Lint against imports that are dead code

[CryZe]

I'm very often seeing people write imports such as this one:
use regex;

Such an import is almost always dead code, because ever since Rust 2018 all external crates are already in scope. In fact pretty much any import that doesn't contain at least one of pub, as ... or :: is very likely dead code. Rust does not currently emit any warning for such an import.

[jonas-schievink]

use regex; on the playground does emit a warning:

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

[CryZe]

That's just the default warning of an unused import, not the specific case I'm talking about. Here's a proper reproduction:

use regex;

fn main() {
    regex::Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
}

Playground

Here, the statement uses regex::Regex which works even without the import. So the import doesn't do anything, but there's no warning.

I've had multiple occurrences now where I had to teach beginners that this isn't necessary, because the compiler fails to point this out.

[ThePuzzlemaker]

I think I'll take a shot at this. @rustbot claim

[ThePuzzlemaker]

Should this be implemented as a new lint or just as an instance of dead_code? I personally think it'd be better as its own lint (something like useless_imports) as dead code is described in rustc_lint_defs as "detect unused, unexported items"

[petrochenkov]

We already have a lint for redundant imports (#58805), it's a part of unused_imports, but it conservatively doesn't fire on imports in mod items.
The reason is that such imports can be used by module-relative paths like super::regex from a child module or crate::regex/self::regex from the current module, but we don't currently track such uses. See the linked PR and issue for more details.

[petrochenkov]

This is also a duplicate of #61640.

[rylev]

Triage: going to close this in favor of #61640.