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

["Gary Guo" <gary@garyguo.net>]

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.

Best,
Gary

> ---
>  rust/pin-init/examples/big_struct_in_place.rs |  2 ++
>  rust/pin-init/internal/src/init.rs            |  6 ++++++
>  rust/pin-init/src/lib.rs                      | 18 ++++++++++++------
>  3 files changed, 20 insertions(+), 6 deletions(-)