Re: [PATCH] rust: pin-init: add references to previously initialized fields

[Boqun Feng <boqun.feng@gmail.com>]

On Sat, Sep 06, 2025 at 12:52:22PM +0200, Danilo Krummrich wrote:
> On Fri Sep 5, 2025 at 7:44 PM CEST, Boqun Feng wrote:
> > On Fri, Sep 05, 2025 at 07:18:25PM +0200, Benno Lossin wrote:
> > [...]
> >> index 606946ff4d7f..1ac0b06fa3b3 100644
> >> --- a/samples/rust/rust_driver_pci.rs
> >> +++ b/samples/rust/rust_driver_pci.rs
> >> @@ -78,8 +78,8 @@ fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> Result<Pin<KBox<Self>
> >>  
> >>          let drvdata = KBox::pin_init(
> >>              try_pin_init!(Self {
> >> -                pdev: pdev.into(),
> >>                  bar <- pdev.iomap_region_sized::<{ Regs::END }>(0, c_str!("rust_driver_pci")),
> >> +                pdev: pdev.into(),
> >
> > Ok, this example is good enough for me to express the concern here: the
> > variable shadowing behavior seems not straightforward (maybe because in
> > normal Rust initalization expression, no binding is created for
> > previous variables, neither do we have a `let` here).
> >
> > Would the future inplace initialization have the similar behavior? I
> > asked because a natural resolution is adding a special syntax like:
> >
> >     let a = ..;
> >
> >     try_pin_init!(Self {
> >         b: a,
> > 	let a = a.into(); // create the new binding here.
> > 	c: a, // <- use the previous initalized `a`.
> >     }
> 
> Can you please clarify the example? I'm a bit confused that this is not a field
> of Self, so currently this can just be written as:
> 

Oh, I could have been more clear: `a` is a field of `Self`, and the
`let` part initalizes it.

> 	try_pin_init!(Self {
> 	   b: a,
> 	   c: a.into,
> 	})
> 
> Of course assuming that a is Clone, as the code above does as well.
> 
> So, if we are concerned by the variable shadowing, which I'm less concerned
> about, maybe we can do this:

I'm not that concerned to block this, but it does look to me like we are
inventing a new way (and even a different syntax because normal Rust
initialization doesn't create new bindings) to create binding, so I
think I should bring it up.

> 
> 	// The "original" `a` and `b`.
> 	let a: A = ...;
> 	let b: B = ...;
> 
> 	try_pin_init!(Self {
> 	   a,                   // Initialize the field only.
> 	   let b <- b,          // Initialize the field and create a `&B` named `b`.
> 	   c: a.into(),         // That's the "original" `a`.
> 	   d <- D::new(b),      // Not the original `b`, but the pin-init one.
> 	})

This looks good to me as well. But I'm curious what the syntax would be
like in the in-place placement language feature in the future.

Regards,
Boqun