Provide more context when an expected type was a type parameter by estebank · Pull Request #160180 · rust-lang/rust

estebank · 2026-07-29T20:27:48Z

This becomes particularly confusing when a type parameter has a default.

error[E0053]: method `foo` has an incompatible type for trait
  --> $DIR/default-type-parameter.rs:8:17
   |
LL | impl Trait for S {
LL |     fn foo() -> u32 { 0 }
   |                 ^^^ expected `()`, found `u32`
   |
note: type in trait
  --> $DIR/default-type-parameter.rs:2:17
   |
LL | trait Trait<T = ()> {
   |             ------ expected to match the default of type parameter `T`
LL |     fn foo() -> T;
   |                 ^
   = note: expected signature `fn() -> ()`
              found signature `fn() -> u32`
help: change the output type to match the trait
   |
LL -     fn foo() -> u32 { 0 }
LL +     fn foo() -> () { 0 }
   |

Fix #67991.

rustbot · 2026-10-05T20:01:12Z

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tiif

Thanks for working on this! I have some non-blocking style nits that I think might be good to have. I am currently reading the code closely, it might take a few days, so I am releasing these comments for earlier feedback.

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tiif · 2026-10-19T23:23:00Z

  --> $DIR/defaults-specialization.rs:8:18
   |
+LL | trait Tr {
+...


non-blocking style nit: will it be possible to have...?

LL | trait Tr { | ...

That change is something to bring up to rust-lang/annotate-snippets-rs. I don't think that rendering is necessarily ideal, because it would be hard to differentiate between the actual code being

trait Tr { ...

and

trait Tr { // this like will be skipped by the diagnostic renderer and replaced with ` ...`

By having it on the other side of the gutter, it makes that a bit clearer.

I see, makes sense.

rustbot · 2026-10-19T23:59:14Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

tiif

Thanks for waiting. I just have a few more questions.

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tiif · 2026-10-25T10:22:20Z

+        && let param = generics.type_param(*param, tcx)
+        && let GenericParamDefKind::Type { has_default: true, .. } = param.kind
+    {
+        context.push(tcx.def_span(trait_m.container_id(tcx)).shrink_to_lo());


What does this context do? Removing this line doesn't seem to change any test.

It will ensure that line that starts the containing trait will be displayed. It would only happen with weirdly formatted code or really long signatures.

tiif · 2026-10-25T10:27:35Z

+            && p.name == kw::SelfUpper
+            && infcx.can_eq(param_env, *ty, impl_trait_ref.self_ty())
+        {
+            format!("expected to match the `Self` default of type parameter `{param_name}`")


We seem to not have test for this, could you help to add one?

tiif · 2026-10-25T10:39:57Z

+        trait_err_span.map(|sp| {
+            let mut span: MultiSpan = sp.into();
+            span.push_span_context(tcx.def_span(trait_m.container_id(tcx)).shrink_to_lo());
+            span.push_span_context(tcx.def_span(trait_m.def_id).shrink_to_lo());


This also doesn't seem to change anything when removed. What is the intention of adding this line?

tiif · 2026-10-25T10:54:16Z

  --> $DIR/defaults-specialization.rs:8:18
   |
+LL | trait Tr {
+...


I see, makes sense.

This becomes particularly confusing when a type parameter has a default. ``` --> $DIR/default-type-parameter.rs:8:17 | LL | fn foo() -> u32 { 0 } | ^^^ expected `()`, found `u32` | note: type in trait --> $DIR/default-type-parameter.rs:2:17 | LL | fn foo() -> T; | ^ = note: expected signature `fn() -> ()` found signature `fn() -> u32` note: expected to match this type parameter --> $DIR/default-type-parameter.rs:1:13 | LL | trait Trait<T = ()> { | ^^^^^^ help: change the output type to match the trait | LL - fn foo() -> u32 { 0 } LL + fn foo() -> () { 0 } ```

…the trait ``` error[E0053]: method `foo` has an incompatible type for trait --> $DIR/async-example-desugared-boxed-in-trait.rs:11:28 | LL | impl MyTrait for i32 { LL | async fn foo(&self) -> i32 { | ^^^ expected `Pin<Box<dyn Future<Output = i32>>>`, found future | note: type in trait --> $DIR/async-example-desugared-boxed-in-trait.rs:7:22 | LL | trait MyTrait { LL | fn foo(&self) -> Pin<Box<dyn Future<Output = i32> + '_>>; | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ = note: expected signature `fn(&_) -> Pin<Box<dyn Future<Output = i32>>>` found signature `fn(&_) -> impl Future<Output = i32>` ```

…type in trait" note ``` error[E0053]: method `foo` has an incompatible type for trait --> $DIR/default-type-parameter.rs:8:17 | LL | impl Trait for S { LL | fn foo() -> u32 { 0 } | ^^^ expected `()`, found `u32` | note: type in trait --> $DIR/default-type-parameter.rs:2:17 | LL | trait Trait<T = ()> { | ------ expected to match the default of type parameter `T` LL | fn foo() -> T; | ^ = note: expected signature `fn() -> ()` found signature `fn() -> u32` help: change the output type to match the trait | LL - fn foo() -> u32 { 0 } LL + fn foo() -> () { 0 } | ```

rustbot · 2026-10-25T16:54:35Z

This PR was rebased onto a different main 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 · 2026-10-25T16:56:48Z

@tiff modified a few tests to exercise the new logic. The context spans are not always needed under normally formatted code, but for more convoluted/long code the thing being pointed at is not on the same line as the item itself, which makes it harder to understand what part of the code the compiler is talking about at a glance. Ensuring the item is in view, helps to disambiguate, specially when there are many types or traits that have associated items with the same name.

estebank · 2026-10-25T16:59:20Z

+LL | trait Trait<
+LL |     T = (),
+   |     ------ expected to match the default of type parameter `T`
+...
+LL |     fn foo()
+LL |     -> T;
+   |        ^


This is an example of the context spans actually doing something.

