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

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Message-ID: <aLshd0_C-1rh3FAg@tardis-2.local>
Date: Fri, 5 Sep 2025 10:44:23 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, Fiona Behrens <me@...enk.dev>,
	Alban Kurti <kurti@...icto.ai>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Krzysztof Wilczy´nski <kwilczynski@...nel.org>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: pin-init: add references to previously initialized
 fields

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`.
    }

(Since I lost tracking of the discussion a bit, maybe there is a
previous discussion I've missed here?)

Regards,
Boqun

>                  index: *info,
>              }),
>              GFP_KERNEL,