Rollup merge of #72677 - chrissimpkins:fix-72574, r=estebank

Fix diagnostics for `@ ..` binding pattern in tuples and tuple structs

Fixes #72574
Associated #72534 #72373

Includes a new suggestion with `Applicability::MaybeIncorrect` confidence level.

### Before

#### tuple

```
error: `..` patterns are not allowed here
 --> src/main.rs:4:19
  |
4 |         (_a, _x @ ..) => {}
  |                   ^^
  |
  = note: only allowed in tuple, tuple struct, and slice patterns

error[E0308]: mismatched types
 --> src/main.rs:4:9
  |
3 |     match x {
  |           - this expression has type `({integer}, {integer}, {integer})`
4 |         (_a, _x @ ..) => {}
  |         ^^^^^^^^^^^^^ expected a tuple with 3 elements, found one with 2 elements
  |
  = note: expected tuple `({integer}, {integer}, {integer})`
             found tuple `(_, _)`

error: aborting due to 2 previous errors
```

#### tuple struct

```
error: `..` patterns are not allowed here
 --> src/main.rs:6:25
  |
6 |         Binder(_a, _x @ ..) => {}
  |                         ^^
  |
  = note: only allowed in tuple, tuple struct, and slice patterns

error[E0023]: this pattern has 2 fields, but the corresponding tuple struct has 3 fields
 --> src/main.rs:6:9
  |
1 | struct Binder(i32, i32, i32);
  | ----------------------------- tuple struct defined here
...
6 |         Binder(_a, _x @ ..) => {}
  |         ^^^^^^^^^^^^^^^^^^^ expected 3 fields, found 2

error: aborting due to 2 previous errors
```

### After

*Note: final output edited during source review discussion, see thread for details*

#### tuple

```
error: `_x @` is not allowed in a tuple
 --> src/main.rs:4:14
  |
4 |         (_a, _x @ ..) => {}
  |              ^^^^^^^ is only allowed in a slice
  |
help: replace with `..` or use a different valid pattern
  |
4 |         (_a, ..) => {}
  |              ^^

error[E0308]: mismatched types
 --> src/main.rs:4:9
  |
3 |     match x {
  |           - this expression has type `({integer}, {integer}, {integer})`
4 |         (_a, _x @ ..) => {}
  |         ^^^^^^^^^^^^^ expected a tuple with 3 elements, found one with 1 element
  |
  = note: expected tuple `({integer}, {integer}, {integer})`
             found tuple `(_,)`

error: aborting due to 2 previous errors
```

#### tuple struct

```
error: `_x @` is not allowed in a tuple struct
 --> src/main.rs:6:20
  |
6 |         Binder(_a, _x @ ..) => {}
  |                    ^^^^^^^ is only allowed in a slice
  |
help: replace with `..` or use a different valid pattern
  |
6 |         Binder(_a, ..) => {}
  |                    ^^

error[E0023]: this pattern has 1 field, but the corresponding tuple struct has 3 fields
 --> src/main.rs:6:9
  |
1 | struct Binder(i32, i32, i32);
  | ----------------------------- tuple struct defined here
...
6 |         Binder(_a, _x @ ..) => {}
  |         ^^^^^^^^^^^^^^^^^^^ expected 3 fields, found 1

error: aborting due to 2 previous errors
```

r? @estebank

Author: JohnTitor

src/librustc_ast_lowering/pat.rs

@@ -3,6 +3,7 @@ use super::{ImplTraitContext, LoweringContext, ParamMode};
 use rustc_ast::ast::*;
 use rustc_ast::ptr::P;
 use rustc_data_structures::stack::ensure_sufficient_stack;
+use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_span::symbol::Ident;
@@ -102,10 +103,36 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // Note that unlike for slice patterns,
             // where `xs @ ..` is a legal sub-slice pattern,
             // it is not a legal sub-tuple pattern.
-            if pat.is_rest() {
-                rest = Some((idx, pat.span));
-                break;
+            match pat.kind {
+                // Found a sub-tuple rest pattern
+                PatKind::Rest => {
+                    rest = Some((idx, pat.span));
+                    break;
+                }
+                // Found a sub-tuple pattern `$binding_mode $ident @ ..`.
+                // This is not allowed as a sub-tuple pattern
+                PatKind::Ident(ref _bm, ident, Some(ref sub)) if sub.is_rest() => {
+                    rest = Some((idx, pat.span));
+                    let sp = pat.span;
+                    self.diagnostic()
+                        .struct_span_err(
+                            sp,
+                            &format!("`{} @` is not allowed in a {}", ident.name, ctx),
+                        )
+                        .span_label(sp, "this is only allowed in slice patterns")
+                        .help("remove this and bind each tuple field independently")
+                        .span_suggestion_verbose(
+                            sp,
+                            &format!("if you don't need to use the contents of {}, discard the tuple's remaining fields", ident),
+                            "..".to_string(),
+                            Applicability::MaybeIncorrect,
+                        )
+                        .emit();
+                    break;
+                }
+                _ => {}
             }
+
             // It was not a sub-tuple pattern so lower it normally.
             elems.push(self.lower_pat(pat));
         }

src/test/ui/issues/issue-72574-1.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let x = (1, 2, 3);
+    match x {
+        (_a, _x @ ..) => {}
+        _ => {}
+    }
+}
+//~^^^^ ERROR `_x @` is not allowed in a tuple

src/test/ui/issues/issue-72574-1.stderr

@@ -0,0 +1,14 @@
+error: `_x @` is not allowed in a tuple
+  --> $DIR/issue-72574-1.rs:4:14
+   |
+LL |         (_a, _x @ ..) => {}
+   |              ^^^^^^^ this is only allowed in slice patterns
+   |
+   = help: remove this and bind each tuple field independently
+help: if you don't need to use the contents of _x, discard the tuple's remaining fields
+   |
+LL |         (_a, ..) => {}
+   |              ^^
+
+error: aborting due to previous error
+

src/test/ui/issues/issue-72574-2.rs

@@ -0,0 +1,10 @@
+struct Binder(i32, i32, i32);
+
+fn main() {
+    let x = Binder(1, 2, 3);
+    match x {
+        Binder(_a, _x @ ..) => {}
+        _ => {}
+    }
+}
+//~^^^^ ERROR `_x @` is not allowed in a tuple struct

src/test/ui/issues/issue-72574-2.stderr

@@ -0,0 +1,14 @@
+error: `_x @` is not allowed in a tuple struct
+  --> $DIR/issue-72574-2.rs:6:20
+   |
+LL |         Binder(_a, _x @ ..) => {}
+   |                    ^^^^^^^ this is only allowed in slice patterns
+   |
+   = help: remove this and bind each tuple field independently
+help: if you don't need to use the contents of _x, discard the tuple's remaining fields
+   |
+LL |         Binder(_a, ..) => {}
+   |                    ^^
+
+error: aborting due to previous error
+