rust: str: add `c_str!` macro

Add `c_str!`, which is a convenience macro that creates a new `CStr`
from a string literal.

It is designed to be similar to a `str` in usage, and it is usable
in const contexts, for instance:

    const X: &CStr = c_str!("Example");
Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/str.rs

@@ -321,6 +321,29 @@ fn index(&self, index: Idx) -> &Self::Output {
     }
 }
 
+/// Creates a new [`CStr`] from a string literal.
+///
+/// The string literal should not contain any `NUL` bytes.
+///
+/// # Examples
+///
+/// ```
+/// # use kernel::c_str;
+/// # use kernel::str::CStr;
+/// const MY_CSTR: &CStr = c_str!("My awesome CStr!");
+/// ```
+#[macro_export]
+macro_rules! c_str {
+    ($str:expr) => {{
+        const S: &str = concat!($str, "\0");
+        const C: &$crate::str::CStr = match $crate::str::CStr::from_bytes_with_nul(S.as_bytes()) {
+            Ok(v) => v,
+            Err(_) => panic!("string contains interior NUL"),
+        };
+        C
+    }};
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;