Extra panic cases.

Just some extra sanity checking, making explicit some values not
possible in code working with token trees -- we shouldn't be seeing
explicit delimiter tokens, because they should be represented as
`TokenTree::Delimited`.

compiler/rustc_ast/src/attr/mod.rs

@@ -327,7 +327,8 @@ fn from_tokens<'a, I>(tokens: &mut iter::Peekable<I>) -> Option<MetaItem>
         I: Iterator<Item = &'a TokenTree>,
     {
         // FIXME: Share code with `parse_path`.
-        let path = match tokens.next().map(|tt| TokenTree::uninterpolate(tt)).as_deref() {
+        let tt = tokens.next().map(|tt| TokenTree::uninterpolate(tt));
+        let path = match tt.as_deref() {
             Some(&TokenTree::Token(
                 Token { kind: ref kind @ (token::Ident(..) | token::PathSep), span },
                 _,
@@ -368,6 +369,12 @@ fn from_tokens<'a, I>(tokens: &mut iter::Peekable<I>) -> Option<MetaItem>
                 token::Nonterminal::NtPath(path) => (**path).clone(),
                 _ => return None,
             },
+            Some(TokenTree::Token(
+                Token { kind: token::OpenDelim(_) | token::CloseDelim(_), .. },
+                _,
+            )) => {
+                panic!("Should be `AttrTokenTree::Delimited`, not delim tokens: {:?}", tt);
+            }
             _ => return None,
         };
         let list_closing_paren_pos = tokens.peek().map(|tt| tt.span().hi());

compiler/rustc_expand/src/config.rs

@@ -214,6 +214,12 @@ fn can_skip(stream: &AttrTokenStream) -> bool {
                 ) => {
                     panic!("Nonterminal should have been flattened: {:?}", tree);
                 }
+                AttrTokenTree::Token(
+                    Token { kind: TokenKind::OpenDelim(_) | TokenKind::CloseDelim(_), .. },
+                    _,
+                ) => {
+                    panic!("Should be `AttrTokenTree::Delimited`, not delim tokens: {:?}", tree);
+                }
                 AttrTokenTree::Token(token, spacing) => {
                     Some(AttrTokenTree::Token(token, spacing)).into_iter()
                 }