From: Benno Lossin Date: Thu, 19 Mar 2026 09:35:25 +0000 (+0100) Subject: rust: pin-init: properly document let binding workaround X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=960c37cbcba78730ee175f4887ffcdf523385c53;p=thirdparty%2Fkernel%2Fstable.git rust: pin-init: properly document let binding workaround 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(init: impl Init) -> impl Init { | ^^^^^^^^^^^^^^^ | note: ...which requires borrow-checking `cast_init`... --> src/lib.rs:1160:1 | 1160 | pub const unsafe fn cast_init(init: impl Init) -> impl Init { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ note: ...which requires const checking `cast_init`... --> src/lib.rs:1160:1 | 1160 | pub const unsafe fn cast_init(init: impl Init) -> impl Init { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ = 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(init: impl Init) -> impl Init { | ^^^^^^^^^^^^^^^ = 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 ] Reviewed-by: Gary Guo Link: https://patch.msgid.link/20260319093542.3756606-3-lossin@kernel.org Signed-off-by: Benno Lossin --- diff --git a/rust/pin-init/examples/big_struct_in_place.rs b/rust/pin-init/examples/big_struct_in_place.rs index c05139927486..de8612a5e27d 100644 --- a/rust/pin-init/examples/big_struct_in_place.rs +++ b/rust/pin-init/examples/big_struct_in_place.rs @@ -1,5 +1,7 @@ // SPDX-License-Identifier: Apache-2.0 OR MIT +#![cfg_attr(USE_RUSTC_FEATURES, feature(lint_reasons))] + use pin_init::*; // Struct with size over 1GiB diff --git a/rust/pin-init/internal/src/init.rs b/rust/pin-init/internal/src/init.rs index 2fe918f4d82a..c1c9400b090a 100644 --- a/rust/pin-init/internal/src/init.rs +++ b/rust/pin-init/internal/src/init.rs @@ -173,6 +173,12 @@ pub(crate) fn expand( }; // SAFETY: TODO let init = unsafe { ::pin_init::#init_from_closure::<_, #error>(init) }; + // FIXME: this let binding is required to avoid a compiler error (cycle when computing the + // opaque type returned by this function) before Rust 1.81. Remove after MSRV bump. + #[allow( + clippy::let_and_return, + reason = "some clippy versions warn about the let binding" + )] init }}) } diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs index b1de166b5626..a513930ee01a 100644 --- a/rust/pin-init/src/lib.rs +++ b/rust/pin-init/src/lib.rs @@ -1144,9 +1144,12 @@ pub const unsafe fn cast_pin_init(init: impl PinInit) -> impl Pin // 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::())) }; - // 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) - #[allow(clippy::let_and_return)] + // FIXME: this let binding is required to avoid a compiler error (cycle when computing the opaque + // type returned by this function) before Rust 1.81. Remove after MSRV bump. + #[allow( + clippy::let_and_return, + reason = "some clippy versions warn about the let binding" + )] res } @@ -1160,9 +1163,12 @@ pub const unsafe fn cast_init(init: impl Init) -> impl Init // 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::())) }; - // 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) - #[allow(clippy::let_and_return)] + // FIXME: this let binding is required to avoid a compiler error (cycle when computing the opaque + // type returned by this function) before Rust 1.81. Remove after MSRV bump. + #[allow( + clippy::let_and_return, + reason = "some clippy versions warn about the let binding" + )] res }