Infer all anonymous lifetimes in assoc consts as `'static` by oli-obk · Pull Request #156508 · rust-lang/rust

oli-obk · 2026-05-12T17:57:51Z

This FCW has been on for almost a year, let's actually do the change that T-lang asked for now that there's no breakage to be expected anymore. I first did a crater run checking if turning it into a hard error breaks anything.

There are two cases that would be a hard error (but totally fine by defaulting to 'static), both based on an old version of minio. One is a personal crate on github with no changes in the last year (https://github.com/PapePathe/daaray_kaamil) and the other is https://github.com/gear-tech/gear, which is developed on github actively and does not have this issue anymore (also old minio dependency that was since updated). The crates.io version is 2 years outdated.

Thus we can pretty much conclude we're not going to cause anyone any confusion or subtle bugs by changing the rules. The rules for associated constants' types are now the same as the rules for free constants' types:

We now treat all anonymous lifetimes as 'static, irrespective of whether other lifetimes are in scope or not. This means the following piece of code starts compiling without warnings or errors again.

struct Foo<'a>(&'a ());

impl<'a> Foo<'a> {
    const STR: &str = "hello, world";
}

Unfortunately it also means that

trait Foo {
    const B: S = S { s: &() };
}

struct S<'a> {
    s: &'a (),
}

now has no warning or error (just like the same const as a free constant would have). It used to emit

error[E0726]: implicit elided lifetime not allowed here
  --> $DIR/missing-lifetime-in-assoc-const-type.rs:7:14
   |
LL |     const B: S = S { s: &() };
   |              ^ expected lifetime parameter
   |
help: indicate the anonymous lifetime
   |
LL |     const B: S<'_> = S { s: &() };
   |               ++++

It is very possible to keep this error around, but it seems unfortunate to have different behaviour between const items and assoc const items.

oli-obk · 2026-05-12T17:57:59Z

@bors try

Turn elided_lifetimes_in_associated_constant FCW into a hard error

rust-bors · 2026-05-12T20:07:16Z

☀️ Try build successful (CI)
Build commit: b28cc4e (b28cc4eb48a6415b914e8ad0308be255ff8d73ed, parent: 2aabf3ce0569dd7c8c2d2a27944850056dcf2566)

oli-obk · 2026-05-13T08:04:41Z

@craterbot check

craterbot · 2026-05-13T08:05:11Z

👌 Experiment pr-156508 created and queued.
🤖 Automatically detected try build b28cc4e
🔍 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 · 2026-05-21T09:59:47Z

🚧 Experiment pr-156508 is now running

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

craterbot · 2026-05-23T15:44:25Z

🎉 Experiment pr-156508 is completed!
📊 3 regressed and 10 fixed (931813 total)
📊 5201 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 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

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-156508/retry-regressed-list.txt ↩

oli-obk · 2026-05-26T09:03:05Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-156508/retry-regressed-list.txt

craterbot · 2026-05-26T09:03:10Z

👌 Experiment pr-156508-1 created and queued.
🤖 Automatically detected try build b28cc4e
⚠️ Try build based on commit eaec6ce, but latest commit is a92df8e. Did you forget to make a new try build?
🔍 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

@petrochenkov

Some cleanups around passing extra lifetime params from the resolver to ast lowering No functional changes, mostly removing information that was never used at any step. Possibly interesting to @petrochenkov and @cjgillot I originally thought this was a necessary cleanup to be able to refactor our `extra_lifetime_params_map` handling, but it turns out that I can also just do rust-lang#156508 which removes all the ugly usage of that side table and makes it possible for that side table to be put into `PerOwnerResolverData`. The changes here still were an improvement on its own, thus this PR.

@petrochenkov

Some cleanups around passing extra lifetime params from the resolver to ast lowering No functional changes, mostly removing information that was never used at any step. Possibly interesting to @petrochenkov and @cjgillot I originally thought this was a necessary cleanup to be able to refactor our `extra_lifetime_params_map` handling, but it turns out that I can also just do rust-lang#156508 which removes all the ugly usage of that side table and makes it possible for that side table to be put into `PerOwnerResolverData`. The changes here still were an improvement on its own, thus this PR.

@petrochenkov

Some cleanups around passing extra lifetime params from the resolver to ast lowering No functional changes, mostly removing information that was never used at any step. Possibly interesting to @petrochenkov and @cjgillot I originally thought this was a necessary cleanup to be able to refactor our `extra_lifetime_params_map` handling, but it turns out that I can also just do rust-lang#156508 which removes all the ugly usage of that side table and makes it possible for that side table to be put into `PerOwnerResolverData`. The changes here still were an improvement on its own, thus this PR.

