rust: task: use `as_char_ptr` instead of `as_ptr().cast()`

From: Gary Guo <[email protected]>

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.

Similarly, the dual from_ptr is also disallowed.

Signed-off-by Gary Guo <[email protected]>

---
Changes since v2:
- Also disallow CStr::from_ptr

---
 .clippy.toml       | 10 ++++++++++
 rust/kernel/str.rs |  3 +++
 2 files changed, 13 insertions(+)

.clippy.toml +10 -0

index 137f41d203de..a51de9a46380 100644
@@ -9,3 +9,13 @@ 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"
+
+[[disallowed-methods]]
+path = "core::ffi::CStr::from_ptr"
+replacement = "kernel::prelude::CStrExt::from_char_ptr"
+reason = "kernel's `char` is always unsigned, use `from_char_ptr` instead"

rust/kernel/str.rs +3 -1

index fa87779d2253..97bf9427af59 100644
@@ -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()
 }

@@ -319,6 +320,7 @@ unsafe fn to_bytes_mut(s: &mut CStr) -> &mut [u8] {
 
 impl CStrExt for CStr {
     #[inline]
+    #[expect(clippy::disallowed_methods, reason = "internal implementation")]
     unsafe fn from_char_ptr<'a>(ptr: *const c_char) -> &'a Self {
         // SAFETY: The safety preconditions are the same as for `CStr::from_ptr`.
         unsafe { CStr::from_ptr(ptr.cast()) }

@@ -334,6 +336,7 @@ 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

From: Gary Guo <[email protected]>

as_char_ptr would provide the correct (unsigned char) type without needing to convert to an intermediate type and cast the pointer.

The as_ptr() function is going to be disallowed by clippy warning, so fix this usage.

This is used only if CONFIG_DEBUG_ATOMIC_SLEEP=y. Instead of conditionally importing CStrExt, import it via prelude instead, and remove other imports that are already available via the prelude.

Reported-by kernel test robot <[email protected]>

Closes https://lore.kernel.org/oe-kbuild-all/[email protected]/ open here

Signed-off-by Gary Guo <[email protected]>