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

.github/workflows/ci.yml

@@ -123,7 +123,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - run: cargo install cargo-hack
       - run: git config user.name "github-runner" && git config user.email "<>"
-      - run: git rebase --exec 'cargo hack --clean-per-run --feature-powerset --exclude-features alloc --exclude-features default --version-range 1.82.. --clean-per-version check --locked' --exec 'cargo clean' --root
+      - run: git rebase --exec 'cargo hack --clean-per-run --feature-powerset --exclude-features alloc --exclude-features default --exclude-features unsafe-pinned --version-range 1.82.. --clean-per-version check --locked' --exec 'cargo clean' --root
   nightly-msrv:
     runs-on: ubuntu-latest
     steps:
@@ -137,7 +137,7 @@ jobs:
       - run: cargo install cargo-hack
       - run: cargo install cargo-expand
       - run: git config user.name "github-runner" && git config user.email "<>"
-      - run: git rebase --exec 'cargo hack --clean-per-run --feature-powerset --version-range 1.78.. --clean-per-version check --locked' --exec 'cargo clean' --root
+      - run: git rebase --exec 'cargo hack --clean-per-run --feature-powerset --exclude-features unsafe-pinned --version-range 1.78.. --clean-per-version check --locked' --exec 'cargo clean' --root
         env:
           RUSTC_BOOTSTRAP: 1
   os-check:

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"

src/lib.rs

@@ -1549,3 +1549,12 @@ impl<T> PinInitable<T> for UnsafeCell<T> {
         unsafe { cast_pin_init(value_init) }
     }
 }
+
+#[cfg(feature = "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) }
+    }
+}