Auto merge of #51201 - estebank:dotdot, r=petrochenkov

Accept `..` in incorrect position to avoid further errors

We currently give a specific message when encountering a `..` anywhere
other than the end of a pattern. Modify the parser to accept it (while
still emitting the error) so that we don't also trigger "missing fields
in pattern" errors afterwards.

Add suggestions to either remove trailing `,` or moving the `..` to the
end.

Follow up to #49268.

src/test/parse-fail/bind-struct-early-modifiers.rs

@@ -13,7 +13,7 @@
 fn main() {
     struct Foo { x: isize }
     match (Foo { x: 10 }) {
-        Foo { ref x: ref x } => {}, //~ ERROR expected `,`, found `:`
+        Foo { ref x: ref x } => {}, //~ ERROR expected `,`
         _ => {}
     }
 }

src/test/ui/issue-49257.rs

@@ -17,6 +17,8 @@ struct Point { x: u8, y: u8 }
 
 fn main() {
     let p = Point { x: 0, y: 0 };
+    let Point { .., y, } = p; //~ ERROR expected `}`, found `,`
     let Point { .., y } = p; //~ ERROR expected `}`, found `,`
-    //~| ERROR pattern does not mention fields `x`, `y`
+    let Point { .., } = p; //~ ERROR expected `}`, found `,`
+    let Point { .. } = p;
 }

src/test/ui/issue-49257.stderr

@@ -1,15 +1,38 @@
 error: expected `}`, found `,`
   --> $DIR/issue-49257.rs:20:19
    |
-LL |     let Point { .., y } = p; //~ ERROR expected `}`, found `,`
-   |                   ^ `..` must be in the last position, and cannot have a trailing comma
+LL |     let Point { .., y, } = p; //~ ERROR expected `}`, found `,`
+   |                 --^
+   |                 | |
+   |                 | expected `}`
+   |                 `..` must be at the end and cannot have a trailing comma
+help: move the `..` to the end of the field list
+   |
+LL |     let Point {  y, .. } = p; //~ ERROR expected `}`, found `,`
+   |                --   ^^^^
 
-error[E0027]: pattern does not mention fields `x`, `y`
-  --> $DIR/issue-49257.rs:20:9
+error: expected `}`, found `,`
+  --> $DIR/issue-49257.rs:21:19
    |
 LL |     let Point { .., y } = p; //~ ERROR expected `}`, found `,`
-   |         ^^^^^^^^^^^^^^^ missing fields `x`, `y`
+   |                 --^
+   |                 | |
+   |                 | expected `}`
+   |                 `..` must be at the end and cannot have a trailing comma
+help: move the `..` to the end of the field list
+   |
+LL |     let Point {  y , .. } = p; //~ ERROR expected `}`, found `,`
+   |                --  ^^^^^^
 
-error: aborting due to 2 previous errors
+error: expected `}`, found `,`
+  --> $DIR/issue-49257.rs:22:19
+   |
+LL |     let Point { .., } = p; //~ ERROR expected `}`, found `,`
+   |                 --^
+   |                 | |
+   |                 | expected `}`
+   |                 | help: remove this comma
+   |                 `..` must be at the end and cannot have a trailing comma
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0027`.