Rollup merge of #45178 - Badel2:comma-after-struct, r=petrochenkov

Better error message for comma after base struct

#41834

This adds a better error for commas after the base struct:
```
let foo = Foo {
    one: 111,
    ..Foo::default(), // This comma is a syntax error
};
```

The current error is a generic `expected one of ...` which isn't beginner-friendly. My error looks like this:

```
error: cannot use a comma after the base struct
  --> tmp/example.rs:26:9
   |
26 |         ..Foo::default(),
   |         ^^^^^^^^^^^^^^^^- help: remove this comma
   |
   = note: the base struct expansion must always be the last field
```

I even added a note for people who don't know why this isn't allowed.

src/libsyntax/parse/parser.rs

@@ -2314,6 +2314,7 @@ impl<'a> Parser<'a> {
 
         while self.token != token::CloseDelim(token::Brace) {
             if self.eat(&token::DotDot) {
+                let exp_span = self.prev_span;
                 match self.parse_expr() {
                     Ok(e) => {
                         base = Some(e);
@@ -2323,6 +2324,16 @@ impl<'a> Parser<'a> {
                         self.recover_stmt();
                     }
                 }
+                if self.token == token::Comma {
+                    let mut err = self.sess.span_diagnostic.mut_span_err(
+                        exp_span.to(self.prev_span),
+                        "cannot use a comma after the base struct",
+                    );
+                    err.span_suggestion_short(self.span, "remove this comma", "".to_owned());
+                    err.note("the base struct must always be the last field");
+                    err.emit();
+                    self.recover_stmt();
+                }
                 break;
             }