rust: sync: change error type of constructor functions

Change the error type of the constructors of `Arc` and `UniqueArc` to be
`AllocError` instead of `Error`. This makes the API more clear as to
what can go wrong when calling `try_new` or its variants.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20230408122429.1103522-4-y86-dev@protonmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: BennoLossin

rust/kernel/sync/arc.rs

@@ -17,11 +17,11 @@
 
 use crate::{
     bindings,
-    error::Result,
     types::{ForeignOwnable, Opaque},
 };
 use alloc::boxed::Box;
 use core::{
+    alloc::AllocError,
     fmt,
     marker::{PhantomData, Unsize},
     mem::{ManuallyDrop, MaybeUninit},
@@ -152,7 +152,7 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 
 impl<T> Arc<T> {
     /// Constructs a new reference counted instance of `T`.
-    pub fn try_new(contents: T) -> Result<Self> {
+    pub fn try_new(contents: T) -> Result<Self, AllocError> {
         // INVARIANT: The refcount is initialised to a non-zero value.
         let value = ArcInner {
             // SAFETY: There are no safety requirements for this FFI call.
@@ -472,15 +472,15 @@ pub struct UniqueArc<T: ?Sized> {
 
 impl<T> UniqueArc<T> {
     /// Tries to allocate a new [`UniqueArc`] instance.
-    pub fn try_new(value: T) -> Result<Self> {
+    pub fn try_new(value: T) -> Result<Self, AllocError> {
         Ok(Self {
             // INVARIANT: The newly-created object has a ref-count of 1.
             inner: Arc::try_new(value)?,
         })
     }
 
     /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
-    pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>> {
+    pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
         Ok(UniqueArc::<MaybeUninit<T>> {
             // INVARIANT: The newly-created object has a ref-count of 1.
             inner: Arc::try_new(MaybeUninit::uninit())?,