extra_unused_type_parameters probably should not warn empty functions

[taiki-e]

Summary

It is a popular pattern to define an empty function with an unused type parameter with trait bounds to ensure that the type implements the trait.
extra_unused_type_parameters probably should not warn this pattern (at least by default).

• https://github.com/dtolnay/proc-macro2/blob/4822d05db17faa0639a3e0200aaf79cdacac16e0/tests/marker.rs#L12
• https://github.com/tokio-rs/tokio/blob/01bb1ecf4dee073da65dfede4c845ac27c555af0/tokio/src/lib.rs#L633
• https://github.com/tokio-rs/axum/blob/main/axum/src/test_helpers/mod.rs#L8-L10

Mentioning @mkrasnitski who implemented this lint in #10028.

Lint Name

extra_unused_type_parameters

Reproducer

I tried this code:

#![warn(clippy::extra_unused_type_parameters)]

#[allow(dead_code)]
fn assert_send<T: Send>() {}

#[allow(dead_code)]
struct S(());

const _: fn() = || {
    assert_send::<S>();
};

I saw this happen:

warning: type parameter goes unused in function definition
 --> src/lib.rs:4:15
  |
4 | fn assert_send<T: Send>() {}
  |               ^^^^^^^^^
  |
  = help: consider removing the parameter
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_type_parameters
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![warn(clippy::extra_unused_type_parameters)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I expected to see this happen: no warning

Version

rustc 1.69.0-nightly (2773383a3 2023-02-10)
binary: rustc
commit-hash: 2773383a314a4b8f481ce2bed12c32de794ffbe9
commit-date: 2023-02-10
host: aarch64-apple-darwin
release: 1.69.0-nightly
LLVM version: 15.0.7

Additional Labels

No response

[mkrasnitski]

Hmm, this makes me think about the best way to address this. Obviously, the original intent of the lint here is more-or-less correct, in that the only valid way to use the function is by adding a turbofish. But in this case that concern is irrelevant.

It's also a bit difficult to pick exactly which features of the function to check for when turning off the lint. I think maybe a sane choice would be: if a parameter has a bound on it and the function is empty, then don't consider that parameter unused. Of course, the question is how to define if a function is empty - should only the body be empty, or should the function take no arguments as well? I think just checking the body might be the better option here.

Open to opinions here - I'm not sure if avoiding false negatives or false positives is more important.