Implement ignore and index metavar expression

crates/mbe/src/expander/transcriber.rs

@@ -103,6 +103,23 @@ fn expand_subtree(
                 err = err.or(e);
                 push_fragment(arena, fragment)
             }
+            Op::Ignore { name, id } => {
+                // Expand the variable, but ignore the result. This registers the repetition count.
+                expand_var(ctx, name, *id);
+            }
+            Op::Index { depth } => {
+                let index = ctx
+                    .nesting
+                    .get(ctx.nesting.len() - 1 - (*depth as usize))
+                    .map_or(0, |nest| nest.idx);
+                arena.push(
+                    tt::Leaf::Literal(tt::Literal {
+                        text: index.to_string().into(),
+                        id: tt::TokenId::unspecified(),
+                    })
+                    .into(),
+                );
+            }
         }
     }
     // drain the elements added in this instance of expand_subtree