Re: [PATCH v2] rust: pin-init: add `#[bind]` attribute to access previously initialized fields

["Danilo Krummrich" <dakr@kernel.org>]

On Wed Sep 10, 2025 at 12:40 PM CEST, Alice Ryhl wrote:
> On Wed, Sep 10, 2025 at 12:36 PM Benno Lossin <lossin@kernel.org> wrote:
>>
>> On Wed Sep 10, 2025 at 12:17 PM CEST, Alice Ryhl wrote:
>> > On Wed, Sep 10, 2025 at 12:07:53PM +0200, Benno Lossin wrote:
>> >> Assigning a field a value in an initializer macro can be marked with the
>> >> `#[bind]` attribute. Doing so creates a `let` binding with the same
>> >> name. This `let` binding has the type `Pin<&mut T>` if the field is
>> >> structurally pinned or `&mut T` otherwise (where `T` is the type of the
>> >> field).
>> >>
>> >> Signed-off-by: Benno Lossin <lossin@kernel.org>
>> >
>> > Is there a reason we can't apply this to all fields and avoid the
>> > attribute?
>>
>> Adding the attribute was due to Boqun's concern on v1 [1]. I think it
>> might be surprising too, but I'm also happy with no attribute.
>>
>> [1]: https://lore.kernel.org/all/aLshd0_C-1rh3FAg@tardis-2.local
>
> IMO the ideal is if it works without an attribute. Perhaps trying that
> in the kernel is a reasonable experiment to find out whether that's
> reasonable to do for the general language feature?
>
>> > Do we have a place that might be able to use this?
>>
>> I didn't find one, but Danilo plans to base some changes on top this
>> cycle that need this.

We can use it in devres right away:

diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
index d04e3fcebafb..97c616a1733d 100644
--- a/rust/kernel/devres.rs
+++ b/rust/kernel/devres.rs
@@ -137,10 +137,11 @@ pub fn new<'a, E>(
     {
         let callback = Self::devres_callback;
 
-        try_pin_init!(&this in Self {
+        try_pin_init!(Self {
             dev: dev.into(),
             callback,
             // INVARIANT: `inner` is properly initialized.
+            #[bind]
             inner <- Opaque::pin_init(try_pin_init!(Inner {
                     devm <- Completion::new(),
                     revoke <- Completion::new(),
@@ -150,8 +151,7 @@ pub fn new<'a, E>(
             //
             // [1] https://github.com/Rust-for-Linux/pin-init/pull/69
             _add_action: {
-                // SAFETY: `this` is a valid pointer to uninitialized memory.
-                let inner = unsafe { &raw mut (*this.as_ptr()).inner };
+                let inner = core::ptr::from_ref(inner.into_ref().get_ref());
 
                 // SAFETY:
                 // - `dev.as_raw()` is a pointer to a valid bound device.
@@ -160,7 +160,7 @@ pub fn new<'a, E>(
                 //    properly initialized, because we require `dev` (i.e. the *bound* device) to
                 //    live at least as long as the returned `impl PinInit<Self, Error>`.
                 to_result(unsafe {
-                    bindings::devm_add_action(dev.as_raw(), Some(callback), inner.cast())
+                    bindings::devm_add_action(dev.as_raw(), Some(callback), inner.cast_mut().cast())
                 }).inspect_err(|_| {
                     let inner = Opaque::cast_into(inner);
 

Together with the initializer code blocks this becomes quite nice. :)

> Danilo, what plans do you have?

Besides that, the plan is [1].

[1] https://lore.kernel.org/all/DCL32RUQ6Z56.1ERY7JBK6O1J6@kernel.org/