Tweak output of missing lifetime on associated type #135602

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

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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 ```

…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 { | ++ ```

``` 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 ``` ``` error[E0106]: missing lifetime specifier --> $DIR/issue-74918-missing-lifetime.rs:9:30 | LL | type Item = IteratorChunk<T, S>; | ^ expected named lifetime parameter | help: consider introducing a named lifetime parameter | LL ~ impl<'a, T, S: Iterator<Item = T>> Iterator for ChunkingIterator<T, S> { LL ~ type Item = IteratorChunk<'a, T, S>; | ```

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 ```

Given an associated item that needs a named lifetime, look at the enclosing `impl` item for one. If there is none, look at the self type and the implemented trait to see if either of those has an anonimous lifetime. If so, suggest adding a named lifetime. ``` 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 | type Item = &T; | ^ this lifetime must come from the implemented type | 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; | ```

Move the previous long message to a note and use a shorter primary message: ``` error: missing lifetime in associated type --> $DIR/missing-lifetime-in-assoc-type-1.rs:9:17 | 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 | 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: consider using the lifetime from the impl block | LL | type Item = &'a T; | ++ ```

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

tests/ui/lifetimes/missing-lifetime-in-assoc-type-1.stderr

+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

tests/ui/lifetimes/missing-lifetime-in-assoc-type-4.stderr

+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

tests/ui/lifetimes/missing-lifetime-in-assoc-type-2.stderr

+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.

rustbot assigned Nadrieril Jan 16, 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 16, 2025

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estebank added 6 commits January 16, 2025 23:07

Add tests for some cases mentioned in rust-lang#135589

d1baed6

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

estebank commented Jan 17, 2025

View reviewed changes

Make "add lifetime" suggestion verbose

e3c1f72

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

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Tweak output of missing lifetime on associated type #135602

Tweak output of missing lifetime on associated type #135602

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estebank commented Jan 17, 2025

Tweak output of missing lifetime on associated type #135602

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Tweak output of missing lifetime on associated type #135602

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