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

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Message-Id: <DBN0JG0QP7TU.PZSZ8NZL5LD6@kernel.org>
Date: Sun, 27 Jul 2025 19:28:15 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Suchit Karunakaran" <suchitkarunakaran@...il.com>, <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>
Cc: <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 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?

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.

---
Cheers,
Benno

> -    // cycle when computing the type returned by this function)
> -    res
> +    unsafe { pin_init_from_closure(|ptr: *mut U| init.__pinned_init(ptr.cast::<T>())) }
>  }