Reject relaxed bounds inside associated type bounds (ATB)

[fmease]

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.

Footnotes

1. I consider this a flaw in the implementation and I've already added a huge FIXME. ↩

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). ↩

[fmease]

@bors try

[bors]

⌛ Trying commit 5c245de with merge de39175...

[bors]

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

[fmease]

@craterbot check

[craterbot]

👌 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]

🚧 Experiment pr-135331 is now running

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

[craterbot]

🎉 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]

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",
    ),
]

[rfcbot]

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

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[lcnr]

The crater run is apparently clean.

except for 2 hobby projects only on GitHub for which @fmease opened fixes back in January which have since been ignored.

[tmandry]

I'd feel more comfortable if we had a more recent crater run than 6 months. I think it is unlikely that that would surface anything to block this PR, maybe prompt some more PRs to existing projects. I would especially want to know if it hits anything on crates.io with deps. We can do that asynchronously with getting this landed, so

@rfcbot reviewed

[fmease]

@bors try

[rust-bors]

⌛ Trying commit 788fb08 with merge 29194e8…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 29194e8 (29194e8f603400afdb2f86c9418e9fccb1628ea0, parent: ba7e63b63871a429533c189adbfb1d9a6337e000)

[fmease]

@craterbot check

[craterbot]

👌 Experiment pr-135331-1 created and queued.
🤖 Automatically detected try build 29194e8
🔍 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]

🚧 Experiment pr-135331-1 is now running

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

[nikomatsakis]

@rfcbot reviewed

[rfcbot]

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

[craterbot]

🎉 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]

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: pacman-repo-builder/arch-pkg-text#25 and PaulmannLighting/le-stream#1.