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Compiler's help suggestion is incorrect when passing Option<String> into a function expecting Option<&str> #107604

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@scizzorz

Description

@scizzorz

Code

fn foo(_s: Option<&str>) {
}

fn main() {
    let s = Some("".to_string());
    foo(s);
}

// AND:

fn foo(_s: Option<&str>) {
}

fn main() {
    let s = Some("".to_string());
    foo(s.map(|x| &*x));
}

Current output

error[E0308]: mismatched types
 --> foo.rs:6:9
  |
6 |     foo(s);
  |     --- ^ expected `&str`, found struct `String`
  |     |
  |     arguments to this function are incorrect
  |
  = note: expected enum `Option<&str>`
             found enum `Option<String>`
note: function defined here
 --> foo.rs:1:4
  |
1 | fn foo(_s: Option<&str>) {
  |    ^^^ ----------------
help: try converting the passed type into a `&str`
  |
6 |     foo(s.map(|x| &*x));
  |          +++++++++++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

// AND:

error[E0515]: cannot return value referencing function parameter `x`
 --> foo.rs:6:19
  |
6 |     foo(s.map(|x| &*x));
  |                   ^^-
  |                   | |
  |                   | `x` is borrowed here
  |                   returns a value referencing data owned by the current function

error: aborting due to previous error

For more information about this error, try `rustc --explain E0515`.

Desired output

I believe the help suggestion should be valid code. I was able to solve this error with:

fn foo(s: Option<&str>) {
}

fn main() {
    let s = Some("".to_string());
    foo(s.as_ref().map(AsRef::as_ref));
}

Rationale and extra context

rustc's help suggestions are often extremely helpful, but in this case the suggested change simply does not work and also doesn't provide any further help or explanation about why it doesn't work.

Other cases

No response

Anything else?

No response

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    A-diagnosticsArea: Messages for errors, warnings, and lintsT-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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