``` rust #![feature(macro_rules)] macro_rules! foo { () => { $e }} fn main() { foo!() } ``` ``` unknown-macro-variable.rs:3:29: 3:30 error: unexpected token: `$` unknown-macro-variable.rs:3 macro_rules! foo { () => { $e }} ^ ``` This is very confusing and does not provide much help to work out what is wrong. (Looks like this was introduced by 38ce6d9eac5d0bcfa0c102bc64393a987b4a43e3.)