rust: replace CStr with core::ffi::CStr

`kernel::ffi::CStr` was introduced in commit d126d23801 ("rust: str:
add `CStr` type") in November 2022 as an upstreaming of earlier work
that was done in May 2021[0]. That earlier work, having predated the
inclusion of `CStr` in `core`, largely duplicated the implementation of
`std::ffi::CStr`.

`std::ffi::CStr` was moved to `core::ffi::CStr` in Rust 1.64 in
September 2022. Hence replace `kernel::str::CStr` with `core::ffi::CStr`
to reduce our custom code footprint, and retain needed custom
functionality through an extension trait.

Add `CStr` to `ffi` and the kernel prelude.

Link: https://github.com/Rust-for-Linux/linux/commit/faa3cbcca03d0dec8f8e43f1d8d5c0860d98a23f [0]
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-16-9378a54385f8@gmail.com
[ Removed assert that would now depend on the Rust version. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

drivers/android/binder/stats.rs

@@ -61,7 +61,7 @@ impl BinderStats {
 
 mod strings {
     use core::str::from_utf8_unchecked;
-    use kernel::str::CStr;
+    use kernel::str::{CStr, CStrExt as _};
 
     extern "C" {
         static binder_command_strings: [*const u8; super::BC_COUNT];

rust/ffi.rs

@@ -46,3 +46,5 @@ alias! {
 }
 
 pub use core::ffi::c_void;
+
+pub use core::ffi::CStr;

rust/kernel/debugfs/entry.rs

@@ -3,7 +3,7 @@
 
 use crate::debugfs::file_ops::FileOps;
 use crate::ffi::c_void;
-use crate::str::CStr;
+use crate::str::{CStr, CStrExt as _};
 use crate::sync::Arc;
 use core::marker::PhantomData;
 

rust/kernel/device.rs

@@ -13,6 +13,7 @@ use core::{marker::PhantomData, ptr};
 
 #[cfg(CONFIG_PRINTK)]
 use crate::c_str;
+use crate::str::CStrExt as _;
 
 pub mod property;
 

rust/kernel/drm/ioctl.rs

@@ -156,7 +156,9 @@ macro_rules! declare_drm_ioctls {
                         Some($cmd)
                     },
                     flags: $flags,
-                    name: $crate::c_str!(::core::stringify!($cmd)).as_char_ptr(),
+                    name: $crate::str::as_char_ptr_in_const_context(
+                        $crate::c_str!(::core::stringify!($cmd)),
+                    ),
                 }
             ),*];
             ioctls

rust/kernel/error.rs

@@ -182,6 +182,8 @@ impl Error {
         if ptr.is_null() {
             None
         } else {
+            use crate::str::CStrExt as _;
+
             // SAFETY: The string returned by `errname` is static and `NUL`-terminated.
             Some(unsafe { CStr::from_char_ptr(ptr) })
         }

rust/kernel/firmware.rs

@@ -4,7 +4,14 @@
 //!
 //! C header: [`include/linux/firmware.h`](srctree/include/linux/firmware.h)
 
-use crate::{bindings, device::Device, error::Error, error::Result, ffi, str::CStr};
+use crate::{
+    bindings,
+    device::Device,
+    error::Error,
+    error::Result,
+    ffi,
+    str::{CStr, CStrExt as _},
+};
 use core::ptr::NonNull;
 
 /// # Invariants

rust/kernel/prelude.rs

@@ -19,7 +19,7 @@ pub use core::{
 
 pub use ::ffi::{
     c_char, c_int, c_long, c_longlong, c_schar, c_short, c_uchar, c_uint, c_ulong, c_ulonglong,
-    c_ushort, c_void,
+    c_ushort, c_void, CStr,
 };
 
 pub use crate::alloc::{flags::*, Box, KBox, KVBox, KVVec, KVec, VBox, VVec, Vec};
@@ -43,7 +43,7 @@ pub use super::static_assert;
 
 pub use super::error::{code::*, Error, Result};
 
-pub use super::{str::CStr, ThisModule};
+pub use super::{str::CStrExt as _, ThisModule};
 
 pub use super::init::InPlaceInit;
 

rust/kernel/seq_file.rs

@@ -4,7 +4,7 @@
 //!
 //! C header: [`include/linux/seq_file.h`](srctree/include/linux/seq_file.h)
 
-use crate::{bindings, c_str, fmt, types::NotThreadSafe, types::Opaque};
+use crate::{bindings, c_str, fmt, str::CStrExt as _, types::NotThreadSafe, types::Opaque};
 
 /// A utility for generating the contents of a seq file.
 #[repr(transparent)]

rust/kernel/sync/condvar.rs

@@ -8,7 +8,7 @@
 use super::{lock::Backend, lock::Guard, LockClassKey};
 use crate::{
     ffi::{c_int, c_long},
-    str::CStr,
+    str::{CStr, CStrExt as _},
     task::{
         MAX_SCHEDULE_TIMEOUT, TASK_FREEZABLE, TASK_INTERRUPTIBLE, TASK_NORMAL, TASK_UNINTERRUPTIBLE,
     },

rust/kernel/sync/lock.rs

@@ -7,7 +7,7 @@
 
 use super::LockClassKey;
 use crate::{
-    str::CStr,
+    str::{CStr, CStrExt as _},
     types::{NotThreadSafe, Opaque, ScopeGuard},
 };
 use core::{cell::UnsafeCell, marker::PhantomPinned, pin::Pin};

rust/kernel/sync/lock/global.rs

@@ -5,7 +5,7 @@
 //! Support for defining statics containing locks.
 
 use crate::{
-    str::CStr,
+    str::{CStr, CStrExt as _},
     sync::lock::{Backend, Guard, Lock},
     sync::{LockClassKey, LockedBy},
     types::Opaque,