Fix pretty printing an AST representing &(mut ident)

[tomprogrammer]

The PR fixes a misguiding help diagnostic in the parser that I reported in #80186. I discovered that the parsers recovery and reporting logic was correct but the pretty printer produced wrong code for the example. (Details in #80186 (comment))

Example:

#![allow(unused_variables)]
fn main() {
    let mut &x = &0;
}

The AST fragment

PatKind::Ref(PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..), Mutability::Not)

was printed to be &mut ident. But this wouldn't round trip through parsing again, because then it would be:

PatKind::Ref(PatKind::Ident(BindingMode::ByValue(Mutability::Not), ..), Mutability::Mut)

Now the pretty-printer prints &(mut ident). Reparsing that code results in the AST fragment

PatKind::Ref(PatKind::Paren(PatKind::Ident(BindingMode::ByValue(Mutability::Mut), ..)), Mutability::Not)

which I think should behave like the original pattern.

Old diagnostic:

error: `mut` must be attached to each individual binding
 --> src/main.rs:3:9
  |
3 |     let mut &x = &0;
  |         ^^^^^^ help: add `mut` to each binding: `&mut x`
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`

New diagnostic:

error: `mut` must be attached to each individual binding
 --> src/main.rs:3:9
  |
3 |     let mut &x = &0;
  |         ^^^^^^ help: add `mut` to each binding: `&(mut x)`
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`

Fixes #80186

[rust-highfive]

r? @davidtwco

(rust-highfive has picked a reviewer for you, use r? to override)

[tomprogrammer]

I didn't find a place to test the help diagnostics of the parser. Are these actually tested?
The change in the pretty-printer can be tested without the parser. I don't know where to add this test, so I appreciate hints where to put the test.

[camelid]

I didn't find a place to test the help diagnostics of the parser. Are these actually tested?
The change in the pretty-printer can be tested without the parser. I don't know where to add this test, so I appreciate hints where to put the test.

Yes, diagnostics are thoroughly tested. Just add code (the code from the issue you opened) to a file in src/test/ui/pattern (or somewhere in src/test/ui). Then run ./x.py test src/test/ui/pattern/<your-file>.rs --bless.

Note that you will need to add //~ ERROR and potentially //~ HELP annotations; read this for help with that.

[tomprogrammer]

I added a minimal test case that should cover the entire issue. All other code in the issue can be reduced to this test case.

[davidtwco]

@bors r+

[bors]

📌 Commit b05ab18 has been approved by davidtwco

[bors]

⌛ Testing commit b05ab18 with merge 1e88a17...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing 1e88a17 to master...