Wrong error when `derive(CoercePointee)` is used with `cfg_attr`'ed `pointee` and 2+ generic argument type

[WaffleLapkin]

rust-analyzer version: rust-analyzer 1.91.0-nightly (425a9c0 2025-08-17)

rustc version: rustc 1.91.0-nightly (425a9c0a0 2025-08-17)

editor or extension: helix

code snippet to reproduce:

#![feature(derive_coerce_pointee)]
use std::marker::CoercePointee;
use std::ptr::NonNull;

#[derive(CoercePointee)]
#[repr(transparent)]
pub struct Ptr<T, #[cfg_attr(all(), pointee)] V: ?Sized> {
    ptr: NonNull<V>,
    _ghost: PhantomData<T>,
}

r-a emits an error saying "exactly one generic type parameter must be marked as #[pointee] to derive CoercePointee", rustc compiles this code successfully.

[ChayimFriedman2]

Probably will be fixed by #20316, which refactors how we handle cfg and cfg_attr passed to macros. But it will have to wait until we complete the migration to the next solver.