linux-kernel - [PATCH 5/6] rust: add `CStr` methods matching `core::ffi::CStr`

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Message-Id: <20250704-core-cstr-prepare-v1-5-a91524037783@gmail.com>
Date: Fri, 04 Jul 2025 16:14:56 -0400
From: Tamir Duberstein <tamird@...il.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>, 
 Viresh Kumar <viresh.kumar@...aro.org>, Danilo Krummrich <dakr@...nel.org>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Nishanth Menon <nm@...com>, Stephen Boyd <sboyd@...nel.org>, 
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
 nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, 
 rust-for-linux@...r.kernel.org, Tamir Duberstein <tamird@...il.com>
Subject: [PATCH 5/6] rust: add `CStr` methods matching `core::ffi::CStr`

Prepare for replacing `CStr` with `core::ffi::CStr` by soft-deprecating
methods which don't exist on `core::ffi::CStr`.

We could keep `as_bytes{,_with_nul}` through an extension trait but
seeing as we have to introduce `as_char_ptr_in_const_context` as a free
function, we may as well introduce `to_bytes{,_with_nul}` here to allow
downstream code to migrate in one cycle rather than two.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Signed-off-by: Tamir Duberstein <tamird@...il.com>
---
 rust/kernel/str.rs | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index f326f0c40ab0..cbb357fc0111 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -175,6 +175,15 @@ macro_rules! b_str {
     }};
 }

+/// Returns a C pointer to the string.
+// It is a free function rather than a method on an extension trait because:
+//
+// - error[E0379]: functions in trait impls cannot be declared const
+#[inline]
+pub const fn as_char_ptr_in_const_context(c_str: &CStr) -> *const c_char {
+    c_str.0.as_ptr()
+}
+
 /// Possible errors when using conversion functions in [`CStr`].
 #[derive(Debug, Clone, Copy)]
 pub enum CStrConvertError {
@@ -294,23 +303,45 @@ pub unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut CStr {
     }

     /// Returns a C pointer to the string.
+    ///
+    /// Using this function in a const context is deprecated in favor of
+    /// [`as_char_ptr_in_const_context`] in preparation for replacing `CStr` with `core::ffi::CStr`
+    /// which does not have this method.
     #[inline]
     pub const fn as_char_ptr(&self) -> *const c_char {
-        self.0.as_ptr()
+        as_char_ptr_in_const_context(self)
     }

     /// Convert the string to a byte slice without the trailing `NUL` byte.
     #[inline]
-    pub fn as_bytes(&self) -> &[u8] {
+    pub fn to_bytes(&self) -> &[u8] {
         &self.0[..self.len()]
     }

+    /// Convert the string to a byte slice without the trailing `NUL` byte.
+    ///
+    /// This function is deprecated in favor of [`Self::to_bytes`] in preparation for replacing
+    /// `CStr` with `core::ffi::CStr` which does not have this method.
+    #[inline]
+    pub fn as_bytes(&self) -> &[u8] {
+        self.to_bytes()
+    }
+
     /// Convert the string to a byte slice containing the trailing `NUL` byte.
     #[inline]
-    pub const fn as_bytes_with_nul(&self) -> &[u8] {
+    pub const fn to_bytes_with_nul(&self) -> &[u8] {
         &self.0
     }

+    /// Convert the string to a byte slice containing the trailing `NUL` byte.
+    ///
+    /// This function is deprecated in favor of [`Self::to_bytes_with_nul`] in preparation for
+    /// replacing `CStr` with `core::ffi::CStr` which does not have this method.
+    #[inline]
+    pub const fn as_bytes_with_nul(&self) -> &[u8] {
+        self.to_bytes_with_nul()
+    }
+
     /// Yields a [`&str`] slice if the [`CStr`] contains valid UTF-8.
     ///
     /// If the contents of the [`CStr`] are valid UTF-8 data, this

-- 
2.50.0