[PATCH v5 2/8] rust: debugfs: add Dir::empty() for conditional debugfs usage

[Timur Tabi <ttabi@nvidia.com>]

Add Dir::empty() which returns a &'static reference to a no-op Dir.
All file and subdirectory operations on this directory silently succeed
without creating actual filesystem entries.

This enables callers to use a single code path regardless of whether
debugfs is available at runtime. For example:

    let dir = optional_debugfs_root.unwrap_or_else(|| Dir::empty());
    dir.scope(data, name, |d, s| { ... })

The returned reference has 'static lifetime, allowing initializers
returned by scope() to be used outside the immediate scope where the
Dir reference was obtained.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
---
 rust/kernel/debugfs.rs | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index facad81e8290..44ff2b37786a 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -110,6 +110,24 @@ pub fn new(name: &CStr) -> Self {
         Dir::create(name, None)
     }
 
+    /// Returns a reference to a static no-op directory that doesn't create any debugfs entries.
+    ///
+    /// All file and subdirectory creation operations on this directory will silently succeed
+    /// without creating actual filesystem entries. This is useful when you want to conditionally
+    /// enable debugfs but use the same code path regardless.
+    pub fn empty() -> &'static Self {
+        #[cfg(CONFIG_DEBUG_FS)]
+        {
+            static EMPTY: Dir = Dir(None);
+            &EMPTY
+        }
+        #[cfg(not(CONFIG_DEBUG_FS))]
+        {
+            static EMPTY: Dir = Dir();
+            &EMPTY
+        }
+    }
+
     /// Creates a subdirectory within this directory.
     ///
     /// # Examples
-- 
2.52.0