properly document let binding workaround

examples/big_struct_in_place.rs

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
+
 use pin_init::*;
 
 // Struct with size over 1GiB

internal/src/init.rs

@@ -189,6 +189,12 @@ 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
     }})
 }

src/lib.rs

@@ -1147,9 +1147,12 @@ 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
 }
 
@@ -1163,9 +1166,12 @@ 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
 }
 

tests/default_error.rs

@@ -1,4 +1,5 @@
 #![allow(dead_code)]
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
 
 use pin_init::{init, Init};
 

tests/ui/expand/no_field_access.expanded.rs

@@ -68,6 +68,9 @@ fn main() {
         let init = unsafe {
             ::pin_init::init_from_closure::<_, ::core::convert::Infallible>(init)
         };
-        init
+        #[allow(
+            clippy::let_and_return,
+            reason = "some clippy versions warn about the let binding"
+        )] init
     };
 }

tests/ui/expand/simple-init.expanded.rs

@@ -30,6 +30,9 @@ fn main() {
         let init = unsafe {
             ::pin_init::init_from_closure::<_, ::core::convert::Infallible>(init)
         };
-        init
+        #[allow(
+            clippy::let_and_return,
+            reason = "some clippy versions warn about the let binding"
+        )] init
     };
 }

tests/underscore.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
+
 use pin_init::{init, Init};
 
 pub struct Foo {