do not suggest changing function signature in trait impls when it matches the one from `Trait` itself

Code

struct Keyword<'a>(&'a str);

struct InvalidKeyword;

impl<'a> std::str::FromStr for Keyword<'a> {
    type Err = InvalidKeyword;
    fn from_str<'b>(s: &'b str) -> Result<Self, Self::Err> {
        // ^ Error: lifetime 'b may not live long enough. Consider adding 'b : 'a here.
        if matches!(s, "let" | "fn" | "struct") {
            Ok(Keyword(s))
        } else {
            Err(InvalidKeyword)
        }
    }
}

Current output

error: lifetime may not live long enough
  --> src/lib.rs:10:13
   |
5  | impl<'a> std::str::FromStr for Keyword<'a> {
   |      -- lifetime `'a` defined here
6  |     type Err = InvalidKeyword;
7  |     fn from_str<'b>(s: &'b str) -> Result<Self, Self::Err> {
   |                 -- lifetime `'b` defined here
...
10 |             Ok(Keyword(s))
   |             ^^^^^^^^^^^^^^ associated function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
   |
   = help: consider adding the following bound: `'b: 'a`

error: could not compile `playground` (lib) due to 1 previous error

Desired output

The same output but without help: consider adding the following bound: `'b: 'a`.

Rationale and extra context

I think its more important to ensure that impl Trait matches signature defined in trait itself then it is to suggest fix to user's code.

This fix would be always wrong unless trait itself is defined in current crate in which case this suggestion should be applied.

Other cases

Rust Version

rustc 1.91.0-nightly

Anything else?

No response

Start by compiling the minimal Rust reproduction from the issue and compare the current diagnostic with the desired output. Trace the compiler diagnostic path that emits the lifetime-bound help for trait implementations, then verify that the suggestion is omitted when the signature comes from the implemented trait while preserving the stated behavior for locally defined traits.