Improve error message for struct expression missing a base expression after ..

Author: fanzier

compiler/rustc_parse/src/parser/expr.rs

@@ -2108,7 +2108,7 @@ impl<'a> Parser<'a> {
                 // AST lowering will then report an error if it's not on the LHS of an assignment.
                 if self.token == token::CloseDelim(token::Brace) {
                     base = Some(self.mk_expr(
-                        self.prev_token.span,
+                        self.prev_token.span.shrink_to_hi(),
                         ExprKind::Underscore,
                         AttrVec::new(),
                     ));

src/test/ui/destructuring-assignment/struct_destructure_fail.rs

@@ -12,4 +12,5 @@ fn main() {
     Struct { a, _ } = Struct { a: 1, b: 2 }; //~ ERROR pattern does not mention field `b`
     //~| ERROR expected identifier, found reserved identifier `_`
     Struct { a, ..d } = Struct { a: 1, b: 2 }; //~ ERROR base expression not allowed here
+    Struct { a, .. }; //~ ERROR base expression required after `..`
 }

src/test/ui/destructuring-assignment/struct_destructure_fail.stderr

@@ -12,6 +12,12 @@ error: base expression not allowed here
 LL |     Struct { a, ..d } = Struct { a: 1, b: 2 };
    |                   ^ help: consider removing this
 
+error: base expression required after `..`
+  --> $DIR/struct_destructure_fail.rs:15:19
+   |
+LL |     Struct { a, .. };
+   |                   ^ add the base expression here
+
 error[E0026]: struct `Struct` does not have a field named `c`
   --> $DIR/struct_destructure_fail.rs:11:20
    |
@@ -24,7 +30,7 @@ error[E0027]: pattern does not mention field `b`
 LL |     Struct { a, _ } = Struct { a: 1, b: 2 };
    |     ^^^^^^^^^^^^^^^ missing field `b`
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0026, E0027.
 For more information about an error, try `rustc --explain E0026`.