Use a simpler atomic operation than the compare_exchange hammer

compiler/rustc_data_structures/src/sync.rs

@@ -107,6 +107,14 @@ cfg_if! {
             }
         }
 
+        impl Atomic<bool> {
+            pub fn fetch_or(&self, val: bool, _: Ordering) -> bool {
+                let result = self.0.get() | val;
+                self.0.set(val);
+                result
+            }
+        }
+
         impl<T: Copy + PartialEq> Atomic<T> {
             #[inline]
             pub fn compare_exchange(&self,

compiler/rustc_parse/src/parser/mod.rs

@@ -1541,11 +1541,9 @@ pub(crate) fn make_unclosed_delims_error(
 }
 
 pub fn emit_unclosed_delims(unclosed_delims: &mut Vec<UnmatchedDelim>, sess: &ParseSess) {
-    let _ = sess.reached_eof.compare_exchange(
+    let _ = sess.reached_eof.fetch_or(
-        false,
         unclosed_delims.iter().any(|unmatched_delim| unmatched_delim.found_delim.is_none()),
         Ordering::Relaxed,
-        Ordering::Relaxed,
     );
     for unmatched in unclosed_delims.drain(..) {
         if let Some(mut e) = make_unclosed_delims_error(unmatched, sess) {