Implement PinInitable for UnsafePinned behind feature flag.

Add the `unsafe-pinned` feature which gates the `PinInitable`
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>

Author: onestacked

Cargo.toml

@@ -21,6 +21,7 @@ pin-init-internal = { path = "./internal", version = "=0.0.5" }
 default = ["std", "alloc"]
 std = []
 alloc = []
+unsafe-pinned = []
 
 [build-dependencies]
 rustc_version = "0.4"

build.rs

@@ -3,6 +3,7 @@ use rustc_version::{version, Version};
 fn main() {
     println!("cargo::rustc-check-cfg=cfg(RUSTC_LINT_REASONS_IS_STABLE)");
     println!("cargo::rustc-check-cfg=cfg(RUSTC_NEW_UNINIT_IS_STABLE)");
+    println!("cargo::rustc-check-cfg=cfg(RUSTC_HAS_UNSAFE_PINNED)");
     if version().unwrap() >= Version::parse("1.81.0").unwrap()
         || version().unwrap() >= Version::parse("1.81.0-nightly").unwrap()
     {
@@ -11,4 +12,7 @@ fn main() {
     if version().unwrap() >= Version::parse("1.82.0").unwrap() {
         println!("cargo:rustc-cfg=RUSTC_NEW_UNINIT_IS_STABLE");
     }
+    if version().unwrap() >= Version::parse("1.88.0-nightly").unwrap() {
+        println!("cargo:rustc-cfg=RUSTC_HAS_UNSAFE_PINNED");
+    }
 }

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", RUSTC_HAS_UNSAFE_PINNED),
+    feature(unsafe_pinned)
+)]
 
 use core::{
     cell::UnsafeCell,
@@ -1556,3 +1560,12 @@ impl<T> PinInitable<T> for MaybeUninit<T> {
         unsafe { cast_pin_init(value_init) }
     }
 }
+
+#[cfg(all(feature = "unsafe-pinned", RUSTC_HAS_UNSAFE_PINNED))]
+impl<T> PinInitable<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) }
+    }
+}