Warn when an import list is empty

[estebank]

For a given file

use std::*;
use std::{};

output the following warnings

warning: unused import: `use std::{};`, #[warn(unused_imports)] on by default
 --> file.rs:2:1
  |
2 | use std::{};
  | ^^^^^^^^^^^^

warning: unused import: `std::*;`, #[warn(unused_imports)] on by default
 --> file.rs:1:5
  |
1 | use std::*;
  |     ^^^^^^^

[rust-highfive]

r? @eddyb

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[petrochenkov]

This should be included into existing unused_imports lint, IMO.
Also, while being reasonable, this doesn't fix #38012, #38012 is about ::{} being accepted after non-modules.

[bors]

☔ The latest upstream changes (presumably #38014) made this pull request unmergeable. Please resolve the merge conflicts.

[eddyb]

r? @jseyfried

[jseyfried]

@estebank Could you make this an unused_imports lint as @petrochenkov suggested?

[estebank]

@jseyfried done!

[jseyfried]

@estebank Thanks!
@bors r+ rollup

[bors]

📌 Commit 58e70e7 has been approved by jseyfried