1 files changed, 8 insertions, 1 deletions
diff --git a/crates/mbe/src/lib.rs b/crates/mbe/src/lib.rs
index 5445f8790fc..d5de56312a3 100644
--- a/crates/mbe/src/lib.rs
+++ b/crates/mbe/src/lib.rs
@@ -122,6 +122,9 @@ impl fmt::Display for CountError {
     }
 }
 
+/// Index of the matched macro arm on successful expansion.
+pub type MatchedArmIndex = Option<u32>;
+
 /// This struct contains AST for a single `macro_rules` definition. What might
 /// be very confusing is that AST has almost exactly the same shape as
 /// `tt::TokenTree`, but there's a crucial difference: in macro rules, `$ident`
@@ -251,7 +254,7 @@ impl DeclarativeMacro {
         new_meta_vars: bool,
         call_site: Span,
         def_site_edition: Edition,
-    ) -> ExpandResult<tt::Subtree<Span>> {
+    ) -> ExpandResult<(tt::Subtree<Span>, MatchedArmIndex)> {
         expander::expand_rules(&self.rules, tt, marker, new_meta_vars, call_site, def_site_edition)
     }
 }
@@ -330,6 +333,10 @@ impl<T, E> ValueResult<T, E> {
         Self { value: Default::default(), err: Some(err) }
     }
 
+    pub fn zip_val<U>(self, other: U) -> ValueResult<(T, U), E> {
+        ValueResult { value: (self.value, other), err: self.err }
+    }
+
     pub fn map<U>(self, f: impl FnOnce(T) -> U) -> ValueResult<U, E> {
         ValueResult { value: f(self.value), err: self.err }
     }