Tweak output of missing lifetime on associated type by estebank · Pull Request #135602 · rust-lang/rust

estebank · 2025-01-16T22:24:25Z

Each commit can be reviewed independently. Address parts of #135589.

When an associated type is missing a lifetime, point at its enclosing impl, whether it has or doesn't have lifetimes defined. If it does have a lifetime, suggest using it.

error: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
  --> $DIR/missing-lifetime-in-assoc-type-1.rs:8:17
   |
LL | impl<'a> IntoIterator for &S {
   |     ---- there is a named lifetime specified on the impl block you could use
...
LL |     type Item = &T;
   |                 ^ this lifetime must come from the implemented type
   |
help: consider using the lifetime from the impl block
   |
LL |     type Item = &'a T;
   |                  ++

error: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
  --> $DIR/missing-lifetime-in-assoc-type-2.rs:5:17
   |
LL | impl IntoIterator for &S {
   |     - you could add a lifetime on the impl block, if the trait or the self type can have one
LL |     type Item = &T;
   |                 ^ this lifetime must come from the implemented type

On unconstrained lifetime on impl block, suggest using it if there's an implicit borrow in the self type

error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
  --> $DIR/missing-lifetime-in-assoc-type-1.rs:4:6
   |
LL | impl<'a> IntoIterator for &S {
   |      ^^ unconstrained lifetime parameter
   |
help: consider using the named lifetime here instead of an implict lifetime
   |
LL | impl<'a> IntoIterator for &'a S {
   |                            ++

Do not suggest introducing lifetime in impl assoc type

Previously we only showed the trait's assoc item if the trait was local, because we were looking for a small span only for the generics, which we don't have for foreign traits. We now use def_span for the item, so we at least provide some context, even if its span is too wide.

error[E0195]: lifetime parameters or bounds on type `IntoIter` do not match the trait declaration
   --> tests/ui/lifetimes/missing-lifetime-in-assoc-type-4.rs:7:18
    |
7   |     type IntoIter<'a> = std::collections::btree_map::Values<'a, i32, T>;
    |                  ^^^^ lifetimes do not match type in trait
    |
   ::: /home/gh-estebank/rust/library/core/src/iter/traits/collect.rs:292:5
    |
292 |     type IntoIter: Iterator<Item = Self::Item>;
    |     ------------------------------------------ lifetimes in impl do not match this type in trait

rustbot · 2025-01-16T22:24:33Z

r? @Nadrieril

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estebank

Attached screenshots as I find that 1) the colors help get a better sense of what the output looks in practice and 2) the lack of spans into the std types makes it harder to get what I was going for.

estebank · 2025-01-17T01:20:41Z

+error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/missing-lifetime-in-assoc-type-1.rs:4:6
+   |
+LL | impl<'a> IntoIterator for &S {
+   |      ^^ unconstrained lifetime parameter
+   |
+help: consider using the named lifetime here instead of an implict lifetime
+   |
+LL | impl<'a> IntoIterator for &'a S {
+   |                            ++


estebank · 2025-01-17T01:21:10Z

+error: missing lifetime in associated type
+  --> $DIR/missing-lifetime-in-assoc-type-4.rs:5:17
+   |
+LL |     type Item = &T;
+   |                 ^ this lifetime must come from the implemented type
+   |
+note: in the trait the associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+help: add a lifetime to the impl block and use it in the self type and associated type
+   |
+LL ~ impl<'a> IntoIterator for &'a S {
+LL ~     type Item = &'a T;
+   |


estebank · 2025-01-17T01:22:10Z

+error[E0261]: use of undeclared lifetime name `'a`
+  --> $DIR/missing-lifetime-in-assoc-type-2.rs:7:57
+   |
+LL | impl IntoIterator for &S {
+   |     - help: consider introducing lifetime `'a` here: `<'a>`
+...
+LL |     type IntoIter = std::collections::btree_map::Values<'a, i32, T>;
+   |                                                         ^^ undeclared lifetime


estebank · 2025-01-17T16:09:08Z

Feel free to review on a per-commit basis and stop early. I can split-off later commits onto their own PRs. They are all together because they end up affecting the same output and build on each other, but they don't need to land together.

bors · 2025-03-07T14:24:06Z

☔ The latest upstream changes (presumably #138151) made this pull request unmergeable. Please resolve the merge conflicts.

Nadrieril · 2025-03-11T22:47:12Z

+        let Some((rib, span)) = self.lifetime_ribs[..i]
+            .iter()
+            .rev()
+            .skip(1)


Why the skip? Could you add a comment explaining it?

can't recall and removing it made no change :)

Nadrieril · 2025-03-11T22:57:08Z

I appear to have massively procrastinated this. I've reviewed the first 4 commits and I have questions that may be addressed with comments.

Nadrieril · 2025-03-18T17:11:59Z

If you don't mind splitting off the other commits to a different PR.

@rustbot author

bors · 2025-04-11T08:06:27Z

☔ The latest upstream changes (presumably #139657) made this pull request unmergeable. Please resolve the merge conflicts.

estebank · 2025-08-12T17:34:02Z

@Nadrieril rebased and left only the first 4 commits. Created #145314 with the rest.

wesleywiser · 2025-09-18T14:42:09Z

Hi @Nadrieril, it looks like this is ready to review and the author has responded to your previous round of feedback when you have a chance. Thanks!

Nadrieril · 2025-10-06T12:50:12Z

@bors r+

apologies for the delay!

bors · 2025-10-06T12:50:16Z

📌 Commit 6fcc9f5 has been approved by Nadrieril

It is now in the queue for this repository.

rustbot · 2025-10-31T20:51:28Z

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.

rustbot · 2025-10-31T20:51:29Z

⚠️ Warning ⚠️

There are issue links (such as #123) in the commit messages of the following commits.
Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.
- 2e01acc

estebank · 2025-10-31T23:53:29Z

@bors r=Nadrieril

bors · 2025-10-31T23:53:32Z

📌 Commit 8d51667 has been approved by Nadrieril

It is now in the queue for this repository.

fhdjeem77-ui · 2025-11-01T19:56:21Z

+            .filter_map(|rib| match rib.kind {
+                LifetimeRibKind::Generics { span, kind: LifetimeBinderKind::ImplBlock, .. } => {
+                    Some((rib, span))
+                }


rustbot assigned Nadrieril Jan 16, 2025

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estebank force-pushed the issue-135589 branch from bcacdc8 to 7b38d31 Compare January 16, 2025 23:23

estebank commented Jan 17, 2025

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rustbot added the WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) label Jan 17, 2025

estebank force-pushed the issue-135589 branch from e3c1f72 to b4682f4 Compare March 7, 2025 19:25

Nadrieril reviewed Mar 11, 2025

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Comment thread compiler/rustc_resolve/src/late.rs

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Comment thread compiler/rustc_resolve/src/late.rs

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estebank force-pushed the issue-135589 branch from b4682f4 to 6fcc9f5 Compare August 12, 2025 17:30

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estebank mentioned this pull request Aug 12, 2025

Tweak output of missing lifetime on associated type #145314

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Zalathar mentioned this pull request Nov 1, 2025

Rollup of 19 pull requests #148351

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Rollup of 19 pull requests #148354

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Rollup of 11 pull requests #148355

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Rollup of 10 pull requests #148356

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bors merged commit 44d6d86 into rust-lang:master Nov 1, 2025

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