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

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Message-Id: <DCMXPGXDXHYT.D9VJ5QBMAVPN@kernel.org>
Date: Mon, 08 Sep 2025 00:51:02 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>
Cc: "Boqun Feng" <boqun.feng@...il.com>, "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 Sun Sep 7, 2025 at 11:39 PM CEST, Danilo Krummrich wrote:
> On Sun Sep 7, 2025 at 11:06 PM CEST, Benno Lossin wrote:
>> On Sun Sep 7, 2025 at 7:29 PM CEST, Danilo Krummrich wrote:
>> I have some ideas of changing the syntax to be more closure-esque:
>>
>>     init!(|this| -> Result<MyStruct, Error> {
>>         let x = 42;
>>         MyStruct {
>>             x,
>>         }
>>     })
>>
>> There we could add another parameter, that would then serve this
>> purpose. We should also probably rename `this` to `slot` & then use
>> `this` for the initialized version.
>
> I think that's a pretty good idea, but the part that I think is a little bit
> confusing remains: `this` will need to have different fields depending on where
> it's accessed.

Yeah (that's also the main issue with the macro implementation).

>> But as I said before, implementing the `this` thing from a macro
>> perspective is rather difficult (I have two ideas on how to do it and
>> both are bad...).
>>
>>> But as you say, that sounds tricky to implement and is probably not very
>>> intuitive either. I'd rather say keep it as it is, if we don't want something
>>> like the `let b <- b` syntax I proposed for formatting reasons.
>>
>> I don't feel like that's conveying the correct thing, it looks as if you
>> are only declaring a local variable.
>
> Yeah, it's not great, but given that it's a custom syntax it also does not
> create wrong expectations I'd say.

I'd say it looks like combining the `<-` operation already used by the
`init!` macro & a `let` binding. Thus introducing a local variable
that's (pin) initialized in-place. Not a field of the current struct.

> Anyways, I'm fine with either. For now we probably want to land the version as
> it is and revisit once you settle on the syntax rework you mentioned above.

I actually came up with a third option that looks best IMO:

    init!(MyStruct {
        x: 42,
        #[with_binding]
        y: 24,
        z: *y,
    })

The `#[with_binding]` attribute makes the macro generate a variable `y`.
`x` & `z` don't give access to their value. (we of course should come up
with a better name).

Any thoughts?

---
Cheers,
Benno