[PATCH v2 4/4] rust: str: implement `Borrow` and `BorrowMut` for `CString`

[Alexandre Courbot <acourbot@nvidia.com>]

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

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/str.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index a927db8e079c3597860880947a03959e1d6d712e..36d46443c269a73fa1246c52ab8558bae3e439ee 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -3,6 +3,7 @@
 //! String representations.
 
 use crate::alloc::{flags::*, AllocError, KVec};
+use core::borrow::{Borrow, BorrowMut};
 use core::fmt::{self, Write};
 use core::ops::{self, Deref, DerefMut, Index};
 
@@ -911,6 +912,54 @@ fn deref_mut(&mut self) -> &mut Self::Target {
     }
 }
 
+/// Allows `CString` to be used as a `Borrow<CStr>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::str::{CStr, CString};
+/// # use kernel::fmt;
+/// struct Foo<B: Borrow<CStr>>(B);
+///
+/// // Owned instance using `CString`.
+/// let foo_owned = Foo(CString::try_from_fmt(fmt!("{}", "abc"))?);
+///
+/// let str_data = b"abc\0";
+/// // Borrowed from `str_data`.
+/// let foo_borrowed = Foo(CStr::from_bytes_with_nul(str_data)?);
+/// # Ok::<(), Error>(())
+/// ```
+impl Borrow<CStr> for CString {
+    fn borrow(&self) -> &CStr {
+        self.deref()
+    }
+}
+
+/// Allows `CString` to be used as a `BorrowMut<CStr>`.
+///
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// # use kernel::str::{CStr, CString};
+/// # use kernel::fmt;
+/// struct Foo<B: BorrowMut<CStr>>(B);
+///
+/// // Owned instance using `CString`.
+/// let foo_owned = Foo(CString::try_from_fmt(fmt!("{}", "abc"))?);
+///
+/// let mut str_data = [b'a', b'b', b'c', 0];
+/// // Borrowed from `str_data`.
+/// let foo_borrowed = Foo(unsafe { CStr::from_bytes_with_nul_unchecked_mut(&mut str_data) });
+/// # Ok::<(), Error>(())
+/// ```
+impl BorrowMut<CStr> for CString {
+    fn borrow_mut(&mut self) -> &mut CStr {
+        self.deref_mut()
+    }
+}
+
 impl<'a> TryFrom<&'a CStr> for CString {
     type Error = AllocError;
 

-- 
2.49.0