[PATCH 1/2] rust: pin-init: improve safety documentation for `impl<T> [Pin]Init<T> for T`

[Benno Lossin <lossin@kernel.org>]

The inner SAFETY comments were missing since commit 5cfe7bef6751 ("rust:
enable `clippy::undocumented_unsafe_blocks` lint").

Also rework the implementation of `__pinned_init` to better justify the
SAFETY comment.

Link: https://github.com/Rust-for-Linux/pin-init/pull/62/commits/df925b2e27d499b7144df7e62b01acb00d4b94b8
Signed-off-by: Benno Lossin <lossin@kernel.org>
---
 rust/pin-init/src/lib.rs | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 9ab34036e6bc..d1c3ca5cfff4 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -1390,20 +1390,22 @@ pub fn pin_init_array_from_fn<I, const N: usize, T, E>(
     unsafe { pin_init_from_closure(init) }
 }
 
-// SAFETY: Every type can be initialized by-value.
+// SAFETY: the `__init` function always returns `Ok(())` and initializes every field of `slot`.
 unsafe impl<T, E> Init<T, E> for T {
     unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
-        // SAFETY: TODO.
+        // SAFETY: `slot` is valid for writes by the safety requirements of this function.
         unsafe { slot.write(self) };
         Ok(())
     }
 }
 
-// SAFETY: Every type can be initialized by-value. `__pinned_init` calls `__init`.
+// SAFETY: the `__pinned_init` function always returns `Ok(())` and initializes every field of
+// `slot`. Additionally, all pinning invariants of `T` are upheld.
 unsafe impl<T, E> PinInit<T, E> for T {
     unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
-        // SAFETY: TODO.
-        unsafe { self.__init(slot) }
+        // SAFETY: `slot` is valid for writes by the safety requirements of this function.
+        unsafe { slot.write(self) };
+        Ok(())
     }
 }
 

base-commit: 1ce98bb2bb30713ec4374ef11ead0d7d3e856766
-- 
2.49.0