extra_unused_type_parameters should not warn external macros

[taiki-e]

Summary

extra_unused_type_parameters currently warns of code generated by external macros, but it should not. (there is no way for the user to fix it.)

Mentioning @mkrasnitski who implemented this lint in #10028.

Lint Name

extra_unused_type_parameters

Reproducer

I tried this code:

# Cargo.toml
[package]
name = "repro"
version = "0.1.0"
edition = "2021"

[dependencies]
static_assertions = "=1.1.0"

// src/lib.rs
#![warn(clippy::extra_unused_type_parameters)]

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

static_assertions::assert_impl_all!(S: Send);

I saw this happen:

warning: type parameter goes unused in function definition
 --> src/lib.rs:6:1
  |
6 | static_assertions::assert_impl_all!(S: 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)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: this warning originates in the macro `static_assertions::assert_impl_all` (in Nightly builds, run with -Z macro-backtrace for more info)

I expected to see this happen: no warnings

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

[chorman0773]

This is still triggering on the latest nightly (I did rustup update and rebuild to check), with a derive macro.

The linted-on code is

#[repr(C)]
#[derive(Zeroable, Pod, Clone, Copy)]
pub struct O65FixedHeader {
    id1: [u8; 2],
    magic: [u8; 3],
    ver: u8,
    mode: u16,
}

Which triggers this output

error: type parameter goes unused in function definition
 --> binfmt/src/o65.rs:1:1
  |
1 | / #[repr(C)]
2 | | #[derive(Zeroable, Pod, Clone, Copy)]
3 | | pub struct O65FixedHeader {
4 | |     id1: [u8; 2],
... |
7 | |     mode: u16,
8 | | }
  | |_^
  |
  = help: consider removing the parameter
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_type_parameters

IMO, I'm suspicious this should trigger on macro-expanded tokens at all, given that a common technique to typecheck some type against a trait is to define pub fn __check<T: MyTrait>(){} then call it with turbofish.

[mkrasnitski]

That's because the changes haven't made it into nightly rust yet. Nightly pulls from clippy's HEAD on a biweekly basis, and the next time it does this is tomorrow, the 23rd. So, #10321 should make it in then, as well as hopefully #10392, if it gets merged in time.