An non-ascii identifier was used in an invalid context.

Erroneous code example:

# #![feature(non_ascii_idents)]

mod řųśť;
// ^ error!
fn main() {}

# #![feature(non_ascii_idents)]

#[no_mangle]
fn řųśť() {}
// ^ error!
fn main() {}

Non-ascii can be used as module names if it is inline or a #[path] attribute is specified. For example:

# #![feature(non_ascii_idents)]

mod řųśť {
    const IS_GREAT: bool = true;
}

fn main() {}