2018 compatibility lint isn't firing for all imports that need to be rewritten

[alexcrichton]

This code:

#![feature(rust_2018_preview)]
#![warn(rust_2018_breakage)]

mod m {
    use foo;

    crate fn bar() {
        foo();
    }
}

fn foo() {}

fn main() {
    m::bar();
}

on nightly produces no warnings, but to compile in the 2018 edition the use statement needs to be rewritten to crate::.