rust: pin-init: change blanket impls for [Pin]Init and add one for Result<T, E>

Remove the error from the blanket implementations `impl<T, E> Init<T, E>
for T` (and also for `PinInit`). Add implementations for `Result<T, E>`.

This allows one to easily construct (un)conditional failing
initializers. It also improves the compatibility with APIs that do not
use pin-init, because users can supply a `Result<T, E>` to a  function
taking an `impl PinInit<T, E>`.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Link: 58612514b2
[ Also fix a compile error in block. - Benno ]
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/all/20250529081027.297648-2-lossin@kernel.org
[ Add title prefix `rust: pin-init`. - Benno ]
Signed-off-by: Benno Lossin <lossin@kernel.org>

rust/kernel/block/mq/tag_set.rs

@@ -9,7 +9,7 @@ use core::pin::Pin;
 use crate::{
     bindings,
     block::mq::{operations::OperationsVTable, request::RequestDataWrapper, Operations},
-    error,
+    error::{self, Result},
     prelude::try_pin_init,
     types::Opaque,
 };
@@ -41,7 +41,7 @@ impl<T: Operations> TagSet<T> {
         // SAFETY: `blk_mq_tag_set` only contains integers and pointers, which
         // all are allowed to be 0.
         let tag_set: bindings::blk_mq_tag_set = unsafe { core::mem::zeroed() };
-        let tag_set = core::mem::size_of::<RequestDataWrapper>()
+        let tag_set: Result<_> = core::mem::size_of::<RequestDataWrapper>()
             .try_into()
             .map(|cmd_size| {
                 bindings::blk_mq_tag_set {
@@ -56,12 +56,14 @@ impl<T: Operations> TagSet<T> {
                     nr_maps: num_maps,
                     ..tag_set
                 }
-            });
+            })
+            .map(Opaque::new)
+            .map_err(|e| e.into());
 
         try_pin_init!(TagSet {
-            inner <- PinInit::<_, error::Error>::pin_chain(Opaque::new(tag_set?), |tag_set| {
+            inner <- tag_set.pin_chain(|tag_set| {
                 // SAFETY: we do not move out of `tag_set`.
-                let tag_set = unsafe { Pin::get_unchecked_mut(tag_set) };
+                let tag_set: &mut Opaque<_> = unsafe { Pin::get_unchecked_mut(tag_set) };
                 // SAFETY: `tag_set` is a reference to an initialized `blk_mq_tag_set`.
                 error::to_result( unsafe { bindings::blk_mq_alloc_tag_set(tag_set.get())})
             }),

rust/pin-init/src/lib.rs

@@ -1391,8 +1391,8 @@ where
 }
 
 // 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> {
+unsafe impl<T> Init<T> for T {
+    unsafe fn __init(self, slot: *mut T) -> Result<(), Infallible> {
         // SAFETY: `slot` is valid for writes by the safety requirements of this function.
         unsafe { slot.write(self) };
         Ok(())
@@ -1401,14 +1401,36 @@ unsafe impl<T, E> Init<T, E> for T {
 
 // 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> {
+unsafe impl<T> PinInit<T> for T {
+    unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), Infallible> {
         // SAFETY: `slot` is valid for writes by the safety requirements of this function.
         unsafe { slot.write(self) };
         Ok(())
     }
 }
 
+// SAFETY: when the `__init` function returns with
+// - `Ok(())`, `slot` was initialized and all pinned invariants of `T` are upheld.
+// - `Err(err)`, slot was not written to.
+unsafe impl<T, E> Init<T, E> for Result<T, E> {
+    unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
+        // SAFETY: `slot` is valid for writes by the safety requirements of this function.
+        unsafe { slot.write(self?) };
+        Ok(())
+    }
+}
+
+// SAFETY: when the `__pinned_init` function returns with
+// - `Ok(())`, `slot` was initialized and all pinned invariants of `T` are upheld.
+// - `Err(err)`, slot was not written to.
+unsafe impl<T, E> PinInit<T, E> for Result<T, E> {
+    unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
+        // SAFETY: `slot` is valid for writes by the safety requirements of this function.
+        unsafe { slot.write(self?) };
+        Ok(())
+    }
+}
+
 /// Smart pointer containing uninitialized memory and that can write a value.
 pub trait InPlaceWrite<T> {
     /// The type `Self` turns into when the contents are initialized.