rust: init: make `PinInit<T, E>` a supertrait of `Init<T, E>`

Remove the blanket implementation of `PinInit<T, E> for I where I:
Init<T, E>`. This blanket implementation prevented custom types that
implement `PinInit`.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20230814084602.25699-12-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init.rs

@@ -799,7 +799,7 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
 ///
 /// [`Arc<T>`]: crate::sync::Arc
 #[must_use = "An initializer must be used in order to create its value."]
-pub unsafe trait Init<T: ?Sized, E = Infallible>: Sized {
+pub unsafe trait Init<T: ?Sized, E = Infallible>: PinInit<T, E> {
     /// Initializes `slot`.
     ///
     /// # Safety
@@ -810,18 +810,6 @@ pub unsafe trait Init<T: ?Sized, E = Infallible>: Sized {
     unsafe fn __init(self, slot: *mut T) -> Result<(), E>;
 }
 
-// SAFETY: Every in-place initializer can also be used as a pin-initializer.
-unsafe impl<T: ?Sized, E, I> PinInit<T, E> for I
-where
-    I: Init<T, E>,
-{
-    unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
-        // SAFETY: `__init` meets the same requirements as `__pinned_init`, except that it does not
-        // require `slot` to not move after init.
-        unsafe { self.__init(slot) }
-    }
-}
-
 /// Creates a new [`PinInit<T, E>`] from the given closure.
 ///
 /// # Safety
@@ -964,6 +952,13 @@ unsafe impl<T, E> Init<T, E> for T {
     }
 }
 
+// SAFETY: Every type can be initialized by-value. `__pinned_init` calls `__init`.
+unsafe impl<T, E> PinInit<T, E> for T {
+    unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
+        unsafe { self.__init(slot) }
+    }
+}
+
 /// Smart pointer that can initialize memory in-place.
 pub trait InPlaceInit<T>: Sized {
     /// Use the given pin-initializer to pin-initialize a `T` inside of a new smart pointer of this

rust/kernel/init/__internal.rs

@@ -32,6 +32,18 @@ where
     }
 }
 
+// SAFETY: While constructing the `InitClosure`, the user promised that it upholds the
+// `__pinned_init` invariants.
+unsafe impl<T: ?Sized, F, E> PinInit<T, E> for InitClosure<F, T, E>
+where
+    F: FnOnce(*mut T) -> Result<(), E>,
+{
+    #[inline]
+    unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
+        (self.0)(slot)
+    }
+}
+
 /// This trait is only implemented via the `#[pin_data]` proc-macro. It is used to facilitate
 /// the pin projections within the initializers.
 ///