Strange "no extern crate" behavior on Rust 2015

[Disasm]

Assume we have rand = "0.6.1" dependency in Cargo.toml. On Rust 2015 the following example works well

fn main() {
    let _ = rand::thread_rng();
}

but the following doesn't.

use rand::thread_rng;

fn main() {
    let _ = thread_rng();
}

On Rust 2018 both examples are totally ok (as expected).
Why do we need "extern crate rand" for the second case but not for the first? Are there any reasons for such inconsistency?

% rustc --version
rustc 1.31.0 (abe02cefd 2018-12-04)

[jsvcycling]

I get the same result on Rust 2015.

error[E0432]: unresolved import `rand`
 --> src\main.rs:1:5
  |
1 | use rand::thread_rng;
  |     ^^^^ maybe a missing `extern crate rand;`?

It works with extern crate rand; though.

[steveklabnik]

cc @petrochenkov

[petrochenkov]

Duplicate or #55478 (which has some more detailed explanation).