Warn about useless imports

[CryZe]

I'm seeing quite a lot of people do the following:

use some_crate;

However due to uniform paths, this doesn't do anything, as the crate is already accessible like that in the current module. This neither disambiguates nor does it have any meaningful effect, yet Rust does not emit a warning.

All of this only applies to uniform paths. So in Rust 2015 this actually is a meaningful import.

[mati865]

Can you provide better example?
Works as expected for:

use rand;

pub fn foo() {}

Warning:

warning: unused import: `rand`
 --> src/lib.rs:1:5
  |
1 | use rand;
  |     ^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

[CryZe]

Yours is the warning about rand not being used at all, which is different from using rand, but importing it anyway when you don't need to.

Here's an example with std on the playground

use std;

fn main() {
    std::iter::once(5);
}

The use std; there doesn't do anything, as std::iter::once(5) works regardless, as std is already accessible in the module. Yet there is no warning.

[CryZe]

Here is where I most recently encountered this in the wild: wasmerio/wasmer#1140

There they have code that roughly looks like this:

use blake3;

fn main() {
    let mut hasher = blake3::Hasher::new();
    ...
}

They use blake3; despite blake3::Hasher::new() working just fine without the use blake3;.

I've seen people make this mistake quite often by now, as this used to be required in Rust 2015 but is now completely redundant in Rust 2018.

[petrochenkov]

This is a duplicate of #61640 and #10178 (which contains some implementation hints), and the lint was partially implemented in #58805.