Make the use of unstable features non-mandatory

Author: yvt

Cargo.toml

@@ -10,4 +10,8 @@ readme = "README.md"
 license = "MIT"
 keywords = ["atomic", "reference", "static"]
 
+[features]
+default = []
+nightly = []
+
 [dependencies]

src/lib.rs

@@ -71,9 +71,15 @@
 //!     assert!(res);
 //! }
 //! ```
+//!
+//! # Cargo Features
+//!
+//!  - `nightly` enables the use of unstable features. Turns [`AtomicRef::new`]
+//!    into `const fn`, making it callable in constant contexts.
+//!
 #![no_std]
-#![feature(const_fn)]
-#![feature(const_if_match)]
+#![cfg_attr(feature = "nightly", feature(const_fn))]
+#![cfg_attr(feature = "nightly", feature(const_if_match))]
 
 use core::sync::atomic::{AtomicPtr, Ordering};
 use core::marker::PhantomData;
@@ -174,12 +180,29 @@ macro_rules! static_atomic_ref {
     () => ();
 }
 
-/// An internal helper function for converting `Option<&'a T>` values to
-/// `*mut T` for storing in the `AtomicUsize`.
-const fn from_opt<'a, T>(p: Option<&'a T>) -> *mut T {
-    match p {
-        Some(p) => p as *const T as *mut T,
-        None => null_mut(),
+macro_rules! const_fn_if_nightly {
+    (
+        $( #[$meta:meta] )*
+        $vis:vis fn $($rest:tt)*
+    ) => {
+        $( #[$meta] )*
+        #[cfg(feature = "nightly")]
+        $vis const fn $($rest)*
+
+        $( #[$meta] )*
+        #[cfg(not(feature = "nightly"))]
+        $vis fn $($rest)*
+    };
+}
+
+const_fn_if_nightly! {
+    /// An internal helper function for converting `Option<&'a T>` values to
+    /// `*mut T` for storing in the `AtomicUsize`.
+    fn from_opt<'a, T>(p: Option<&'a T>) -> *mut T {
+        match p {
+            Some(p) => p as *const T as *mut T,
+            None => null_mut(),
+        }
     }
 }
 
@@ -190,20 +213,22 @@ unsafe fn to_opt<'a, T>(p: *mut T) -> Option<&'a T> {
 }
 
 impl<'a, T> AtomicRef<'a, T> {
-    /// Creates a new `AtomicRef`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use atomic_ref::AtomicRef;
-    ///
-    /// static VALUE: i32 = 10;
-    /// let atomic_ref = AtomicRef::new(Some(&VALUE));
-    /// ```
-    pub const fn new(p: Option<&'a T>) -> AtomicRef<'a, T> {
-        AtomicRef {
-            data: AtomicPtr::new(from_opt(p)),
-            _marker: PhantomData,
+    const_fn_if_nightly! {
+        /// Creates a new `AtomicRef`.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use atomic_ref::AtomicRef;
+        ///
+        /// static VALUE: i32 = 10;
+        /// let atomic_ref = AtomicRef::new(Some(&VALUE));
+        /// ```
+        pub fn new(p: Option<&'a T>) -> AtomicRef<'a, T> {
+            AtomicRef {
+                data: AtomicPtr::new(from_opt(p)),
+                _marker: PhantomData,
+            }
         }
     }