New false positive expl_impl_clone_on_copy lint in expansion of custom tokens

[dtolnay]

The following began emitting lints in nightly-2021-03-14. Fine in nightly-2021-03-13 and earlier.

#![deny(clippy::all, clippy::pedantic)]

syn::custom_keyword!(test);

error: you are implementing `Clone` explicitly on a `Copy` type
 --> src/main.rs:3:1
  |
3 | syn::custom_keyword!(test);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/main.rs:1:22
  |
1 | #![deny(clippy::all, clippy::pedantic)]
  |                      ^^^^^^^^^^^^^^^^
  = note: `#[deny(clippy::expl_impl_clone_on_copy)]` implied by `#[deny(clippy::pedantic)]`
note: consider deriving `Clone` or removing `Copy`
 --> src/main.rs:3:1
  |
3 | syn::custom_keyword!(test);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#expl_impl_clone_on_copy
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

The warning is a false positive because there is not a way to accomplish the same effect (impl in downstream crate conditional on Syn's clone-impls feature) with derive.

syn/src/custom_keyword.rs

Lines 193 to 215 in 92b1129

	// Not public API.
	#[cfg(feature = "clone-impls")]
	#[doc(hidden)]
	#[macro_export]
	macro_rules! impl_clone_for_custom_keyword {
	($ident:ident) => {
	impl $crate::__private::Copy for $ident {}
	impl $crate::__private::Clone for $ident {
	fn clone(&self) -> Self {
	*self
	}
	}
	};
	}
	// Not public API.
	#[cfg(not(feature = "clone-impls"))]
	#[doc(hidden)]
	#[macro_export]
	macro_rules! impl_clone_for_custom_keyword {
	($ident:ident) => {};
	}