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rust: pin-init: properly document let binding workaround

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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: bb3e96f3e9
[ 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 <gary@garyguo.net>
Link: https://patch.msgid.link/20260319093542.3756606-3-lossin@kernel.org
Signed-off-by: Benno Lossin <lossin@kernel.org>
This commit is contained in:
Benno Lossin 2026-03-19 10:35:25 +01:00
parent 002a121b16
commit 960c37cbcb
3 changed files with 20 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
rust/pin-init/examples/big_struct_in_place.rs
View File

@ -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

6
rust/pin-init/internal/src/init.rs
View File

@ -173,6 +173,12 @@ fn assert_zeroable<T: ?::core::marker::Sized>(_: *mut T)
};
// 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
}})
}

18
rust/pin-init/src/lib.rs
View File

@ -1144,9 +1144,12 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), 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)
#[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 @@ unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), 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)
#[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
}