The tracking issue for this feature is: #45388

The crate_visibility_modifier feature allows the crate keyword to be used as a visibility modifier synonymous to pub(crate), indicating that a type (function, &c.) is to be visible to the entire enclosing crate, but not to other crates.

#![allow(unused_variables)]
#![feature(crate_visibility_modifier)]

fn main() {
crate struct Foo {
    bar: usize,
}
}