rust: pin-init: Implement Wrapper for UnsafePinned behind feature flag.

Add the `unsafe-pinned` feature which gates the `Wrapper`
implementation of the `core::pin::UnsafePinned` struct.

For now this is just a cargo feature, but once `core::pin::UnsafePinned`
is stable a config flag can be added to allow the usage of this
implementation in the linux kernel.

Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: Rust-for-Linux/pin-init@99cb193
[ Fixed commit authorship. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: onestacked

rust/pin-init/src/lib.rs

@@ -269,6 +269,10 @@
 #![forbid(missing_docs, unsafe_op_in_unsafe_fn)]
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(feature = "alloc", feature(allocator_api))]
+#![cfg_attr(
+    all(feature = "unsafe-pinned", CONFIG_RUSTC_HAS_UNSAFE_PINNED),
+    feature(unsafe_pinned)
+)]
 
 use core::{
     cell::UnsafeCell,
@@ -1557,3 +1561,11 @@ impl<T> Wrapper<T> for MaybeUninit<T> {
         unsafe { cast_pin_init(value_init) }
     }
 }
+
+#[cfg(all(feature = "unsafe-pinned", CONFIG_RUSTC_HAS_UNSAFE_PINNED))]
+impl<T> Wrapper<T> for core::pin::UnsafePinned<T> {
+    fn pin_init<E>(init: impl PinInit<T, E>) -> impl PinInit<Self, E> {
+        // SAFETY: `UnsafePinned<T>` has a compatible layout to `T`.
+        unsafe { cast_pin_init(init) }
+    }
+}