rust: error: impl Debug for Error with errname() integration

Integrate the `Error` type with `errname()` by providing a new
`name()` method.

Then, implement `Debug` for the type using the new method.

[ Miguel: under `CONFIG_SYMBOLIC_ERRNAME=n`, `errname()` is a
  `static inline`, so added a helper to support that case,
  like we had in the `rust` branch. Also moved `#include` up
  and reworded commit message for clarity. ]

Co-developed-by: Wedson Almeida Filho <walmeida@microsoft.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Co-developed-by: Sven Van Asbroeck <thesven73@gmail.com>
Signed-off-by: Sven Van Asbroeck <thesven73@gmail.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230531174450.3733220-1-aliceryhl@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindings/bindings_helper.h

@@ -6,6 +6,7 @@
  * Sorted alphabetically.
  */
 
+#include <linux/errname.h>
 #include <linux/slab.h>
 #include <linux/refcount.h>
 #include <linux/wait.h>

rust/helpers.c

@@ -21,6 +21,7 @@
 #include <linux/bug.h>
 #include <linux/build_bug.h>
 #include <linux/err.h>
+#include <linux/errname.h>
 #include <linux/refcount.h>
 #include <linux/mutex.h>
 #include <linux/spinlock.h>
@@ -110,6 +111,12 @@ long rust_helper_PTR_ERR(__force const void *ptr)
 }
 EXPORT_SYMBOL_GPL(rust_helper_PTR_ERR);
 
+const char *rust_helper_errname(int err)
+{
+	return errname(err);
+}
+EXPORT_SYMBOL_GPL(rust_helper_errname);
+
 struct task_struct *rust_helper_get_current(void)
 {
 	return current;

rust/kernel/error.rs

@@ -4,12 +4,15 @@
 //!
 //! C header: [`include/uapi/asm-generic/errno-base.h`](../../../include/uapi/asm-generic/errno-base.h)
 
+use crate::str::CStr;
+
 use alloc::{
     alloc::{AllocError, LayoutError},
     collections::TryReserveError,
 };
 
 use core::convert::From;
+use core::fmt;
 use core::num::TryFromIntError;
 use core::str::Utf8Error;
 
@@ -133,6 +136,42 @@ impl Error {
         // SAFETY: self.0 is a valid error due to its invariant.
         unsafe { bindings::ERR_PTR(self.0.into()) as *mut _ }
     }
+
+    /// Returns a string representing the error, if one exists.
+    #[cfg(not(testlib))]
+    pub fn name(&self) -> Option<&'static CStr> {
+        // SAFETY: Just an FFI call, there are no extra safety requirements.
+        let ptr = unsafe { bindings::errname(-self.0) };
+        if ptr.is_null() {
+            None
+        } else {
+            // SAFETY: The string returned by `errname` is static and `NUL`-terminated.
+            Some(unsafe { CStr::from_char_ptr(ptr) })
+        }
+    }
+
+    /// Returns a string representing the error, if one exists.
+    ///
+    /// When `testlib` is configured, this always returns `None` to avoid the dependency on a
+    /// kernel function so that tests that use this (e.g., by calling [`Result::unwrap`]) can still
+    /// run in userspace.
+    #[cfg(testlib)]
+    pub fn name(&self) -> Option<&'static CStr> {
+        None
+    }
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.name() {
+            // Print out number if no name can be found.
+            None => f.debug_tuple("Error").field(&-self.0).finish(),
+            // SAFETY: These strings are ASCII-only.
+            Some(name) => f
+                .debug_tuple(unsafe { core::str::from_utf8_unchecked(name) })
+                .finish(),
+        }
+    }
 }
 
 impl From<AllocError> for Error {