Fixup path fragments upon MBE transcription

crates/mbe/src/expander.rs

@@ -123,4 +123,14 @@ enum Fragment {
     /// proc-macro delimiter=none. As we later discovered, "none" delimiters are
     /// tricky to handle in the parser, and rustc doesn't handle those either.
     Expr(tt::TokenTree),
+    /// There are roughly two types of paths: paths in expression context, where a
+    /// separator `::` between an identifier and its following generic argument list
+    /// is mandatory, and paths in type context, where `::` can be omitted.
+    ///
+    /// Unlike rustc, we need to transform the parsed fragments back into tokens
+    /// during transcription. When the matched path fragment is a type-context path
+    /// and is trasncribed as an expression-context path, verbatim transcription
+    /// would cause a syntax error. We need to fix it up just before transcribing;
+    /// see `transcriber::fix_up_and_push_path_tt()`.
+    Path(tt::TokenTree),
 }