add nightly_const_generics flag

Cargo.toml

@@ -23,6 +23,9 @@ alloc = []
 # https://github.com/rust-lang/rust/issues/54279
 nightly_slice_partition_dedup = []
 
+# use const generics for arrays
+nightly_const_generics = []
+
 [badges]
 appveyor = { repository = "Lokathor/tinyvec" }
 travis-ci = { repository = "Lokathor/tinyvec" }

src/array.rs

@@ -34,6 +34,21 @@ pub trait Array {
   fn as_slice_mut(&mut self) -> &mut [Self::Item];
 }
 
+#[cfg(feature = "nightly_const_generics")]
+impl<T: Default, const N: usize> Array for [T; N] {
+  type Item = T;
+  const CAPACITY: usize = N;
+  #[inline(always)]
+  fn as_slice(&self) -> &[T] {
+    &*self
+  }
+  #[inline(always)]
+  fn as_slice_mut(&mut self) -> &mut [T] {
+    &mut *self
+  }
+}
+
+#[cfg(not(feature = "nightly_const_generics"))]
 macro_rules! impl_array_for_len {
   ($($len:expr),+ $(,)?) => {
     $(impl<T: Default> Array for [T; $len] {
@@ -51,6 +66,7 @@ macro_rules! impl_array_for_len {
   }
 }
 
+#[cfg(not(feature = "nightly_const_generics"))]
 impl_array_for_len! {
   0, /* The oft-forgotten 0-length array! */
   1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,

src/lib.rs

@@ -4,6 +4,10 @@
   feature = "nightly_slice_partition_dedup",
   feature(slice_partition_dedup)
 )]
+#![cfg_attr(
+  feature = "nightly_const_generics",
+  feature(const_generics)
+)]
 #![warn(clippy::missing_inline_in_public_items)]
 #![warn(clippy::must_use_candidate)]
 #![warn(missing_docs)]