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<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 ]
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>

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

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
     }})
 }

rust/pin-init/src/lib.rs

@@ -1144,9 +1144,12 @@ pub const unsafe fn cast_pin_init<T, U, E>(init: impl PinInit<T, E>) -> 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::<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 @@ 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)
-    #[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
 }