@petrochenkov

Rollup merge of #156960 - oli-obk:push-rrmqnqvwtmsq, r=tiif Some cleanups around passing extra lifetime params from the resolver to ast lowering No functional changes, mostly removing information that was never used at any step. Possibly interesting to @petrochenkov and @cjgillot I originally thought this was a necessary cleanup to be able to refactor our `extra_lifetime_params_map` handling, but it turns out that I can also just do #156508 which removes all the ugly usage of that side table and makes it possible for that side table to be put into `PerOwnerResolverData`. The changes here still were an improvement on its own, thus this PR.

craterbot · 2026-06-07T08:18:10Z

🚧 Experiment pr-156508-1 is now running

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

craterbot · 2026-06-07T15:35:58Z

🎉 Experiment pr-156508-1 is completed!
📊 2 regressed and 1 fixed (5012 total)
📊 946 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 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

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-156508-1/retry-regressed-list.txt ↩

rustbot · 2026-06-07T15:38:09Z

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

rustbot · 2026-06-07T15:38:12Z

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oli-obk · 2026-06-07T15:41:21Z

@rfcbot fcp merge types

See the main comment of this PR for a description of the changes and rationale

rust-rfcbot · 2026-06-07T15:41:23Z

@oli-obk 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.

ChayimFriedman2 · 2026-06-07T16:13:37Z

        warn_since: None,
        deny_since: None,
    },
-    Lint {


This file is autogenerated, please revert the change.

View changes since the review

lcnr · 2026-06-08T09:35:32Z

How come

trait Foo {
    const B: S = S { s: &() };
}

struct S<'a> {
    s: &'a (),
}

does not lint because S has an implicitly elided lifetime 🤔 I always forget the adt lifetime elision rules. I think in fn sigs we always require S<'_>, do we have these rules written down somewhere?

oli-obk · 2026-06-08T09:37:50Z

that's unrelated to this change. The reason it doesn't lint is "we gave up at some point due to too much churn and/or the person driving it lost momentum", so the lint has very random rules where things apply

lcnr · 2026-06-08T09:50:31Z

Is it? We currently error here and you explicitly state

It is very possible to keep this error around, but it seems unfortunate to have different behaviour between const items and assoc const items.

I would prefer to keep these errors around. For associated consts their lifetimes can very much not be 'static, so having it be more explicit that there are any lifetimes at all feels valuable to me.

I don't have a strong opinion on what the behavior for free constants should be, but for assoc consts I do somewhat care about this

oli-obk · 2026-06-08T13:45:03Z

The churn was in the ecosystem for elided lifetimes. In this specific case we can indeed just turn it into a hard error, but then it's also a hard error if you use S<'_>.

These are entirely separate lints, but as long as we don't make it conditional on whether lifetimes are in scope, I'm fine with other solutions

I'll add a commit on top of this one so you can see the diff

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 12, 2026

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rust-bors Bot pushed a commit that referenced this pull request May 12, 2026

Auto merge of #156508 - oli-obk:boom, r=<try>

b28cc4e

Turn elided_lifetimes_in_associated_constant FCW into a hard error

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oli-obk force-pushed the boom branch from eaec6ce to a92df8e Compare May 13, 2026 10:34

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels May 23, 2026

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oli-obk force-pushed the boom branch from a92df8e to 6b78b13 Compare May 26, 2026 09:05

oli-obk changed the title ~~Turn elided_lifetimes_in_associated_constant FCW into a hard error~~ Infer all anonymous lifetimes in assoc consts as 'static May 26, 2026

oli-obk mentioned this pull request May 26, 2026

Some cleanups around passing extra lifetime params from the resolver to ast lowering #156960

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oli-obk added 2 commits May 30, 2026 10:09

Turn elided_lifetimes_in_associated_constant FCW into a hard error

b2dfdd2

Infer all anonymous lifetimes in (assoc) consts as 'static

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oli-obk force-pushed the boom branch from f672174 to 7ebcdfd Compare May 30, 2026 08:20

oli-obk mentioned this pull request Jun 3, 2026

Architectural groundwork for expansion-time evaluation rust-lang/rust-project-goals#620

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Jun 7, 2026

oli-obk marked this pull request as ready for review June 7, 2026 15:38

rustbot assigned JonathanBrouwer Jun 7, 2026

rust-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 7, 2026

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