implement and use diagnostic_snapshot

Author: TaKO8Ki

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -179,6 +179,13 @@ impl<'a> Parser<'a> {
         &self.sess.span_diagnostic
     }
 
+    pub(super) fn diagnostic_snapshot(&self) -> Self {
+        let mut snapshot = self.clone();
+        // initialize unclosed_delims to avoid duplicate errors.
+        snapshot.unclosed_delims = vec![];
+        snapshot
+    }
+
     pub(super) fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
         self.sess.source_map().span_to_snippet(span)
     }

compiler/rustc_parse/src/parser/path.rs

@@ -625,8 +625,7 @@ impl<'a> Parser<'a> {
         } else if self.check_type() {
             // Parse type argument.
             let is_const_fn = self.look_ahead(1, |t| t.kind == token::OpenDelim(token::Paren));
-            let mut snapshot = self.clone();
-            snapshot.unclosed_delims = vec![];
+            let mut snapshot = self.diagnostic_snapshot();
             match self.parse_ty() {
                 Ok(ty) => GenericArg::Type(ty),
                 Err(err) => {