Re: [PATCH 2/5] rust: pin-init: properly document let binding workaround

["Benno Lossin" <lossin@kernel.org>]

On Thu Mar 19, 2026 at 12:04 PM CET, Gary Guo wrote:
> On Thu Mar 19, 2026 at 9:35 AM GMT, Benno Lossin wrote:
>> The three let bindings (in the bodies of `cast_init`, `cast_pin_init`
>> and the `init!` macro) are used to avoid the following compiler error in
>> Rust 1.78.0, 1.79.0, 1.80.0, 1.80.1, and 1.81.0 (just showing the one
>> for `cast_init`, the others are similar):
>> 
>>     error[E0391]: cycle detected when computing type of opaque `cast_init::{opaque#0}`
>>         --> src/lib.rs:1160:66
>>          |
>>     1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
>>          |                                                                  ^^^^^^^^^^^^^^^
>>          |
>>     note: ...which requires borrow-checking `cast_init`...
>>         --> src/lib.rs:1160:1
>>          |
>>     1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
>>          | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>     note: ...which requires const checking `cast_init`...
>>         --> src/lib.rs:1160:1
>>          |
>>     1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<U, E> {
>>          | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>          = note: ...which requires computing whether `cast_init::{opaque#0}` is freeze...
>>          = note: ...which requires evaluating trait selection obligation `cast_init::{opaque#0}: core::marker::Freeze`...
>>          = note: ...which again requires computing type of opaque `cast_init::{opaque#0}`, completing the cycle
>>     note: cycle used when computing type of `cast_init::{opaque#0}`
>>         --> src/lib.rs:1160:66
>>          |
>>     1160 | pub const unsafe fn cast_init<T, U, E>(init: impl Init<T, E>) -> impl Init<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
>> 
>> Once we raise the nightly-MSRV above 1.81, we can remove this
>> workaround.
>> 
>> Link: https://github.com/Rust-for-Linux/pin-init/commit/bb3e96f3e9a4f5fca80a22af883c7e5aa90f0893
>> [ Moved this commit after the previous one to avoid a build failure due
>>   to unstable features. Changed the cfg to use `USE_RUSTC_FEAUTURES`.
>>   - Benno ]
>> Signed-off-by: Benno Lossin <lossin@kernel.org>
>
> Reviewed-by: Gary Guo <gary@garyguo.net>
>
> Perhaps we should import review approvals on GitHub as R-bs.

Yeah we should. A quick internet search didn't reveal a magic tool to
me. Do you know of any?

Cheers,
Benno