add cast_[pin_]init functions to change the initialized type

These functions cast the given pointer from one type to another. They
are particularly useful when initializing transparent wrapper types.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: BennoLossin

src/lib.rs

@@ -1216,6 +1216,28 @@ pub const unsafe fn init_from_closure<T: ?Sized, E>(
     __internal::InitClosure(f, PhantomData)
 }
 
+/// Changes the to be initialized type.
+///
+/// # Safety
+///
+/// - `T` must be castable to `U`
+pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
+    // SAFETY: initialization delegated to a valid initializer. Cast is valid by function safety
+    // requirements.
+    unsafe { pin_init_from_closure(|ptr: *mut U| init.__pinned_init(ptr.cast::<T>())) }
+}
+
+/// Changes the to be initialized type.
+///
+/// # Safety
+///
+/// - `T` must be castable to `U`
+pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
+    // SAFETY: initialization delegated to a valid initializer. Cast is valid by function safety
+    // requirements.
+    unsafe { init_from_closure(|ptr: *mut U| init.__init(ptr.cast::<T>())) }
+}
+
 /// An initializer that leaves the memory uninitialized.
 ///
 /// The initializer is a no-op. The `slot` memory is not changed.