A warning about redundant imports should be separate from `unused_imports`

[anforowicz]

Problem

There is a set of related problems:

• Because the same warning name covers both, it is impossible to ignore redundant imports, while still warning for unused imports.
• It is difficult to author libraries that work with std and with no_std, because the prelude contents are different. In particular an unconditional use alloc::vec::Vec causes a warning (or error with -Dwarnings): error: the item Vec is imported redundantly

AFAIU the new warning has been introduced for issue #117448 in the changes here: #117772

This problem has been originally found in https://crbug.com/326247202. (In https://crbug.com/326247202#comment10 and https://crbug.com/326247202#comment11 we discussed disabling the new warning, but it doesn't seem possible without also disabling unused_imports.)

Steps

$ cat src/main.rs
mod my_library {
    #[derive(Debug)]
    pub struct Foo(pub i32);

    #[derive(Debug)]
    pub struct Bar(pub i32);
}

mod prelude_from_my_library {
    pub use super::my_library::Foo;

    // The `main` below compiles fine without the next `use`.
    // But adding reimport of `Bar` to the prelude will trigger
    // a redundant-import warning.
    //
    // Comment out the next line to simulate a previous version of the prelude
    // (where the warning doesn't trigger).
    pub use super::my_library::Bar;
}

use prelude_from_my_library::*;
use my_library::Bar;

fn main() {
    dbg!(Foo(123));
    dbg!(Bar(456));
}

$ cargo run
warning: unused import: `super::my_library::Bar`
  --> src/main.rs:15:13
   |
15 |     pub use super::my_library::Bar;
   |             ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: `redundant-import-repro` (bin "redundant-import-repro") generated 1 warning (run `cargo fix --bin "redundant-import-repro"` to apply 1 suggestion)
[...]

Possible Solution(s)

Maybe it would be helpful to have 2 separate warnings: the old unused_imports warning, and a separate redundant_imports warning?

Version

$ rustc --version --verbose
rustc 1.78.0-nightly (ee9c7c940 2024-02-14)
binary: rustc
commit-hash: ee9c7c940c07d8b67c9a6b2ec930db70dcd23a46
commit-date: 2024-02-14
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly

[petrochenkov]

I'm going to close this to centralize the discussion in #121708.