return_self_not_must_use strange interaction with trait methods

Summary

Should this run on traits at all?
Currently whether the trait is public or not affects the warning.

Lint Name

return_self_not_must_use

Reproducer

I tried this code:

pub trait Trait1 {
    fn set_config1(self) -> Self; // always warns
}

trait Trait2 {
    fn set_config2(self) -> Self; // never warns
}

#[must_use]
struct Builder1;

impl Trait1 for Builder1 {
    fn set_config1(self) -> Self {
        Builder1
    }
}


impl Trait2 for Builder1 {
    fn set_config2(self) -> Self {
        Builder1
    }
}

struct Builder2;

impl Trait1 for Builder2 {
    fn set_config1(self) -> Self {
        Builder2
    }
}

impl Trait2 for Builder2 {
    fn set_config2(self) -> Self {
        Builder2
    }
}

fn main() {
    Builder1.set_config1();
    Builder1.set_config2();
    Builder2.set_config1();
    Builder2.set_config2();
}

I saw this happen:

warning: missing `#[must_use]` attribute on a method returning `Self`
 --> src/main.rs:2:5
  |
2 |     fn set_config1(self) -> Self; // always warns
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = 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 `Builder1` that must be used
  --> src/main.rs:40:5
   |
40 |     Builder1.set_config1();
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default

warning: unused `Builder1` that must be used
  --> src/main.rs:41:5
   |
41 |     Builder1.set_config2();
   |     ^^^^^^^^^^^^^^^^^^^^^^^

I expected to see this happen:

I'm not really sure if we should be warning on traits at all, but we should be consistent about it regardless of if the trait is public or not.

Version

rustc 1.59.0-nightly (399ba6bb3 2022-01-03)
binary: rustc
commit-hash: 399ba6bb377ce02224b57c4d6e127e160fa76b34
commit-date: 2022-01-03
host: x86_64-unknown-linux-gnu
release: 1.59.0-nightly
LLVM version: 13.0.0

Additional Labels

No response

Start by locating the implementation and tests for the return_self_not_must_use lint, then run the Rust reproducer to compare public and private traits. Determine the intended behavior for trait methods, make handling consistent, and add regression coverage for the shown combinations of traits and builders.