Spurious unused import warning with trait used transitively via use super::*

[khuey]

use std::rc::Rc;
use std::ops::Deref;

mod foo {
    use super::*;

    pub struct Bar {
        pub thing: Rc<u32>,
    }
    
    impl Bar {
        pub fn baz(&self) -> &u32 {
            self.thing.deref()
        }
    }
}

fn main() {
    let use_bar = foo::Bar {
        thing: Rc::new(42),
    };
    println!("value {}", use_bar.baz());
}

triggers

warning: unused import: `std::ops::Deref`, #[warn(unused_imports)] on by default
 --> <anon>:2:5
  |
2 | use std::ops::Deref;
  |     ^^^^^^^^^^^^^^^

This happens on both stable and nightly. The warning goes away if use super::* is replaced by use super::{Rc, Deref}. And the sample of course fails to compile if you remove the use std::ops::Deref.

I ran into this while trying to take advantage of serde-rs/serde#765 by defining a bunch of modules with a common set of imports and a use super::* in each module for simplicity.

[Wolvereness]

I also ran across this with the write trait if another example is needed, https://play.rust-lang.org/?gist=296cf1f7d10dbc7a31509e531ac4760f&version=stable

[tjpalmer]

I also ran across this problem while trying out this code: https://github.com/KeepCalmAndLearnRust/pratt-parser

Specifically, the line complained about is here: https://github.com/KeepCalmAndLearnRust/pratt-parser/blob/d0fdf4c408f6626484fbd0da24f7bcd041952747/src/parser.rs#L3

A compiler that cries wolf can break trust.

[ExpHP]

Same as #45268. I'm building a list of linkbacks there for whoever wants to add half a dozen bug reports to their trophy case.

(too bad I chose that issue, this one has a much more accurate title!)

[sanxiyn]

Yes, this is a much better title, so I retitled the merged issue. Closing as a duplicate.