[PATCH v2] rust: impl_flags: add method to return underlying integer

[Andreas Hindborg <a.hindborg@kernel.org>]

Add a `bits` method to return a copy of the underlying integer used
to represent the flags. This is useful when passing flag values to
C APIs.

The visibility of the generated `bits` method is controlled by an optional
visibility specifier in the inner-field position of the bitmask struct
declaration. Write `pub` there (e.g., `pub struct Foo(pub u32)`) to make
`bits` callable from outside the defining module; omit it to keep the bit
pattern hidden.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
@Daniel, I dropped your tag since the patch changed a bit.

Changes in v2:
- Rename the accessor from `into_inner` to `bits`, matching the
  name used by the `bitflags` crate (Gary).
- Tie the visibility of `bits` to the visibility of the inner
  field of the bitmask struct, so callers must explicitly write
  `pub` to expose the bit pattern outside the defining module (Gary).
- Extend the macro's doctest to declare the inner field `pub`
  and exercise the `bits` accessor.
- Link to v1: https://msgid.link/20260212-impl-flags-inner-v1-1-1e2edc96e470@kernel.org

To: Miguel Ojeda <ojeda@kernel.org>
To: Boqun Feng <boqun@kernel.org>
To: Gary Guo <gary@garyguo.net>
To: Björn Roy Baron <bjorn3_gh@protonmail.com>
To: Benno Lossin <lossin@kernel.org>
To: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
To: Danilo Krummrich <dakr@kernel.org>
Cc: rust-for-linux@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 rust/kernel/impl_flags.rs | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/impl_flags.rs b/rust/kernel/impl_flags.rs
index e2bd7639da12..1d39d54540dc 100644
--- a/rust/kernel/impl_flags.rs
+++ b/rust/kernel/impl_flags.rs
@@ -16,6 +16,12 @@
 ///   ([`::core::ops::BitOr`], [`::core::ops::BitAnd`], etc.).
 /// - Utility methods such as `.contains()` to check flags.
 ///
+/// An optional visibility specifier in the inner-field position of the
+/// bitmask struct controls the visibility of the generated `bits`
+/// accessor. Declare it `pub` (e.g., `pub struct Permissions(pub u32)`)
+/// to expose the raw integer outside the defining module — useful when
+/// passing the value to C APIs. Omit it to keep `bits` private.
+///
 /// # Examples
 ///
 /// ```
@@ -24,7 +30,7 @@
 /// impl_flags!(
 ///     /// Represents multiple permissions.
 ///     #[derive(Debug, Clone, Default, Copy, PartialEq, Eq)]
-///     pub struct Permissions(u32);
+///     pub struct Permissions(pub u32);
 ///
 ///     /// Represents a single permission.
 ///     #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -65,12 +71,18 @@
 /// let negated = !read_only;
 /// assert!(negated.contains(Permission::Write));
 /// assert!(!negated.contains(Permission::Read));
+///
+/// // The `pub` in `pub struct Permissions(pub u32)` makes `bits` callable
+/// // from outside the defining module, returning the raw integer
+/// // representation — useful for passing the value to a C API.
+/// let raw: u32 = read_only.bits();
+/// assert_eq!(raw, Permission::Read as u32);
 /// ```
 #[macro_export]
 macro_rules! impl_flags {
     (
         $(#[$outer_flags:meta])*
-        $vis_flags:vis struct $flags:ident($ty:ty);
+        $vis_flags:vis struct $flags:ident($inner_vis:vis $ty:ty);
 
         $(#[$outer_flag:meta])*
         $vis_flag:vis enum $flag:ident {
@@ -267,6 +279,17 @@ pub fn contains_any(self, flags: $flags) -> bool {
             pub fn contains_all(self, flags: $flags) -> bool {
                 (self.0 & flags.0) == flags.0
             }
+
+            /// Return a copy of the inner representation of the flags.
+            ///
+            /// The visibility of this method is controlled by the optional
+            /// visibility specifier in the inner-field position of the
+            /// bitmask struct declaration.
+            #[inline]
+            #[allow(dead_code)]
+            $inner_vis fn bits(self) -> $ty {
+                self.0
+            }
         }
     };
 }

---
base-commit: 7fd2df204f342fc17d1a0bfcd474b24232fb0f32
change-id: 20260212-impl-flags-inner-c61974b27b18

Best regards,
--  
Andreas Hindborg <a.hindborg@kernel.org>