Attributes on impl parameters are not forwarded to body

[azriel91]

Heya, I was implementing a trait function with #[cfg(..)] in its parameters, and encountered the following error:

// within trait impl
async fn cfg_param(
    &self,
    #[cfg(not(feature = "something"))] param: u8,
    #[cfg(feature = "something")] _unused: u8,
    //                            ^^^^^^^
    // cannot find value `_unused` in this scope
) -> u8 {
    #[cfg(not(feature = "something"))]
    { param }
    #[cfg(feature = "something")]
    { 1 }
}

The expanded code body shows:

let __self = self;
let param = param;
let _unused = _unused;
let __ret: u8 = { #[cfg(not(feature = "something"))] { param } };
#[allow(unreachable_code)] __ret

Since _unused is not present in the function signature, the expanded code should likely just omit it.

227 forwards the attributes, such that the expanded code returns:

let __self = self;
#[cfg(not(feature = "something"))]
let param = param;
let __ret: u8 = { #[cfg(not(feature = "something"))] { param } };
#[allow(unreachable_code)] __ret

The _unused statement isn't present since it's been removed by the expansion.