rust: kernel: str: Implement Debug for CString

Make it possible to use a `CString` with the `pr_*` macros directly. That
is, instead of:

    pr_debug!("trying to open {:?}\n", &*filename);

we can now write:

    pr_debug!("trying to open {:?}\n", filename);

Signed-off-by: Asahi Lina <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Reworded to use Alice's commit message as discussed. ]
Signed-off-by: Miguel Ojeda <[email protected]>

rust/kernel/str.rs

@@ -608,6 +608,12 @@ impl<'a> TryFrom<&'a CStr> for CString {
     }
 }
 
+impl fmt::Debug for CString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&**self, f)
+    }
+}
+
 /// A convenience alias for [`core::format_args`].
 #[macro_export]
 macro_rules! fmt {