[PATCH v3] rust: error: implement Display for Error

[Arnav Sharma]

Implement the fmt::Display trait for the kernel Error type.

While the existing Debug implementation already outputs the error name
(e.g., 'EPERM'), implementing Display allows errors to be formatted using
the standard '{}' display marker. This provides a more idiomatic Rust API
and allows Error to be used seamlessly with macros that expect the
Display trait.

Signed-off-by: Arnav Sharma <arnav4324@xxxxxxxxx>
---
Changes in v3:
- Fixed doctest to prevent panics when CONFIG_SYMBOLIC_ERRNAME is disabled.
- Corrected commit message regarding the output of the existing Debug implementation.
- Retained `write!` for the fallback error case. Applying `f.pad()` to dynamically 
  generated text requires memory allocation, which is unsafe in error formatting paths.

 rust/kernel/error.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index 05cf869ac090..4ab7d7d8ccad 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -216,6 +216,28 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     }
 }
 
+impl fmt::Display for Error {
+    /// Displays the error name if available, otherwise the numeric error code.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::prelude::*;
+    /// let err = EPERM;
+    /// // Formats to "EPERM" or "Unknown error 1" depending on CONFIG_SYMBOLIC_ERRNAME.
+    /// let _ = fmt!("{err}");
+    /// ```
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.name() {
+            // Print out the numeric error code if no name can be found.
+            None => write!(f, "Unknown error {}", -self.0.get()),
+            // SAFETY: These strings are ASCII-only.
+            Some(name) => f.pad(unsafe {
+                core::str::from_utf8_unchecked(name.to_bytes())
+            }),
+        }
+    }
+}
 impl From<AllocError> for Error {
     #[inline]
     fn from(_: AllocError) -> Error {
-- 
2.43.0