gistfile1.rs

#[feature(macro_rules)]

extern mod expr;

macro_rules! run(
        ($expr:expr $env:expr) => {
                let (v, n) = (expr.run)(&mut env);
                expr = n;
                v
        }
)

Compiler error:
error: macro definitions are not stable enough for use and are subject to change
/home/sven/Desktop/prob/src/expr/test.rs:5 macro_rules! run(
                                           ^~~~~~~~~~~
/home/sven/Desktop/prob/src/expr/test.rs:5:0: 5:11 note: add #[feature(macro_rules)] to the crate attributes to enable
/home/sven/Desktop/prob/src/expr/test.rs:5 macro_rules! run(

btw the solution is to add a ; at the end of the #[... line