}`, `trait Trait =;` (sic!) doesn't mean `trait Trait = Sized;` (never did!) and as a logical consequence `trait Trait = ?Sized;` is meaningless and should be forbidden. * Don't permit `?Sized` in more places (e.g., supertrait bounds, trait object types) if feature `more_maybe_bounds` is enabled. That internal feature is only meant to allow the user to define & use *new* default traits (that have fewer rules to follow for now to ease experimentation). * Unconditionally check that the `Trait` in `?Trait` is a default trait. Previously, we would only perform this check in selected places which was very brittle and led to bugs slipping through. * Slightly improve diagnostics.

fmease added the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Jan 10, 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 Jan 10, 2025

fmease added S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 10, 2025

bors added a commit to rust-lang-ci/rust that referenced this pull request Jan 10, 2025

Auto merge of rust-lang#135331 - fmease:ban-assoc-ty-unbounds, r=<try>

de39175

[crater-only] Ban assoc ty unbounds cc rust-lang#135229 r? ghost

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. labels Jan 10, 2025

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fmease added S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 11, 2025

fmease mentioned this pull request Jun 18, 2025

More robustly deal with relaxed bounds and improve their diagnostics #142693

Merged

rust-bors bot added a commit that referenced this pull request Jun 20, 2025

Auto merge of #142693 - fmease:unbound-bettering, r=<try>

bfa10de

More robustly deal with relaxed bounds and improve their diagnostics Scaffolding for #135229 (CC #135331) Fixes #136944. Fixes #142718.

rust-bors bot added a commit that referenced this pull request Jun 28, 2025

Auto merge of #142693 - fmease:unbound-bettering, r=<try>

6893bb5

More robustly deal with relaxed bounds and improve their diagnostics Scaffolding for #135229 (CC #135331) Fixes #136944. Fixes #142718.

fmease added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Jul 18, 2025

fmease changed the title ~~[crater-only] Ban assoc ty unbounds~~ Reject relaxed bounds inside associated type bounds (ATB, TraitRef<AssocTy: …>) Jul 18, 2025

fmease changed the title ~~Reject relaxed bounds inside associated type bounds (ATB, TraitRef<AssocTy: …>)~~ [STACKED] Reject relaxed bounds inside associated type bounds (ATB, TraitRef<AssocTy: …>) Jul 18, 2025

fmease changed the title ~~[STACKED] Reject relaxed bounds inside associated type bounds (ATB, TraitRef<AssocTy: …>)~~ [STACKED] Reject relaxed bounds inside associated type bounds (ATB) Jul 18, 2025

fmease force-pushed the ban-assoc-ty-unbounds branch from 5c245de to e7050cb Compare July 18, 2025 10:58

fmease removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. S-experimental Status: Ongoing experiment that does not require reviewing and won't be merged in its current state. labels Jul 18, 2025

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This was referenced Jul 31, 2025

Incoming breakage: Remove unnecessary ?Sized bounds pacman-repo-builder/arch-pkg-text#25

Closed

Incoming breakage: Remove unnecessary ?Sized bound PaulmannLighting/le-stream#1

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rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Aug 9, 2025

rfcbot added the to-announce Announce this issue on triage meeting label Aug 9, 2025

rustbot mentioned this pull request Aug 9, 2025

Tracking issue for release notes of #135331: Reject relaxed bounds inside associated type bounds (ATB) #145165

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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 11, 2025

Zalathar mentioned this pull request Aug 11, 2025

Rollup of 5 pull requests #145240

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bors merged commit 64aea00 into rust-lang:master Aug 11, 2025
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rustbot added this to the 1.91.0 milestone Aug 11, 2025

fmease deleted the ban-assoc-ty-unbounds branch August 11, 2025 12:32

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Mark-Simulacrum mentioned this pull request Sep 25, 2025

Draft release notes for 1.91 #147010

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fmease mentioned this pull request Oct 16, 2025

Further tighten up relaxed bounds #147734

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fmease mentioned this pull request Oct 22, 2025

regression: this relaxed bound is not permitted here (accepted breakage; downstream patched) #147963

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Reject relaxed bounds inside associated type bounds (ATB) #135331

Reject relaxed bounds inside associated type bounds (ATB) #135331

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