Rust-clippy: Feature request: warn about redundant imports

[PonasKovas]

For example

use std::mem::drop;

Should give a warning, because drop is in the std prelude and it's already in scope.

use std::io::prelude::*;
use std::io::Write;

Should also give a warning, because Write is in io's prelude and is already imported.

[ebroto]

I would like to tackle this one, but I'm not sure about the interactions with the existing WILDCARD_IMPORTS lint.

I guess it would be coherent to only trigger this lint for prelude imports? Also, what would be the category of this lint? I've seen other A-unnecessary lints classified under complexity, but maybe this one should be pedantic as it kind of overlaps with WILDCARD_IMPORTS?

[flip1995]

An overlap of these lints shouldn't matter that much, since WILDCARD_IMPORTS is pedantic. I would put this lint in style or complexity.

[ebroto]

Someone correct me if I'm wrong, but I think this is not doable currently due to ty::TyCtxt::names_imported_by_glob_use only returning names actually imported by the glob use.

If there is a non-glob import introducing the name, that seems to get precedence, and there is no way to know from the lint if the glob also could have introduced that same name.

[flip1995]

Is this a problem though? if there is an import use foo::* and an import use foo::bar, the bar is either used and gets returned by ty::TyCtxt::names_imported_by_glob_use or it is not used and the unused_imports lint will trigger. Or does it not get returned if there is a non-glob import?

[ebroto]

It seems to be the latter (does not get returned if there is a non-glob import).

With this code...

use std::io::prelude::*;
use std::io::Write;

struct S {}
impl Write for S {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        Ok(42)
    }

    fn flush(&mut self) -> std::io::Result<()> {
        Ok(())
    }
}

Adding a dbg!() call on the returned value from

cx.tcx.names_imported_by_glob_use(item.hir_id.owner.to_def_id());

I get:

[clippy_lints/src/redundant_imports.rs:43] &used_imports = {}

If I comment out the non-glob import, I get:

[clippy_lints/src/redundant_imports.rs:43] &used_imports = {
    "Write",
}

[flip1995]

Or yeah, in that case, this is really limited/hard to implement.

[ebroto]

I feel then this is a bit out of my league for now, I will leave it so someone else can address it.