Point at fn bound that introduced lifetime obligation #146011

estebank · 2025-08-29T19:52:08Z

The last note is new

error[E0597]: `c` does not live long enough
  --> $DIR/without-precise-captures-we-are-powerless.rs:19:20
   |
LL | fn simple<'a>(x: &'a i32) {
   |           -- lifetime `'a` defined here
...
LL |     let c = async move || { println!("{}", *x); };
   |         - binding `c` declared here
LL |     outlives::<'a>(c());
   |     ---------------^---
   |     |              |
   |     |              borrowed value does not live long enough
   |     argument requires that `c` is borrowed for `'a`
LL |     outlives::<'a>(call_once(c));
LL | }
   | - `c` dropped here while still borrowed
   |
note: requirement that `c` is borrowed for `'a` introduced here
  --> $DIR/without-precise-captures-we-are-powerless.rs:7:33
   |
LL | fn outlives<'a>(_: impl Sized + 'a) {}
   |                                 ^^

When encountering a ConstraintCategory::Predicate in a funtion call, point at the Span for that Predicate to explain where the lifetime obligation originates from.

CC #55307.

rustbot · 2025-08-29T19:52:13Z

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lcnr · 2025-08-31T09:56:10Z

tests/ui/async-await/async-closures/without-precise-captures-we-are-powerless.stderr

   | - `c` dropped here while still borrowed
   |
-note: requirement that `c` is borrowed for `'a` introduced here
+note: requirement for `'a` introduced here


what is a requirement for 'a, I feel like the requirement is for the APIT here, not 'a. Is it not possible to keep sth like "requirement that outlives " introduced here"?

Changed to "requirement that the value outlives 'a introduced here".

lcnr · 2025-09-17T15:35:20Z

tests/ui/borrowck/issue-17545.stderr

   |        argument requires that borrow lasts for `'a`
+   |
+note: requirement that the value outlives `'a` introduced here
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL


how come this doesn't actually show the predicate which caused this?

because the span is in std, which don't get displayed when the sources aren't found (like when path remap is being used, which is done for all tests).

lcnr · 2025-09-17T15:37:42Z

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

+                        err.span_note(
+                            pred,
+                            format!("requirement that the value outlives `{region_name}` introduced here"),
+                        );


if we report this for all preds while always using region_name it'd be reported twice here, would it?

fn outlives_indir<'a: 'b, 'b, T: 'a>(x: T) {} fn foo<'b>() { outlives_indir::<'_, 'b, _>(&mut 1u32); }

in a sense it would be

requirement that the value outlives {other_region} introduced here

requirement that {other_region} outlives {region_name} introduced here

ahhh. you break. please make this a if let Some(pred_span) = ... :<

actually, idk if that's much easier to read, pls either use an if let = iter.find(..) instead of a loop here or add a comment that we only print this note for the first occurrance

Changed it to be if let Some(span) = iter.filter_map(...).next().

lcnr

r=me after nit

View changes since this review

``` error[E0597]: `c` does not live long enough --> $DIR/without-precise-captures-we-are-powerless.rs:19:20 | LL | fn simple<'a>(x: &'a i32) { | -- lifetime `'a` defined here ... LL | let c = async move || { println!("{}", *x); }; | - binding `c` declared here LL | outlives::<'a>(c()); | ---------------^--- | | | | | borrowed value does not live long enough | argument requires that `c` is borrowed for `'a` LL | outlives::<'a>(call_once(c)); LL | } | - `c` dropped here while still borrowed | note: requirement that `c` is borrowed for `'a` introduced here --> $DIR/without-precise-captures-we-are-powerless.rs:7:33 | LL | fn outlives<'a>(_: impl Sized + 'a) {} | ^^ ``` When encountering a `ConstraintCategory::Predicate` in a funtion call, point at the `Span` for that `Predicate` to explain where the lifetime obligation originates from.

``` error[E0716]: temporary value dropped while borrowed --> $DIR/multiple-sources-for-outlives-requirement.rs:5:38 | LL | fn foo<'b>() { | -- lifetime `'b` defined here LL | outlives_indir::<'_, 'b, _>(&mut 1u32); | ---------------------------------^^^^-- temporary value is freed at the end of this statement | | | | | creates a temporary value which is freed while still in use | argument requires that borrow lasts for `'b` | note: requirements that the value outlives `'b` introduced here --> $DIR/multiple-sources-for-outlives-requirement.rs:1:23 | LL | fn outlives_indir<'a: 'b, 'b, T: 'a>(_x: T) {} | ^^ ^^ ```

rustbot · 2025-09-28T21:14:06Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

estebank · 2025-09-28T21:15:36Z

tests/ui/regions/multiple-sources-for-outlives-requirement.stderr

+note: requirements that the value outlives `'b` introduced here
+  --> $DIR/multiple-sources-for-outlives-requirement.rs:1:23
+   |
+LL | fn outlives_indir<'a: 'b, 'b, T: 'a>(_x: T) {}
+   |                       ^^         ^^


