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Misleading Suggestion when Returning a Reference to a Bare Trait from a Function  #127689

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@veera-sivarajan

Description

@veera-sivarajan

Code

// edition: 2021
trait Trait {}

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

fn main() {}

Current output

error[E0106]: missing lifetime specifier
 --> <source>:5:13
  |
5 | fn fun() -> &Trait {
  |             ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
  |
5 | fn fun() -> &'static Trait {
  |              +++++++

error[E0782]: trait objects must include the `dyn` keyword
 --> <source>:5:14
  |
5 | fn fun() -> &Trait {
  |              ^^^^^
  |
help: add `dyn` keyword before this trait
  |
5 | fn fun() -> &dyn Trait {
  |              +++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0106, E0782.
For more information about an error, try `rustc --explain E0106`.

Desired output

error[E0782]: trait objects must include the `dyn` keyword
 --> <source>:5:14
  |
5 | fn fun() -> &Trait {
  |              ^^^^^
  |
help: add `dyn` keyword before this trait
  |
5 | fn fun() -> &dyn Trait {
  |              +++

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0106, E0782.
For more information about an error, try `rustc --explain E0106`.

Rationale and extra context

The error E0106 suggests to add a static lifetime but this is incorrect. If and when #127692 is merged, the error E0782 can provide accurate and helpful messages and the former should be suppressed in favor of E0782.

But this can be tricky as E0106 is emitted at rustc_resolve which doesn't have any context about type of values.

Rust Version

rustc 1.79.0

Anything else?

No response

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    A-diagnosticsArea: Messages for errors, warnings, and lintsA-dyn-traitArea: trait objects, vtable layoutD-incorrectDiagnostics: A diagnostic that is giving misleading or incorrect information.D-verboseDiagnostics: Too much output caused by a single piece of incorrect code.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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