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regression test for Trait<A><B> in "consider further restricting this bound" suggestion #158454

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Merged
rust-bors merged 2 commits into from
Jun 28, 2026
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regression test for Trait<A><B> in "consider further restricting this bound" suggestion #158454

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9260f2e
fix: don't produce invalid `Trait<A><B>` in "consider further restric…
Albab-Hasan Jun 26, 2026
dde5f49
fix: add regression test for invalid `Trait<A><B>` bound suggestion
Albab-Hasan Jun 27, 2026
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27 changes: 27 additions & 0 deletions tests/ui/suggestions/restrict-bound-already-has-generic-args.rs
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// Regression test for https://github.com/rust-lang/rust/issues/142803.

@workingjubilee workingjubilee Jun 26, 2026

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This regression test seems insufficient to me, but even so it exhibits the desired behavior without any compiler changes applied. Specifically, this is currently the output of the lines in question when considering the latest stable:

             found associated type `<impl Pair<Left = A> as Pair>::Right`
help: consider further restricting this bound
   |
18 | fn frob<A, B>(pair: impl Pair<Left = A, Right = B>) -> impl Pair<Left = A, Right = B> {
   |                                       +++++++++++

For more information about this error, try `rustc --explain E0271`.

This was fixed somewhere between 1.95 and 1.96, as it is specifically 1.96 that has the desired output.

@Albab-Hasan Would you be so kind as to bisect rustc to determine the speciifc commit at which this behavior was fixed? This regression test may still be needed if the fix was incidental, with some updates to the error matching to make sure it verifies that the specific behavior we want is obtained without relying on direct verification of the stderr. Otherwise, it may just be duplicating another test.

@Albab-Hasan Albab-Hasan Jun 27, 2026

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mb the fix landed in #154655 ("Fix associated type bound suggestion span issue", fixes #145586) merged 2026-04-01, nightly-2026-04-02. that PR touches rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs. my fix for #142803 was incidental.

its test covers a different pattern (Visitor/Deserializer with lifetimes) so the test case here is not a duplicate.

since the fix is incidental ill drop the code change to diagnostics.rs (now unreachable) and update the test to use inline //~ annotations targeting the suggestion text directly so it explicitly guards the merged-args behavior without relying on full stderr.


trait Pair {
type Left;
type Right;

fn split(self) -> (Self::Left, Self::Right);
}

impl<A, B> Pair for (A, B) {
type Left = A;
type Right = B;

fn split(self) -> (Self::Left, Self::Right) {
self
}
}

fn frob<A, B>(pair: impl Pair<Left = A>) -> impl Pair<Left = A, Right = B> {
//~^ ERROR type mismatch
//~| HELP consider further restricting this bound
//~| SUGGESTION , Right = B
let (left, right) = pair.split();
(left, right)
}

fn main() {}
24 changes: 24 additions & 0 deletions tests/ui/suggestions/restrict-bound-already-has-generic-args.stderr
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error[E0271]: type mismatch resolving `<(A, <impl Pair<Left = A> as Pair>::Right) as Pair>::Right == B`
--> $DIR/restrict-bound-already-has-generic-args.rs:19:45
|
LL | fn frob<A, B>(pair: impl Pair<Left = A>) -> impl Pair<Left = A, Right = B> {
| - expected this type parameter ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type mismatch resolving `<(A, <impl Pair<Left = A> as Pair>::Right) as Pair>::Right == B`
...
LL | (left, right)
| ------------- return type was inferred to be `(A, <impl Pair<Left = A> as Pair>::Right)` here
|
note: expected this to be `B`
--> $DIR/restrict-bound-already-has-generic-args.rs:12:18
|
LL | type Right = B;
| ^
= note: expected type parameter `B`
found associated type `<impl Pair<Left = A> as Pair>::Right`
help: consider further restricting this bound
|
LL | fn frob<A, B>(pair: impl Pair<Left = A, Right = B>) -> impl Pair<Left = A, Right = B> {
| +++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0271`.
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