rust: init: add `PinnedDrop` trait and macros

The `PinnedDrop` trait that facilitates destruction of pinned types.
It has to be implemented via the `#[pinned_drop]` macro, since the
`drop` function should not be called by normal code, only by other
destructors. It also only works on structs that are annotated with
`#[pin_data(PinnedDrop)]`.

Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-10-y86-dev@protonmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/rust/kernel/init/__internal.rs b/rust/kernel/init/__internal.rs
index a3389a684296..774cb620afa7 100644
--- a/rust/kernel/init/__internal.rs
+++ b/rust/kernel/init/__internal.rs
@@ -161,3 +161,18 @@ impl<T: ?Sized> Drop for DropGuard<T> {
         }
     }
 }
+
+/// Token used by `PinnedDrop` to prevent calling the function without creating this unsafely
+/// created struct. This is needed, because the `drop` function is safe, but should not be called
+/// manually.
+pub struct OnlyCallFromDrop(());
+
+impl OnlyCallFromDrop {
+    /// # Safety
+    ///
+    /// This function should only be called from the [`Drop::drop`] function and only be used to
+    /// delegate the destruction to the pinned destructor [`PinnedDrop::drop`] of the same type.
+    pub unsafe fn new() -> Self {
+        Self(())
+    }
+}