[PATCH v2 3/4] rust: alloc: implement `Borrow` and `BorrowMut` for `KBox`

[Alexandre Courbot]

Implement `Borrow<T>` and `BorrowMut<T>` for `KBox<T>`. This allows
`KBox<T>` to be used in generic APIs asking for types implementing those
traits. `T` and `&mut T` also implement those traits allowing users to
use either owned, borrowed and heap-owned values.

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
 rust/kernel/alloc/kbox.rs | 61 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c386ff771d506a2eb4c211a93ea9b59bc04c93f5..0e3cbe809a878766ee8f8508ed0d30b4f8afaa07 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -6,6 +6,7 @@
 use super::allocator::{KVmalloc, Kmalloc, Vmalloc};
 use super::{AllocError, Allocator, Flags};
 use core::alloc::Layout;
+use core::borrow::{Borrow, BorrowMut};
 use core::fmt;
 use core::marker::PhantomData;
 use core::mem::ManuallyDrop;
@@ -499,6 +500,66 @@ fn deref_mut(&mut self) -> &mut T {
     }
 }
 
+/// Allows `Box<T>` to be used as a `Borrow<T>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::alloc::KBox;
+/// struct Foo<B: Borrow<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `KBox`.
+/// let foo_box = Foo(KBox::new(1, GFP_KERNEL)?);
+///
+/// let i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T, A> Borrow<T> for Box<T, A>
+where
+    T: ?Sized,
+    A: Allocator,
+{
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
+/// Allows `Box<T>` to be used as a `BorrowMut<T>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// # use kernel::alloc::KBox;
+/// struct Foo<B: BorrowMut<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `KBox`.
+/// let foo_box = Foo(KBox::new(1, GFP_KERNEL)?);
+///
+/// let mut i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&mut i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T, A> BorrowMut<T> for Box<T, A>
+where
+    T: ?Sized,
+    A: Allocator,
+{
+    fn borrow_mut(&mut self) -> &mut T {
+        self.deref_mut()
+    }
+}
+
 impl<T, A> fmt::Display for Box<T, A>
 where
     T: ?Sized + fmt::Display,

-- 
2.49.0