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

r? @tiif

rustbot has assigned @tiif.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 77 candidates
Random selection from 19 candidates

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.

View changes since this review

tiif · 2026-10-18T23:36:28Z

Could you add the issue number here and comment on what this test is about?

Is the link enough?

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.

rustbot · 2026-10-19T23:59:14Z

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

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 · 2026-10-20T20:32:25Z

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.

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

This comment has been minimized.

Sign in to view

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

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 5, 2026

rustbot assigned tiif Oct 5, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 5, 2026

This comment has been minimized.

Sign in to view

tiif requested changes Oct 19, 2026

View reviewed changes

rustbot 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 Oct 19, 2026

estebank added 7 commits October 20, 2026 20:28

Only point at param with default type

b322796

Tweak message

4836331

fix test

44e8dec

Fix rebase

fec5c13

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

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

Navigation Menu

Repository navigation

Provide more context when an expected type was a type parameter - #160180

Provide more context when an expected type was a type parameter#160180
estebank wants to merge 7 commits into
rust-lang:mainfrom
estebank:issue-67991

estebank commented Jul 29, 2026 •

edited

Loading

Uh oh!

This comment has been minimized.

rustbot commented Oct 5, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

tiif left a comment •

edited by rustbot

Loading

Uh oh!

tiif Oct 18, 2026

Uh oh!

estebank Oct 20, 2026

Uh oh!

tiif Oct 19, 2026

Uh oh!

estebank Oct 20, 2026

Uh oh!

Uh oh!

rustbot commented Oct 19, 2026

Uh oh!

rustbot commented Oct 20, 2026

Uh oh!

This branch has not been deployed

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

Conversation

estebank commented Jul 29, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment has been minimized.

rustbot commented Oct 5, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

tiif left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

tiif Oct 18, 2026

Choose a reason for hiding this comment

Uh oh!

estebank Oct 20, 2026

Choose a reason for hiding this comment

Uh oh!

tiif Oct 19, 2026

Choose a reason for hiding this comment

Uh oh!

estebank Oct 20, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

rustbot commented Oct 19, 2026

Uh oh!

rustbot commented Oct 20, 2026

Uh oh!

This branch has not been deployed

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

estebank commented Jul 29, 2026 •

edited

Loading

tiif left a comment •

edited by rustbot

Loading