linux-kernel - Re: [PATCH] rust/pin-init: remove workaround for type inference cycle

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Message-ID: <CAO9wTFisWa5qY2qQE+BEdD40vacGwtzw=2xxPnV0V_w=Gt4jwg@mail.gmail.com>
Date: Sun, 27 Jul 2025 23:14:43 +0530
From: Suchit K <suchitkarunakaran@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, 
	gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, 
	aliceryhl@...gle.com, tmgross@...ch.edu, dakr@...nel.org, me@...enk.dev, 
	chrisi.schrefl@...il.com, rust-for-linux@...r.kernel.org, 
	skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust/pin-init: remove workaround for type inference cycle

On Sun, 27 Jul 2025 at 22:58, Benno Lossin <lossin@...nel.org> wrote:
>
> On Sun Jul 27, 2025 at 7:02 PM CEST, Suchit Karunakaran wrote:
> > The `cast_pin_init` and `cast_init` functions previously used an
> > intermediate `let` binding before returning the result expression to work
> > around a Rust compiler issue causing type inference cycles. With the
> > minimum Rust compiler version for the kernel now at 1.78.0, where this
> > issue is fixed, the workaround is no longer needed. This patch removes the
> > unnecessary `let` variables and returns the expressions directly.
> >
> > Signed-off-by: Suchit Karunakaran <suchitkarunakaran@...il.com>
>
> I still encounter the cycle when compiling with 1.78.0, which version
> did you test this with?
>

Oops, I just realised that I compiled it locally and forgot that I had
installed version 1.78.0 on my VM and not on my local system. I
actually compiled it with version 1.88.0. Sorry for the inconvenience.

> One of The errors I see after applying the patch on top of
> `pin-init-next`:
>
>     error[E0391]: cycle detected when computing type of opaque `cast_pin_init::{opaque#0}`
>         --> rust/pin-init/src/lib.rs:1278:73
>          |
>     1278 | pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
>          |                                                                         ^^^^^^^^^^^^^^^^^^
>          |
>     note: ...which requires borrow-checking `cast_pin_init`...
>         --> rust/pin-init/src/lib.rs:1278:1
>          |
>     1278 | pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
>          | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>     note: ...which requires promoting constants in MIR for `cast_pin_init`...
>         --> rust/pin-init/src/lib.rs:1278:1
>          |
>     1278 | pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
>          | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>     note: ...which requires const checking `cast_pin_init`...
>         --> rust/pin-init/src/lib.rs:1278:1
>          |
>     1278 | pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
>          | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>          = note: ...which requires computing whether `cast_pin_init::{opaque#0}` is freeze...
>          = note: ...which requires evaluating trait selection obligation `cast_pin_init::{opaque#0}: core::marker::Freeze`...
>          = note: ...which again requires computing type of opaque `cast_pin_init::{opaque#0}`, completing the cycle
>     note: cycle used when computing type of `cast_pin_init::{opaque#0}`
>         --> rust/pin-init/src/lib.rs:1278:73
>          |
>     1278 | pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
>          |                                                                         ^^^^^^^^^^^^^^^^^^
>          = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
>
> > ---
> >  rust/pin-init/src/lib.rs | 10 ++--------
> >  1 file changed, 2 insertions(+), 8 deletions(-)
> >
> > diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
> > index 62e013a5cc20..cc244eeb19cd 100644
> > --- a/rust/pin-init/src/lib.rs
> > +++ b/rust/pin-init/src/lib.rs
> > @@ -1278,10 +1278,7 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
> >  pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> impl PinInit<U, E> {
> >      // SAFETY: initialization delegated to a valid initializer. Cast is valid by function safety
> >      // requirements.
> > -    let res = unsafe { pin_init_from_closure(|ptr: *mut U| init.__pinned_init(ptr.cast::<T>())) };
> > -    // FIXME: remove the let statement once the nightly-MSRV allows it (1.78 otherwise encounters a
>
> Do note the comment mentioning that it is needed for version 1.78 here,
> so I think this patch still needs to wait until we bump the minimum.
>

Yup got it. I'll send the patch after the minimum version is
increased. Thanks for reviewing.