"function defined here" notes shouldn't show `mut`s

[scottmcm]

In #90886 I changed the definition of fold such that it no longer needed the mut on the f parameter, and was surprised to get a UI test failure (in https://github.com/rust-lang/rust/blob/master/src/test/ui/issues/issue-3044.rs) from it. The update from bless:

Playground link with the same as that UI test: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=548bf44a26aa1406bafb496f9341bf29

It doesn't seem to be showing the note in playground, though?!?

The current output is:

 note: associated function defined here
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
   |
LL |     fn fold<B, F>(mut self, init: B, mut f: F) -> B
   |        ^^^^

Ideally the output should look like:

 note: associated function defined here
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
   |
LL |     fn fold<B, F>(self, init: B, f: F) -> B
   |        ^^^^

because the muts are not relevant to the caller of the function.

[scottmcm]

Having hit submit on that, I'm now thinking I'm wrong. It's showing the line of code, so showing the line as anything other than what it actually is would be more confusing for local stuff.

So maybe this is actually more about "please show the signature not the line of code" for things that aren't "local" (in some sense)?

Or maybe you should just close this. Feel free to do that if my rambling here isn't actionable.