Fix internal compiler error on malformed match arm pattern.

Issue: 54379

src/libsyntax/parse/parser.rs

@@ -3866,6 +3866,9 @@ impl<'a> Parser<'a> {
             // check that a comma comes after every field
             if !ate_comma {
                 let err = self.struct_span_err(self.prev_span, "expected `,`");
+                if let Some(mut delayed) = delayed_err {
+                    delayed.emit();
+                }
                 return Err(err);
             }
             ate_comma = false;

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

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+struct MyStruct {
+    pub s1: Option<String>,
+}
+
+fn main() {
+    let thing = MyStruct { s1: None };
+
+    match thing {
+        MyStruct { .., Some(_) } => {},
+        _ => {}
+    }
+}

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

@@ -0,0 +1,24 @@
+error: expected `}`, found `,`
+  --> $DIR/issue-54379.rs:18:22
+   |
+LL |         MyStruct { .., Some(_) } => {},
+   |                    --^
+   |                    | |
+   |                    | expected `}`
+   |                    `..` must be at the end and cannot have a trailing comma
+
+error: expected `,`
+  --> $DIR/issue-54379.rs:18:24
+   |
+LL |         MyStruct { .., Some(_) } => {},
+   |                        ^^^^
+
+error[E0027]: pattern does not mention field `s1`
+  --> $DIR/issue-54379.rs:18:9
+   |
+LL |         MyStruct { .., Some(_) } => {},
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ missing field `s1`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0027`.