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estebank · 2026-10-25T16:59:47Z

+LL |     K = Self,
+   |     -------- expected to match the `Self` default of type parameter `K`


I'm surprised I even wrote logic for this case without a test. Good catch. IIRC, it is a common case for the math traits like Add<Rhs = Self>.

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tiif

The context span stuff is interesting to know, thank you! I pushed a commit to fix a typo to avoid another round of review cycle. It's time to land this.

@bors r+ rollup

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rust-bors · 2026-11-01T04:03:16Z

📌 Commit 9ec7923 has been tentatively approved by tiif

It will be put into the queue for this repository once PR CI succeeds.

…uwer Rollup of 6 pull requests Successful merges: - #138143 (rustdoc: add unstable option `--crate-list-heading` to customize the sidebar crate list.) - #165251 (Restore aarch64-apple-1 jobs with free disk space) - #160180 (Provide more context when an expected type was a type parameter) - #165262 (Replace `AbiAlign` with `Align` in function returns) - #165265 (Bless mir-opt tests again, again) - #165266 (Revert recent changes to async_drop)

Provide more context when an expected type was a type parameter This becomes particularly confusing when a type parameter has a default. ``` error[E0053]: method `foo` has an incompatible type for trait --> $DIR/default-type-parameter.rs:8:17 | LL | impl Trait for S { LL | fn foo() -> u32 { 0 } | ^^^ expected `()`, found `u32` | note: type in trait --> $DIR/default-type-parameter.rs:2:17 | LL | trait Trait<T = ()> { | ------ expected to match the default of type parameter `T` LL | fn foo() -> T; | ^ = note: expected signature `fn() -> ()` found signature `fn() -> u32` help: change the output type to match the trait | LL - fn foo() -> u32 { 0 } LL + fn foo() -> () { 0 } | ``` Fix rust-lang#67991.

Rollup merge of #160180 - estebank:issue-67991, r=tiif Provide more context when an expected type was a type parameter This becomes particularly confusing when a type parameter has a default. ``` error[E0053]: method `foo` has an incompatible type for trait --> $DIR/default-type-parameter.rs:8:17 | LL | impl Trait for S { LL | fn foo() -> u32 { 0 } | ^^^ expected `()`, found `u32` | note: type in trait --> $DIR/default-type-parameter.rs:2:17 | LL | trait Trait<T = ()> { | ------ expected to match the default of type parameter `T` LL | fn foo() -> T; | ^ = note: expected signature `fn() -> ()` found signature `fn() -> u32` help: change the output type to match the trait | LL - fn foo() -> u32 { 0 } LL + fn foo() -> () { 0 } | ``` Fix #67991.

…uwer Rollup of 6 pull requests Successful merges: - rust-lang/rust#138143 (rustdoc: add unstable option `--crate-list-heading` to customize the sidebar crate list.) - rust-lang/rust#165251 (Restore aarch64-apple-1 jobs with free disk space) - rust-lang/rust#160180 (Provide more context when an expected type was a type parameter) - rust-lang/rust#165262 (Replace `AbiAlign` with `Align` in function returns) - rust-lang/rust#165265 (Bless mir-opt tests again, again) - rust-lang/rust#165266 (Revert recent changes to async_drop)

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 Jul 29, 2026

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estebank force-pushed the issue-67991 branch from 29a66c7 to 2358254 Compare October 5, 2026 20:00

estebank marked this pull request as ready for review October 5, 2026 20:01

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estebank force-pushed the issue-67991 branch from c542446 to fec5c13 Compare October 20, 2026 20:32

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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 Oct 20, 2026

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estebank added 7 commits October 25, 2026 16:29

Only point at param with default type

b1476a0

Tweak message

b1cf8bd

fix test

b6b1d7a

Fix rebase

b9b3dba

estebank force-pushed the issue-67991 branch from fec5c13 to e51323b Compare October 25, 2026 16:54

Excercise all new diagnostic changes in existing tests

2885a32

estebank force-pushed the issue-67991 branch from e51323b to 2885a32 Compare October 25, 2026 16:58

estebank commented Oct 25, 2026

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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 Oct 26, 2026

tiif reviewed Nov 1, 2026

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Comment thread tests/ui/compare-method/default-type-parameter.rs Resolved

Fix typo in test description

9ec7923

tiif approved these changes Nov 1, 2026

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rust-bors Bot 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 Nov 1, 2026

JonathanBrouwer mentioned this pull request Nov 1, 2026

Rollup of 6 pull requests #165270

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JonathanBrouwer mentioned this pull request Nov 1, 2026

Rollup of 12 pull requests #165281

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rust-bors Bot merged commit c1afed8 into rust-lang:main Nov 1, 2026
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rustbot added this to the 1.101.0 milestone Nov 1, 2026

		LL \| K = Self,
		\| -------- expected to match the `Self` default of type parameter `K`

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