Don't require nightly for use_spin feature

Author: phil-opp

Cargo.toml

@@ -15,11 +15,10 @@ homepage = "http://os.phil-opp.com/kernel-heap.html#a-better-allocator"
 default = ["use_spin", "const_mut_refs"]
 use_spin = ["spinning_top"]
 alloc_ref = []
-const_mut_refs = []
+const_mut_refs = ["spinning_top/nightly"]
 
 [dependencies.spinning_top]
 version = "0.1.0"
-features = ["nightly"]
 optional = true
 
 [package.metadata.release]

src/lib.rs

@@ -194,10 +194,17 @@ pub struct LockedHeap(Spinlock<Heap>);
 #[cfg(feature = "use_spin")]
 impl LockedHeap {
     /// Creates an empty heap. All allocate calls will return `None`.
+    #[cfg(feature = "const_mut_refs")]
     pub const fn empty() -> LockedHeap {
         LockedHeap(Spinlock::new(Heap::empty()))
     }
 
+    /// Creates an empty heap. All allocate calls will return `None`.
+    #[cfg(not(feature = "const_mut_refs"))]
+    pub fn empty() -> LockedHeap {
+        LockedHeap(Spinlock::new(Heap::empty()))
+    }
+
     /// Creates a new heap with the given `bottom` and `size`. The bottom address must be valid
     /// and the memory in the `[heap_bottom, heap_bottom + heap_size)` range must not be used for
     /// anything else. This function is unsafe because it can cause undefined behavior if the