Generic parameter definitions are not cfg-stripped before #[derive] macros are invoked #75930
Description
The following code (repository at https://github.com/Aaron1011/derive-bug):
// lib.rs use proc_macro::TokenStream; #[proc_macro_derive(MyDerive)] pub fn my_derive(input: TokenStream) -> TokenStream { println!("Derive input: {}", input); TokenStream::new() } // main.rs #[derive(derive_bug::MyDerive)] struct Foo<#[cfg(FALSE)] T> { #[cfg(FALSE)] field: T, val: String, array: [u8; { #[cfg(FALSE)] struct Bar; 0 }] } fn main() {}produces the following output:
Derive input: struct Foo < #[cfg(FALSE)] T > { val : String, array : [u8 ; { 0 }], } Note how all #[cfg(FALSE)] targets are stripped, except for the generic parameter T. This seems quite inconsistent.
Tested on rustc 1.47.0-nightly (de521cbb3 2020-08-21)