feat: Make unbounded a const function

This PR makes `ConcurrentQueue::unbounded` a const function. It'd be great if `bounded` could be `const` as well, but this would likely require static memory allocation support in const functions, which is currently not allowed by the compiler. This would enable https://github.com/smol-rs/async-executor/pull/112 to be directly constructable in a const context (i.e. static/thread_local variable initialization without OnceLock). It might also allow unbounded `async_channel`s to be constructed in a similar context.

Co-authored-by: Taiki Endo <te316e89@gmail.com>

src/lib.rs

@@ -75,6 +75,24 @@ mod unbounded;
 
 mod sync;
 
+/// Make the given function const if the given condition is true.
+macro_rules! const_fn {
+    (
+        const_if: #[cfg($($cfg:tt)+)];
+        $(#[$($attr:tt)*])*
+        $vis:vis const fn $($rest:tt)*
+    ) => {
+        #[cfg($($cfg)+)]
+        $(#[$($attr)*])*
+        $vis const fn $($rest)*
+        #[cfg(not($($cfg)+))]
+        $(#[$($attr)*])*
+        $vis fn $($rest)*
+    };
+}
+
+pub(crate) use const_fn;
+
 /// A concurrent queue.
 ///
 /// # Examples
@@ -131,18 +149,21 @@ impl<T> ConcurrentQueue<T> {
         }
     }
 
-    /// Creates a new unbounded queue.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use concurrent_queue::ConcurrentQueue;
-    ///
-    /// let q = ConcurrentQueue::<i32>::unbounded();
-    /// ```
-    pub fn unbounded() -> ConcurrentQueue<T> {
-        ConcurrentQueue(Inner::Unbounded(Unbounded::new()))
-    }
+    const_fn!(
+        const_if: #[cfg(not(loom))];
+        /// Creates a new unbounded queue.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use concurrent_queue::ConcurrentQueue;
+        ///
+        /// let q = ConcurrentQueue::<i32>::unbounded();
+        /// ```
+        pub const fn unbounded() -> ConcurrentQueue<T> {
+            ConcurrentQueue(Inner::Unbounded(Unbounded::new()))
+        }
+    );
 
     /// Attempts to push an item into the queue.
     ///

src/unbounded.rs

@@ -4,6 +4,7 @@ use core::ptr;
 
 use crossbeam_utils::CachePadded;
 
+use crate::const_fn;
 use crate::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
 use crate::sync::cell::UnsafeCell;
 #[allow(unused_imports)]
@@ -148,19 +149,22 @@ pub struct Unbounded<T> {
 }
 
 impl<T> Unbounded<T> {
-    /// Creates a new unbounded queue.
-    pub fn new() -> Unbounded<T> {
-        Unbounded {
-            head: CachePadded::new(Position {
-                block: AtomicPtr::new(ptr::null_mut()),
-                index: AtomicUsize::new(0),
-            }),
-            tail: CachePadded::new(Position {
-                block: AtomicPtr::new(ptr::null_mut()),
-                index: AtomicUsize::new(0),
-            }),
+    const_fn!(
+        const_if: #[cfg(not(loom))];
+        /// Creates a new unbounded queue.
+        pub const fn new() -> Unbounded<T> {
+            Unbounded {
+                head: CachePadded::new(Position {
+                    block: AtomicPtr::new(ptr::null_mut()),
+                    index: AtomicUsize::new(0),
+                }),
+                tail: CachePadded::new(Position {
+                    block: AtomicPtr::new(ptr::null_mut()),
+                    index: AtomicUsize::new(0),
+                }),
+            }
         }
-    }
+    );
 
     /// Pushes an item into the queue.
     pub fn push(&self, value: T) -> Result<(), PushError<T>> {