rust: debugfs: Add support for read-only files

Extends the `debugfs` API to support creating read-only files. This
is done via the `Dir::read_only_file` method, which takes a data object
that implements the `Writer` trait.

The file's content is generated by the `Writer` implementation, and the
file is automatically removed when the returned `File` handle is
dropped.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250904-debugfs-rust-v11-2-7d12a165685a@google.com
[ Fixup build failure when CONFIG_DEBUGFS=n. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/debugfs/traits.rs

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2025 Google LLC.
+
+//! Traits for rendering or updating values exported to DebugFS.
+
+use crate::sync::Mutex;
+use core::fmt::{self, Debug, Formatter};
+
+/// A trait for types that can be written into a string.
+///
+/// This works very similarly to `Debug`, and is automatically implemented if `Debug` is
+/// implemented for a type. It is also implemented for any writable type inside a `Mutex`.
+///
+/// The derived implementation of `Debug` [may
+/// change](https://doc.rust-lang.org/std/fmt/trait.Debug.html#stability)
+/// between Rust versions, so if stability is key for your use case, please implement `Writer`
+/// explicitly instead.
+pub trait Writer {
+    /// Formats the value using the given formatter.
+    fn write(&self, f: &mut Formatter<'_>) -> fmt::Result;
+}
+
+impl<T: Writer> Writer for Mutex<T> {
+    fn write(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        self.lock().write(f)
+    }
+}
+
+impl<T: Debug> Writer for T {
+    fn write(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        writeln!(f, "{self:?}")
+    }
+}