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

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

[Suchit Karunakaran]

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@gmail.com>
---
 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
-    // cycle when computing the type returned by this function)
-    res
+    unsafe { pin_init_from_closure(|ptr: *mut U| init.__pinned_init(ptr.cast::<T>())) }
 }
 
 /// Changes the to be initialized type.
@@ -1294,10 +1291,7 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
 pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
     // SAFETY: initialization delegated to a valid initializer. Cast is valid by function safety
     // requirements.
-    let res = unsafe { init_from_closure(|ptr: *mut U| init.__init(ptr.cast::<T>())) };
-    // FIXME: remove the let statement once the nightly-MSRV allows it (1.78 otherwise encounters a
-    // cycle when computing the type returned by this function)
-    res
+    unsafe { init_from_closure(|ptr: *mut U| init.__init(ptr.cast::<T>())) }
 }
 
 /// An initializer that leaves the memory uninitialized.
-- 
2.50.1

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

[Benno Lossin]

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@gmail.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>())) }
>  }

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

[Suchit K]

On Sun, 27 Jul 2025 at 22:58, Benno Lossin <lossin@kernel.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@gmail.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.

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

[Benno Lossin]

On Sun Jul 27, 2025 at 7:44 PM CEST, Suchit K wrote:
> On Sun, 27 Jul 2025 at 22:58, Benno Lossin <lossin@kernel.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@gmail.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.

No worries.

>> 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.

If you do, then please send the patch upstream at [1], that makes things
easier for me, thanks!

[1]: https://github.com/rust-for-linux/pin-init

---
Cheers,
Benno

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

[Suchit K]

On Sun, 27 Jul 2025 at 23:28, Benno Lossin <lossin@kernel.org> wrote:
>
> On Sun Jul 27, 2025 at 7:44 PM CEST, Suchit K wrote:
> > On Sun, 27 Jul 2025 at 22:58, Benno Lossin <lossin@kernel.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@gmail.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.
>
> No worries.
>
> >> 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.
>
> If you do, then please send the patch upstream at [1], that makes things
> easier for me, thanks!
>
> [1]: https://github.com/rust-for-linux/pin-init
>

Sure, I’ll send the patch both here and upstream at [1]. Thanks!