Why do I need to use self::core::ops?

Question

I'm trying to use Mul from core.

This is suggested by the compiler and works:

extern crate core;

use self::core::ops::Mul;

but why doesn't

extern crate core;

use core::ops::Mul;

work?

I get the error error: unresolved import `core::ops::Mul`. Did you mean `self::core::ops`?

Answer 1

An extern crate x; loads x into the current namespace. use statements are absolute paths unless they start with self::, so if you put your extern crate core; anywhere but the crate root then you need to specify an absolute path or use self::.

mod foo {
    mod bar {
        extern crate core;
        use foo::bar::core::ops::Mul;
        // or `use self::core::ops::Mul;`
        // or even `use super::bar::core::ops::Mul;` if you’re mad
        // but not `use core::ops::Mul;`.
    }
}

… but as a general rule you shouldn’t use core directly anyway. All the stable stuff from it is available in std which is included automatically.