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

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Message-Id: <DCLNSNWA7AT7.19OWOXUMJ5ZRJ@kernel.org>
Date: Sat, 06 Sep 2025 12:52:22 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "Benno Lossin" <lossin@...nel.org>, "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>, "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 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:

	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:

	// 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.
	})