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

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

On Sat, Sep 06, 2025 at 06:57:04PM -0700, Boqun Feng wrote:
> 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.
> > 	})

Another idea is using `&this`:

 	try_pin_init!(&this in Self {
 	   a,                   // Initialize the field only.
 	   b <- b,              // Initialize the field only.
 	   c: a.into(),         // That's the "original" `a`.
 	   d <- D::new(this->b),      // Not the original `b`, but the pin-init one.
 	})

, like a special field projection during initialization.

Regards,
Boqun

> 
> 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