Incorrect Suggestion when Returning a Bare Trait from a Function

[veera-sivarajan]

Code

// edition 2021
trait Trait {}

fn fun() -> Trait {
    todo!()
}

fn main() {}

Current output

error[E0746]: return type cannot have an unboxed trait object
 --> <source>:5:13
  |
5 | fn fun() -> Trait {
  |             ^^^^^ doesn't have a size known at compile-time
  |
help: box the return type, and wrap all of the returned values in `Box::new`
  |
5 ~ fn fun() -> Box<Trait> {
6 ~     Box::new(todo!())
  |

error[E0782]: trait objects must include the `dyn` keyword
 --> <source>:5:13
  |
5 | fn fun() -> Trait {
  |             ^^^^^
  |
help: use `impl Trait` to return an opaque type, as long as you return a single underlying type
  |
5 | fn fun() -> impl Trait {
  |             ++++
help: alternatively, you can return an owned trait object
  |
5 | fn fun() -> Box<dyn Trait> {
  |             +++++++      +

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0746, E0782.
For more information about an error, try `rustc --explain E0746`.

Desired output

error[E0782]: trait objects must include the `dyn` keyword
 --> <source>:5:13
  |
5 | fn fun() -> Trait {
  |             ^^^^^
  |
help: use `impl Trait` to return an opaque type, as long as you return a single underlying type
  |
5 | fn fun() -> impl Trait {
  |             ++++
help: alternatively, you can return an owned trait object
  |
5 | fn fun() -> Box<dyn Trait> {
  |             +++++++      +

Rationale and extra context

This error report has two issues:

1. E0746 incorrectly suggests to return a Box<Trait>.
2. Both the errors say the same thing and E0746 should be suppressed in favor of E0782.

Other cases

No response

Rust Version

rustc 1.79.0

Anything else?

No response

[veera-sivarajan]

@rustbot label +D-incorrect +D-verbose +A-trait-objects

[progval]

Additionally, the E0746 suggestion is doubly incorrect when the trait is returned from an async function: the suggestions are not syntactically valid:

trait Trait {}

async fn fun() -> Trait {
    todo!()
}

fn main() {}

causes:

error[E0746]: return type cannot have an unboxed trait object
 --> src/main.rs:3:25
  |
3 |   async fn fun() -> Trait {
  |  _________________________^
4 | |     todo!()
5 | | }
  | |_^ doesn't have a size known at compile-time
  |
help: consider returning an `impl Trait` instead of a `dyn Trait`
  |
3 | async fn fun() -> Trait impl {
  |                         ++++
help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
  |
3 ~ async fn fun() -> Trait Box<dyn {
4 |     todo!()
5 ~ }>
  |

error[E0782]: trait objects must include the `dyn` keyword
 --> src/main.rs:3:19
  |
3 | async fn fun() -> Trait {
  |                   ^^^^^
  |
help: add `dyn` keyword before this trait
  |
3 | async fn fun() -> dyn Trait {
  |                   +++
help: you might have meant to write a bound here
  |
1 | : Trait {
  | ~

Some errors have detailed explanations: E0746, E0782.
For more information about an error, try `rustc --explain E0746`.
error: could not compile `playground` (bin "playground") due to 2 previous errors

[GrigorenkoPV]

As of the latest master (ecf2d1f), the error message is now:

error[E0782]: expected a type, found a trait
 --> src/main.rs:4:13
  |
4 | fn fun() -> Trait {
  |             ^^^^^
  |
help: use `impl Trait` to return an opaque type, as long as you return a single underlying type
  |
4 | fn fun() -> impl Trait {
  |             ++++
help: alternatively, you can return an owned trait object
  |
4 | fn fun() -> Box<dyn Trait> {
  |             +++++++      +

which looks ideal, IMO. So this issue can probably be closed now?

The async case is still a bit wrong, but less broken than before:

error[E0782]: expected a type, found a trait
 --> src/main.rs:3:19
  |
3 | async fn fun() -> Trait {
  |                   ^^^^^
  |
help: you can add the `dyn` keyword if you want a trait object
  |
3 | async fn fun() -> dyn Trait {
  |                   +++
help: you might have meant to write a bound here
  |
1 | : Trait {
  | ~

but maybe this should be moved into a separate issue?

[veera-sivarajan]

Yup, thank you!