examples: conditionally enable feature(lint_reasons)

`lint_reasons` is unstable in Rust 1.80 and earlier, enable it
conditionally in the examples to allow compiling them with older
compilers.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: BennoLossin

examples/linked_list.rs

@@ -2,6 +2,7 @@
 
 #![allow(clippy::undocumented_unsafe_blocks)]
 #![cfg_attr(feature = "alloc", feature(allocator_api))]
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
 
 use core::{
     cell::Cell,

examples/mutex.rs

@@ -2,6 +2,7 @@
 
 #![allow(clippy::undocumented_unsafe_blocks)]
 #![cfg_attr(feature = "alloc", feature(allocator_api))]
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
 #![allow(clippy::missing_safety_doc)]
 
 use core::{

examples/pthread_mutex.rs

@@ -3,6 +3,8 @@
 // inspired by <https://github.com/nbdd0121/pin-init/blob/trunk/examples/pthread_mutex.rs>
 #![allow(clippy::undocumented_unsafe_blocks)]
 #![cfg_attr(feature = "alloc", feature(allocator_api))]
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
+
 #[cfg(not(windows))]
 mod pthread_mtx {
     #[cfg(feature = "alloc")]

examples/static_init.rs

@@ -2,6 +2,7 @@
 
 #![allow(clippy::undocumented_unsafe_blocks)]
 #![cfg_attr(feature = "alloc", feature(allocator_api))]
+#![cfg_attr(not(RUSTC_LINT_REASONS_IS_STABLE), feature(lint_reasons))]
 
 use core::{
     cell::{Cell, UnsafeCell},