[PATCH v2 10/12] rust: init: make `PinInit<T, E>` a supertrait of `Init<T, E>`

[Benno Lossin <benno.lossin@proton.me>]

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

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
---
 rust/kernel/init.rs            | 20 +++++++-------------
 rust/kernel/init/__internal.rs | 12 ++++++++++++
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index fa1ebdbf5f4b..3c7cd36a424b 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -805,7 +805,7 @@ 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
@@ -816,18 +816,6 @@ 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
@@ -968,6 +956,12 @@ unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
         Ok(())
     }
 }
+// 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 {
diff --git a/rust/kernel/init/__internal.rs b/rust/kernel/init/__internal.rs
index 7abd1fb65e41..12e195061525 100644
--- a/rust/kernel/init/__internal.rs
+++ b/rust/kernel/init/__internal.rs
@@ -32,6 +32,18 @@ unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
     }
 }
 
+// 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.
 ///
-- 
2.41.0