I tried this code on the latest nightly:

#![feature(decl_macro)] trait Trait { type T; } macro trait_impl { () => { type T = (); } } impl Trait for i32 { trait_impl!(); type T = (); }

I expected to see this happen: an error about type T being defined twice in the same impl. (This is what happens with macro_rules! trait_impl.)

Instead, this happened: the code just compiles. The same also works when considering a function rather than a type.