2 files changed, 27 insertions, 12 deletions
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 35a745ccdee..0913ed86147 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -527,8 +527,17 @@ impl Token {
         // all span information.
         //
         // As a result, some AST nodes are annotated with the token
-        // stream they came from. Attempt to extract these lossless
-        // token streams before we fall back to the stringification.
+        // stream they came from. Here we attempt to extract these
+        // lossless token streams before we fall back to the
+        // stringification.
+        //
+        // During early phases of the compiler, though, the AST could
+        // get modified directly (e.g. attributes added or removed) and
+        // the internal cache of tokens my not be invalidated or
+        // updated. Consequently if the "lossless" token stream
+        // disagrees with our actuall stringification (which has
+        // historically been much more battle-tested) then we go with
+        // the lossy stream anyway (losing span information).
         let mut tokens = None;
 
         match nt.0 {
@@ -555,13 +564,17 @@ impl Token {
             _ => {}
         }
 
-        tokens.unwrap_or_else(|| {
-            nt.1.force(|| {
-                // FIXME(jseyfried): Avoid this pretty-print + reparse hack
-                let source = pprust::token_to_string(self);
-                parse_stream_from_source_str(FileName::MacroExpansion, source, sess, Some(span))
-            })
-        })
+        let tokens_for_real = nt.1.force(|| {
+            // FIXME(#43081): Avoid this pretty-print + reparse hack
+            let source = pprust::token_to_string(self);
+            parse_stream_from_source_str(FileName::MacroExpansion, source, sess, Some(span))
+        });
+        if let Some(tokens) = tokens {
+            if tokens.eq_unspanned(&tokens_for_real) {
+                return tokens
+            }
+        }
+        return tokens_for_real
     }
 }
 
diff --git a/src/libsyntax/tokenstream.rs b/src/libsyntax/tokenstream.rs
index 3a7a1b9a669..6ac04b3cdf6 100644
--- a/src/libsyntax/tokenstream.rs
+++ b/src/libsyntax/tokenstream.rs
@@ -118,7 +118,7 @@ impl TokenTree {
             (&TokenTree::Token(_, ref tk), &TokenTree::Token(_, ref tk2)) => tk == tk2,
             (&TokenTree::Delimited(_, ref dl), &TokenTree::Delimited(_, ref dl2)) => {
                 dl.delim == dl2.delim &&
-                dl.stream().trees().zip(dl2.stream().trees()).all(|(tt, tt2)| tt.eq_unspanned(&tt2))
+                dl.stream().eq_unspanned(&dl2.stream())
             }
             (_, _) => false,
         }
@@ -240,12 +240,14 @@ impl TokenStream {
 
     /// Compares two TokenStreams, checking equality without regarding span information.
     pub fn eq_unspanned(&self, other: &TokenStream) -> bool {
-        for (t1, t2) in self.trees().zip(other.trees()) {
+        let mut t1 = self.trees();
+        let mut t2 = other.trees();
+        for (t1, t2) in t1.by_ref().zip(t2.by_ref()) {
             if !t1.eq_unspanned(&t2) {
                 return false;
             }
         }
-        true
+        t1.next().is_none() && t2.next().is_none()
     }
 
     /// Precondition: `self` consists of a single token tree.