False positive unused_import warning on std::string::String

[Kinrany]

This (very contrived) example emits a warning:

mod foo {
    pub struct String;
    
    #[derive(Debug)]
    pub struct Number;
}

fn main() {
    use std::string::String;
    use foo::*;
    
    let n = Number;
    let s = String::new();

    println!("{:?} {}", n, s);
}

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4333aa0dec7b94c3d2942e7380894003

warning: the item `String` is imported redundantly
  --> src/main.rs:9:9
   |
9  |     use std::string::String;
   |         ^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

This is a false positive because removing use std::string::String results in a compilation error:

error[E0599]: no function or associated item named `new` found for struct `foo::String` in the current scope
  --> src/main.rs:13:21
   |
2  |     pub struct String;
   |     ------------------ function or associated item `new` not found for this
...
13 |     let s = String::new();
   |                     ^^^ function or associated item not found in `foo::String`

Meta

$ rustc --version --verbose
rustc 1.42.0 (b8cedc004 2020-03-09)
binary: rustc
commit-hash: b8cedc00407a4c56a3bda1ed605c6fc166655447
commit-date: 2020-03-09
host: x86_64-pc-windows-msvc
release: 1.42.0
LLVM version: 9.0

Same for 1.43.0-beta.6 and 1.44.0-nightly (2020-04-21 45d050cde277b22a7558).

[dillona]

It seems this regressed somewhere between 1.34.0 and 1.35.0

[dillona]

More accurately, it looks like the false positive has been present since the introduction of the warning in #58805

[Kinrany]

I was actually a little surprised that

use std::string::String;
use foo::*;

did not cause a compilation error due to two imports of the same name at the same level.

[Kreijstal]

I got something similar while implementing ops::Div

warning: the item `panic` is imported redundantly
   --> src\main.rs:187:13
    |
47  | use core::*;
    |     ------- the item `panic` is already imported here
...
187 |         use core::panic;
    |             ^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

when removed, it errors

error[E0659]: `panic` is ambiguous
   --> src\main.rs:189:46
    |
189 |             (_, Number(a)) if a.is_zero() => panic!("no division by zero"),
    |                                              ^^^^^ ambiguous name
    |
    = note: ambiguous because of a conflict between a name from a glob import and an outer scope during import 
or macro resolution
    = note: `panic` could refer to a macro from prelude
note: `panic` could also refer to the macro imported here
   --> src\main.rs:47:5
    |
47  | use core::*;
    |     ^^^^^^^
    = help: consider adding an explicit import of `panic` to disambiguate
    = help: or use `crate::panic` to refer to this macro unambiguously

error[E0659]: `panic` is ambiguous
   --> src\main.rs:191:17
    |
191 |                 panic!("Infinity/Infinity division")
    |                 ^^^^^ ambiguous name
    |
    = note: ambiguous because of a conflict between a name from a glob import and an outer scope during import 
or macro resolution
    = note: `panic` could refer to a macro from prelude
note: `panic` could also refer to the macro imported here
   --> src\main.rs:47:5
    |
47  | use core::*;
    |     ^^^^^^^
    = help: consider adding an explicit import of `panic` to disambiguate
    = help: or use `crate::panic` to refer to this macro unambiguously

[petrochenkov]

This issue no longer reproduces and can be closed when a corresponding test is added.