The last commit is independent and changes it to point at every requirement, as previously this error was pointing only at the second span, which even though is correct, it isn't very understandable to someone who doesn't have a good grasp of Rust yet. What do you think @lcnr? I can drop the last commit and make it point just at the 'a here.

hmm, I agree that this seems better than just pointing at one of them🤔

lcnr · 2025-09-30T09:10:45Z

tests/ui/regions/multiple-sources-for-outlives-requirement.stderr

+note: requirements that the value outlives `'b` introduced here
+  --> $DIR/multiple-sources-for-outlives-requirement.rs:1:23
+   |
+LL | fn outlives_indir<'a: 'b, 'b, T: 'a>(_x: T) {}
+   |                       ^^         ^^


hmm, I agree that this seems better than just pointing at one of them🤔

lcnr · 2025-09-30T09:12:59Z

@bors r+ rollup

bors · 2025-09-30T09:13:02Z

📌 Commit c5313fe has been approved by lcnr

It is now in the queue for this repository.

… r=lcnr Point at fn bound that introduced lifetime obligation The last note is new ``` error[E0597]: `c` does not live long enough --> $DIR/without-precise-captures-we-are-powerless.rs:19:20 | LL | fn simple<'a>(x: &'a i32) { | -- lifetime `'a` defined here ... LL | let c = async move || { println!("{}", *x); }; | - binding `c` declared here LL | outlives::<'a>(c()); | ---------------^--- | | | | | borrowed value does not live long enough | argument requires that `c` is borrowed for `'a` LL | outlives::<'a>(call_once(c)); LL | } | - `c` dropped here while still borrowed | note: requirement that `c` is borrowed for `'a` introduced here --> $DIR/without-precise-captures-we-are-powerless.rs:7:33 | LL | fn outlives<'a>(_: impl Sized + 'a) {} | ^^ ``` When encountering a `ConstraintCategory::Predicate` in a funtion call, point at the `Span` for that `Predicate` to explain where the lifetime obligation originates from. CC rust-lang#55307.

Rollup of 5 pull requests Successful merges: - #140916 (Fix unuseful span in type error in some format_args!() invocations) - #146011 (Point at fn bound that introduced lifetime obligation) - #146649 (cmse: fix 'region variables should not be hashed') - #147109 (Rename various "concrete opaque type" things to say "hidden type") - #147167 (Don't condition RUSTDOC_LIBDIR on `--no-doc`) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #146011 - estebank:lifetime-obligation-span, r=lcnr Point at fn bound that introduced lifetime obligation The last note is new ``` error[E0597]: `c` does not live long enough --> $DIR/without-precise-captures-we-are-powerless.rs:19:20 | LL | fn simple<'a>(x: &'a i32) { | -- lifetime `'a` defined here ... LL | let c = async move || { println!("{}", *x); }; | - binding `c` declared here LL | outlives::<'a>(c()); | ---------------^--- | | | | | borrowed value does not live long enough | argument requires that `c` is borrowed for `'a` LL | outlives::<'a>(call_once(c)); LL | } | - `c` dropped here while still borrowed | note: requirement that `c` is borrowed for `'a` introduced here --> $DIR/without-precise-captures-we-are-powerless.rs:7:33 | LL | fn outlives<'a>(_: impl Sized + 'a) {} | ^^ ``` When encountering a `ConstraintCategory::Predicate` in a funtion call, point at the `Span` for that `Predicate` to explain where the lifetime obligation originates from. CC #55307.

rustbot assigned lcnr Aug 29, 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 Aug 29, 2025

estebank mentioned this pull request Aug 29, 2025

Misleading/unhelpful error message from borrow checker #55307

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lcnr reviewed Aug 31, 2025

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lcnr added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 17, 2025

estebank added 5 commits September 28, 2025 20:55

Point at lifetime requirement origin in more cases

7a0319f

reword note

4973903

Address review comment

58f5260

estebank force-pushed the lifetime-obligation-span branch from 79982d7 to c5313fe Compare September 28, 2025 21:14

estebank commented Sep 28, 2025

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estebank added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 28, 2025

lcnr approved these changes Sep 30, 2025

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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 Sep 30, 2025

This was referenced Sep 30, 2025

Rollup of 3 pull requests #147183

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Rollup of 5 pull requests #147186

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bors merged commit 1aa426b into rust-lang:master Sep 30, 2025
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rustbot added this to the 1.92.0 milestone Sep 30, 2025

Point at fn bound that introduced lifetime obligation #146011

Point at fn bound that introduced lifetime obligation #146011

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