Rollup merge of rust-lang#69236 - Centril:mut-parens-at-recovery, r=e…

…stebank

parse: recover `mut (x @ y)` as `(mut x @ mut y)`.

Follow up to rust-lang#68992 (comment) and rust-lang#63945.

Specifically, when given `let mut (x @ y)` we recover with `let (mut x @ mut y)` as the suggestion:

```rust
error: `mut` must be attached to each individual binding
  --> $DIR/mut-patterns.rs:12:9
   |
LL |     let mut (x @ y) = 0;
   |         ^^^^^^^^^^^ help: add `mut` to each binding: `(mut x @ mut y)`
   |
   = note: `mut` may be followed by `variable` and `variable @ pattern`
```

r? @matthewjasper @estebank

src/librustc_parse/parser/pat.rs

@@ -503,17 +503,18 @@ impl<'a> Parser<'a> {
  // Parse the pattern we hope to be an identifier.
  let mut pat = self.parse_pat(Some("identifier"))?;
 
- // Add `mut` to any binding in the parsed pattern.
- let changed_any_binding = Self::make_all_value_bindings_mutable(&mut pat);
-
- // Unwrap; If we don't have `mut $ident`, error.
- let pat = pat.into_inner();
- match &pat.kind {
- PatKind::Ident(..) => {}
- _ => self.ban_mut_general_pat(mut_span, &pat, changed_any_binding),
+ // If we don't have `mut $ident (@ pat)?`, error.
+ if let PatKind::Ident(BindingMode::ByValue(m @ Mutability::Not), ..) = &mut pat.kind {
+ // Don't recurse into the subpattern.
+ // `mut` on the outer binding doesn't affect the inner bindings.
+ *m = Mutability::Mut;
+ } else {
+ // Add `mut` to any binding in the parsed pattern.
+ let changed_any_binding = Self::make_all_value_bindings_mutable(&mut pat);
+ self.ban_mut_general_pat(mut_span, &pat, changed_any_binding);
  }
 
- Ok(pat.kind)
+ Ok(pat.into_inner().kind)
  }
 
  /// Recover on `mut ref? ident @ pat` and suggest
@@ -542,14 +543,10 @@ impl<'a> Parser<'a> {
  }
 
  fn visit_pat(&mut self, pat: &mut P<Pat>) {
- if let PatKind::Ident(ref mut bm, ..) = pat.kind {
- if let BindingMode::ByValue(ref mut m @ Mutability::Not) = bm {
- *m = Mutability::Mut;
- }
+ if let PatKind::Ident(BindingMode::ByValue(m @ Mutability::Not), ..) = &mut pat.kind
+ {
  self.0 = true;
- // Don't recurse into the subpattern, mut on the outer
- // binding doesn't affect the inner bindings.
- return;
+ *m = Mutability::Mut;
  }
  noop_visit_pat(pat, self);
  }

src/test/ui/parser/mut-patterns.rs

@@ -9,6 +9,8 @@ pub fn main() {
  let mut _ = 0; //~ ERROR `mut` must be followed by a named binding
  let mut (_, _) = (0, 0); //~ ERROR `mut` must be followed by a named binding
 
+ let mut (x @ y) = 0; //~ ERROR `mut` must be attached to each individual binding
+
  let mut mut x = 0;
  //~^ ERROR `mut` on a binding may not be repeated
  //~| remove the additional `mut`s

src/test/ui/parser/mut-patterns.stderr

@@ -14,36 +14,44 @@ LL | let mut (_, _) = (0, 0);
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`
 
+error: `mut` must be attached to each individual binding
+ --> $DIR/mut-patterns.rs:12:9
+ |
+LL | let mut (x @ y) = 0;
+ | ^^^^^^^^^^^ help: add `mut` to each binding: `(mut x @ mut y)`
+ |
+ = note: `mut` may be followed by `variable` and `variable @ pattern`
+
 error: `mut` on a binding may not be repeated
- --> $DIR/mut-patterns.rs:12:13
+ --> $DIR/mut-patterns.rs:14:13
  |
 LL | let mut mut x = 0;
  | ^^^ help: remove the additional `mut`s
 
 error: `mut` must be attached to each individual binding
- --> $DIR/mut-patterns.rs:17:9
+ --> $DIR/mut-patterns.rs:19:9
  |
 LL | let mut Foo { x: x } = Foo { x: 3 };
  | ^^^^^^^^^^^^^^^^ help: add `mut` to each binding: `Foo { x: mut x }`
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`
 
 error: `mut` must be attached to each individual binding
- --> $DIR/mut-patterns.rs:21:9
+ --> $DIR/mut-patterns.rs:23:9
  |
 LL | let mut Foo { x } = Foo { x: 3 };
  | ^^^^^^^^^^^^^ help: add `mut` to each binding: `Foo { mut x }`
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`
 
 error: `mut` on a binding may not be repeated
- --> $DIR/mut-patterns.rs:26:13
+ --> $DIR/mut-patterns.rs:28:13
  |
 LL | let mut mut yield(become, await) = r#yield(0, 0);
  | ^^^ help: remove the additional `mut`s
 
 error: expected identifier, found reserved keyword `yield`
- --> $DIR/mut-patterns.rs:26:17
+ --> $DIR/mut-patterns.rs:28:17
  |
 LL | let mut mut yield(become, await) = r#yield(0, 0);
  | ^^^^^ expected identifier, found reserved keyword
@@ -54,7 +62,7 @@ LL | let mut mut r#yield(become, await) = r#yield(0, 0);
  | ^^^^^^^
 
 error: expected identifier, found reserved keyword `become`
- --> $DIR/mut-patterns.rs:26:23
+ --> $DIR/mut-patterns.rs:28:23
  |
 LL | let mut mut yield(become, await) = r#yield(0, 0);
  | ^^^^^^ expected identifier, found reserved keyword
@@ -65,7 +73,7 @@ LL | let mut mut yield(r#become, await) = r#yield(0, 0);
  | ^^^^^^^^
 
 error: expected identifier, found keyword `await`
- --> $DIR/mut-patterns.rs:26:31
+ --> $DIR/mut-patterns.rs:28:31
  |
 LL | let mut mut yield(become, await) = r#yield(0, 0);
  | ^^^^^ expected identifier, found keyword
@@ -76,23 +84,23 @@ LL | let mut mut yield(become, r#await) = r#yield(0, 0);
  | ^^^^^^^
 
 error: `mut` must be attached to each individual binding
- --> $DIR/mut-patterns.rs:26:9
+ --> $DIR/mut-patterns.rs:28:9
  |
 LL | let mut mut yield(become, await) = r#yield(0, 0);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add `mut` to each binding: `r#yield(mut r#become, mut r#await)`
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`
 
 error: `mut` must be attached to each individual binding
- --> $DIR/mut-patterns.rs:35:9
+ --> $DIR/mut-patterns.rs:37:9
  |
 LL | let mut W(mut a, W(b, W(ref c, W(d, B { box f }))))
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add `mut` to each binding: `W(mut a, W(mut b, W(ref c, W(mut d, B { box mut f }))))`
  |
  = note: `mut` may be followed by `variable` and `variable @ pattern`
 
 error: expected identifier, found `x`
- --> $DIR/mut-patterns.rs:42:21
+ --> $DIR/mut-patterns.rs:44:21
  |
 LL | let mut $p = 0;
  | ^^ expected identifier
@@ -102,5 +110,5 @@ LL | foo!(x);
  |
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 12 previous errors
+error: aborting due to 13 previous errors