fix(collapsible_match): exclude binding modes from struct field pattern suggestions

[ada4a]

Fixes #13287

changelog: [collapsible_match]: exclude binding modes from struct field pattern suggestions

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[ada4a]

I could additionally mention that the user might need to push ref( mut) to somewhere inside the replaced binding, e.g., in a case like:

  fn struct_field_pat_with_binding_mode(err: Option<Error>) {
      if let Some(Error { ref token, .. }) = err {
          if let Some(t) = token {
              println!("token used as a ref");
          }
      }
  }

the required change will be

fn struct_field_pat_with_binding_mode(err: Option<Error>) {
    if let Some(Error { token: Some(ref t), .. }) = err {
        println!("token used as a ref");
    }
}

But firstly, this is actually not required in the example provided in the original issue -- when in this situation:

fn struct_field_pat_with_binding_mode(err: Option<Error>) {
    if let Some(Error { ref token, .. }) = err {
        if let Some(Token::Name) = token {
            //~^ collapsible_match
            println!("token used as a ref");
        }
    }
}

we inline token:

fn struct_field_pat_with_binding_mode(err: Option<Error>) {
    if let Some(Error { token: Some(Token::Name), .. }) = err {
        println!("token used as a ref");
    }
}

we not only don't need to put a ref anywhere, but AFAICT wherever we put we get a compile error (and that would also be the case if err was Option<&Error>).

I guess we could employ smarter heuristics to only add the "you'll need to push the ref( mut)" note if we see that the pattern to be replaced with doesn't contain any bindings.

But I ultimately decided not to bother, given that we already don't do any of that in an existing test case -- this case:

rust-clippy/tests/ui/collapsible_match2.rs

Lines 63 to 71 in 5ac9657

	match Some(&[1]) {
	Some(ref s) => match s {
	//~^ collapsible_match
	[n] => foo(n),
	_ => (),
	},
	_ => (),
	}
	}

gets a message that doesn't acknowledge the ref at all:

rust-clippy/tests/ui/collapsible_match2.stderr

Lines 99 to 106 in 5ac9657

	help: the outer pattern can be modified to include the inner pattern
	--> tests/ui/collapsible_match2.rs:64:14
	|
	LL | Some(ref s) => match s {
	| ^^^^^ replace this binding
	LL |
	LL | [n] => foo(n),
	| ^^^ with this pattern

[dswij]

Thank you!

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