[PATCH v2 2/2] rust: disallow use of `CStr::as_ptr`

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

As kernel always use unsigned char and not the platform ABI's default, an
user should always use `as_char_ptr` provided via `CStrExt` instead.
Therefore configure `disallow-methods` feature of clippy to catch incorrect
usage.

Acked-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 .clippy.toml       | 5 +++++
 rust/kernel/str.rs | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/.clippy.toml b/.clippy.toml
index 137f41d203de..4654b522ae03 100644
--- a/.clippy.toml
+++ b/.clippy.toml
@@ -9,3 +9,8 @@ disallowed-macros = [
     # it here, see: https://github.com/rust-lang/rust-clippy/issues/11303.
     { path = "kernel::dbg", reason = "the `dbg!` macro is intended as a debugging tool", allow-invalid = true },
 ]
+
+[[disallowed-methods]]
+path = "core::ffi::CStr::as_ptr"
+replacement = "kernel::prelude::CStrExt::as_char_ptr"
+reason = "kernel's `char` is always unsigned, use `as_char_ptr` instead"
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index fa87779d2253..08b8e2ebc8ad 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -189,6 +189,7 @@ macro_rules! b_str {
 //
 // - error[E0379]: functions in trait impls cannot be declared const
 #[inline]
+#[expect(clippy::disallowed_methods, reason = "internal implementation")]
 pub const fn as_char_ptr_in_const_context(c_str: &CStr) -> *const c_char {
     c_str.as_ptr().cast()
 }
@@ -334,6 +335,7 @@ unsafe fn from_bytes_with_nul_unchecked_mut(bytes: &mut [u8]) -> &mut Self {
     }
 
     #[inline]
+    #[expect(clippy::disallowed_methods, reason = "internal implementation")]
     fn as_char_ptr(&self) -> *const c_char {
         self.as_ptr().cast()
     }
-- 
2.51.2