Only call parse_token_tree once.

This code currently uses a `while` loop and gathers token trees into a
vector, but it only succeeds in the case where there is a single token
tree. We can instead just call `parse_token_tree` once and then look for
`Eof` to detect the success case.

Author: nnethercote

src/librustdoc/clean/render_macro_matchers.rs

@@ -76,14 +76,13 @@ fn snippet_equal_to_token(tcx: TyCtxt<'_>, matcher: &TokenTree) -> Option<String
         };
 
     // Reparse a single token tree.
-    let mut reparsed_trees = Vec::new();
-    while parser.token != token::Eof {
-        reparsed_trees.push(parser.parse_token_tree());
+    if parser.token == token::Eof {
+        return None;
     }
-    if reparsed_trees.len() != 1 {
+    let reparsed_tree = parser.parse_token_tree();
+    if parser.token != token::Eof {
         return None;
     }
-    let reparsed_tree = reparsed_trees.pop().unwrap();
 
     // Compare against the original tree.
     if reparsed_tree.eq_unspanned(matcher) { Some(snippet) } else { None }