E0412 is correct but E0782 is emitted (in trait definitions)

[Colonial-Dev]

Code

trait Foo {
    type Clone;
    fn foo() -> Clone
}

Current output

error[E0782]: trait objects must include the `dyn` keyword
 --> src/main.rs:4:17
  |
4 |     fn foo() -> Clone;
  |                 ^^^^^
  |
help: add `dyn` keyword before this trait
  |
4 |     fn foo() -> dyn Clone;
  |                 +++

For more information about this error, try `rustc --explain E0782`.

Desired output

Obviously this isn't helpful, as Clone isn't object safe. The correct error is E0412:

error[E0412]: cannot find type `Clone` in this scope
 --> src/main.rs:4:17
  |
4 |     fn foo() -> Clone;
  |                 ^^^
  |
help: you might have meant to use the associated type
  |
4 |     fn foo() -> Self::Clone;
  |                 ++++++

For more information about this error, try `rustc --explain E0412`.

Rationale and extra context

E0412 is correctly emitted if the associated type name is changed to not collide with any traits currently in scope. So:

trait Foo {
    type Bar;
    fn foo() -> Bar
}

Errors as expected.

I've also seen this confuse new users "in the wild" on Rustcord - that's what originally inspired me to file this issue.

Other cases

No response

Anything else?

No response

[trevyn]

@rustbot claim

[trevyn]

It looks like #119752 (in the queue) changes this to E0038 without a suggestion (for edition 2021):

error[E0038]: the trait `Clone` cannot be made into an object
  --> /Users/e/Documents/GitHub/rust/tests/ui/suggestions/issue-116434.rs:5:17
   |
LL |     fn foo() -> Clone;
   |                 ^^^^^ `Clone` cannot be made into an object
   |
   = note: the trait cannot be made into an object because it requires `Self: Sized`
   = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>

A suggestion to -> impl Clone is also valid (and with the above pull appears for edition < 2021), but perhaps your -> Self::Clone makes more sense in context, if not exactly... great.

Do you have any links or can share how this is tripping up other users?

[trevyn]

Ah, apparently it's possible to produce multiple suggestions. I'll poke at it and see what comes out.

[Colonial-Dev]

Do you have any links or can share how this is tripping up other users?

It's been a while, but what I saw on the Discord went something like this:

trait DbHandle {
    /* irrelevant, but not object safe */ 
} 

// Some lines away... 

trait DbInterface {
    // (presumably assuming the namespaces don't overlap) 
    type DbHandle;

    fn handle() -> DbHandle;
} 

The user then gets the "please add dyn" message, which they follow.

The compiler then tells them "sorry you can't dyn that," and they wind up in the Discord very confused about the mixed signals.

[trevyn]

The problematic dyn suggestion is also gone from nightly: playground

The type (trait) DbHandle is in scope, so I don't think E0412 (cannot find type in this scope) is correct.

If it provides the correct suggestion though, it would definitely be an improvement to include that suggestion in the output. I don't see a clear way to get there from the E0038 code path though. :(

[trevyn]

Making some progress on this, currently blocked on #120164