`return_self_not_must_use` is reported for types that already have a `#[must_use]` attribute

Summary

Clippy reports return_self_must_use for traits in a generic context, even if the return type already has #[must_use].

Lint Name

return_self_must_use

Reproducer

I tried this code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=c6a72eb40b979421206de10daf27d85d

use std::fmt::Display;
use std::error::Error;
pub type BoxError = Box<dyn Error + Send + Sync>;
pub type MyResult<T> = Result<T, BoxError>;

pub trait MyResultExt<T, E> {
    fn log_err(self, msg: &str) -> Self
    where
        E: Display;
}

impl<T> MyResultExt<T, BoxError> for MyResult<T> {
    fn log_err(self, _: &str) -> Self {
        self
    }
}

fn main() {
    Ok(()).log_err("oops");
}

I saw this happen:

warning: missing `#[must_use]` attribute on a method returning `Self`
 --> src/main.rs:7:5
  |
7 | /     fn log_err(self, msg: &str) -> Self
8 | |     where
9 | |         E: Display;
  | |___________________^
  |
  = note: `#[warn(clippy::return_self_not_must_use)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#return_self_not_must_use
warning: unused `std::result::Result` that must be used
  --> src/main.rs:19:5
   |
19 |     Ok(()).log_err("oops");
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default
   = note: this `Result` may be an `Err` variant, which should be handled

I expected to see this happen: No return_self_not_must_use warning, because the compiler already reports unused_must_use for this code.

Version

1.60.0-nightly (2022-01-13 22e491ac7ed454d34669)

Additional Labels

No response

Start by locating the implementation and existing tests for the return_self_must_use lint, then reproduce the supplied generic-trait example with the Rust playground configuration. Trace how the lint determines whether a return type is already #[must_use]. Done means the redundant lint warning is absent for this case while the lint still reports applicable non-must-use returns.