Fix span for incorrect pattern field and add label

compiler/rustc_parse/src/parser/pat.rs

@@ -795,6 +795,7 @@ impl<'a> Parser<'a> {
         }
         self.bump();
         let (fields, etc) = self.parse_pat_fields().unwrap_or_else(|mut e| {
+            e.span_label(path.span, "while parsing the fields for this pattern");
             e.emit();
             self.recover_stmt();
             (vec![], true)
@@ -844,7 +845,7 @@ impl<'a> Parser<'a> {
 
             // check that a comma comes after every field
             if !ate_comma {
-                let err = self.struct_span_err(self.prev_token.span, "expected `,`");
+                let err = self.struct_span_err(self.token.span, "expected `,`");
                 if let Some(mut delayed) = delayed_err {
                     delayed.emit();
                 }

src/test/ui/parser/bind-struct-early-modifiers.stderr

@@ -1,8 +1,10 @@
 error: expected `,`
-  --> $DIR/bind-struct-early-modifiers.rs:4:19
+  --> $DIR/bind-struct-early-modifiers.rs:4:20
    |
 LL |         Foo { ref x: ref x } => {},
-   |                   ^
+   |         ---        ^
+   |         |
+   |         while parsing the fields for this pattern
 
 error: aborting due to previous error
 

src/test/ui/parser/issue-10392.stderr

@@ -2,7 +2,9 @@ error: expected identifier, found `,`
   --> $DIR/issue-10392.rs:6:13
    |
 LL |     let A { , } = a();
-   |             ^ expected identifier
+   |         -   ^ expected identifier
+   |         |
+   |         while parsing the fields for this pattern
 
 error: aborting due to previous error
 

src/test/ui/parser/issue-63135.stderr

@@ -35,7 +35,9 @@ error: expected one of `!` or `[`, found `}`
   --> $DIR/issue-63135.rs:3:16
    |
 LL | fn i(n{...,f #
-   |                ^ expected one of `!` or `[`
+   |      -         ^ expected one of `!` or `[`
+   |      |
+   |      while parsing the fields for this pattern
 
 error: aborting due to 5 previous errors
 

src/test/ui/resolve/issue-54379.stderr

@@ -8,10 +8,12 @@ LL |         MyStruct { .., Some(_) } => {},
    |                    `..` must be at the end and cannot have a trailing comma
 
 error: expected `,`
-  --> $DIR/issue-54379.rs:9:24
+  --> $DIR/issue-54379.rs:9:28
    |
 LL |         MyStruct { .., Some(_) } => {},
-   |                        ^^^^
+   |         --------           ^
+   |         |
+   |         while parsing the fields for this pattern
 
 error: aborting due to 2 previous errors