[PATCH v2] rust: update `kernel::c_str!` documentation

[Tamir Duberstein <tamird@kernel.org>]

Now that all literals are C-Strings, update the documentation to explain
that use of this macro should be limited to non-literal strings.

Link: https://github.com/Rust-for-Linux/linux/issues/1075
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@kernel.org>
---
This patch completes the work of replacing our custom `CStr` with
upstream's.
---
Changes in v2:
- Drop rename, keep only documentation update. (Gary Guo)
- Add example of misuse to documentation. (Gary Guo)
- Link to v1: https://patch.msgid.link/20260302-cstr-rename-macro-v1-1-a269fe4dc3f0@kernel.org
---
 rust/kernel/str.rs | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index fa87779d2253..6dae82e7d875 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -376,19 +376,32 @@ fn as_ref(&self) -> &BStr {
     }
 }
 
-/// Creates a new [`CStr`] from a string literal.
+/// Creates a new [`CStr`] at compile time.
 ///
-/// The string literal should not contain any `NUL` bytes.
+/// Rust supports C string literals since Rust 1.77, and they should be used instead of this macro
+/// where possible. This macro exists to allow static *non-literal* C strings to be created at
+/// compile time. This is most often used in other macros.
+///
+/// # Panics
+///
+/// This macro panics if the operand contains an interior `NUL` byte.
 ///
 /// # Examples
 ///
 /// ```
 /// # use kernel::c_str;
 /// # use kernel::str::CStr;
-/// const MY_CSTR: &CStr = c_str!("My awesome CStr!");
+/// // This is allowed, but `c"literal"` should be preferred for literals.
+/// const BAD: &CStr = c_str!("literal");
+///
+/// // `c_str!` is still needed for static non-literal C strings.
+/// const GOOD: &CStr = c_str!(concat!(file!(), ":", line!(), ": My CStr!"));
 /// ```
 #[macro_export]
 macro_rules! c_str {
+    // NB: we could write `($str:lit) => compile_error!("use a C string literal instead");` here but
+    // that would trigger when the literal is at the top of several macro expansions. That would be
+    // too limiting to macro authors.
     ($str:expr) => {{
         const S: &str = concat!($str, "\0");
         const C: &$crate::str::CStr = match $crate::str::CStr::from_bytes_with_nul(S.as_bytes()) {

---
base-commit: 11439c4635edd669ae435eec308f4ab8a0804808
change-id: 20260302-cstr-rename-macro-64201be6c969

Best regards,
--  
Tamir Duberstein <tamird@kernel.org>