The `#[pointee]` attribute is required even if there is only one generic parameter

[Darksonn]

Currently, the following code triggers a compilation failure.

#![feature(derive_smart_pointer)]
use core::marker::SmartPointer;

#[repr(transparent)]
#[derive(SmartPointer)]
struct MyArc<T: ?Sized> {
    inner: *mut T,
}

error: At least one generic type should be designated as `#[pointee]` in order to derive `SmartPointer` traits
 --> src/lib.rs:5:10
  |
5 | #[derive(SmartPointer)]
  |          ^^^^^^^^^^^^
  |
  = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)

However, the RFC says that specifying #[pointee] when the struct has only one type parameter is allowed, but not required.

See also this patch on the LKML.

Tracking issue: #123430
cc @dingxiangfei2009

[dingxiangfei2009]

@rustbot claim

[Darksonn]

Oh, also, the error message says "At least one generic" but it's not valid to have several pointees.