Add unstable-const-fn feature to make new() functions const.

.travis.yml

@@ -22,6 +22,9 @@ matrix:
     - rust: nightly
       env:
       - FEATURES='array-sizes-33-128 array-sizes-129-255'
+    - rust: nightly
+      env:
+      - FEATURES='unstable-const-fn'
     - name: "miri"
       script: sh ci/miri.sh
 branches:

Cargo.toml

@@ -37,6 +37,7 @@ harness = false
 [features]
 default = ["std"]
 std = []
+unstable-const-fn = []
 
 array-sizes-33-128 = []
 array-sizes-129-255 = []

src/array.rs

@@ -34,11 +34,13 @@ pub unsafe trait Array {
 }
 
 pub trait Index : PartialEq + Copy {
+    const ZERO: Self;
     fn to_usize(self) -> usize;
     fn from(_: usize) -> Self;
 }
 
 impl Index for () {
+    const ZERO: Self = ();
     #[inline(always)]
     fn to_usize(self) -> usize { 0 }
     #[inline(always)]
@@ -46,6 +48,7 @@ impl Index for () {
 }
 
 impl Index for bool {
+    const ZERO: Self = false;
     #[inline(always)]
     fn to_usize(self) -> usize { self as usize }
     #[inline(always)]
@@ -53,6 +56,7 @@ impl Index for bool {
 }
 
 impl Index for u8 {
+    const ZERO: Self = 0;
     #[inline(always)]
     fn to_usize(self) -> usize { self as usize }
     #[inline(always)]
@@ -60,6 +64,7 @@ impl Index for u8 {
 }
 
 impl Index for u16 {
+    const ZERO: Self = 0;
     #[inline(always)]
     fn to_usize(self) -> usize { self as usize }
     #[inline(always)]
@@ -67,6 +72,7 @@ impl Index for u16 {
 }
 
 impl Index for u32 {
+    const ZERO: Self = 0;
     #[inline(always)]
     fn to_usize(self) -> usize { self as usize }
     #[inline(always)]
@@ -74,6 +80,7 @@ impl Index for u32 {
 }
 
 impl Index for usize {
+    const ZERO: Self = 0;
     #[inline(always)]
     fn to_usize(self) -> usize { self }
     #[inline(always)]

src/array_string.rs

@@ -58,11 +58,22 @@ impl<A> ArrayString<A>
     /// assert_eq!(&string[..], "foo");
     /// assert_eq!(string.capacity(), 16);
     /// ```
+    #[cfg(not(feature="unstable-const-fn"))]
     pub fn new() -> ArrayString<A> {
         unsafe {
             ArrayString {
                 xs: MaybeUninitCopy::uninitialized(),
-                len: Index::from(0),
+                len: Index::ZERO,
+            }
+        }
+    }
+
+    #[cfg(feature="unstable-const-fn")]
+    pub const fn new() -> ArrayString<A> {
+        unsafe {
+            ArrayString {
+                xs: MaybeUninitCopy::uninitialized(),
+                len: Index::ZERO,
             }
         }
     }

src/lib.rs

@@ -14,12 +14,19 @@
 //!   - Optional
 //!   - Enable more array sizes (see [Array] for more information)
 //!
+//! - `unstable-const-fn`
+//!   - Optional
+//!   - Makes [`ArrayVec::new`] and [`ArrayString::new`] `const fn`s,
+//!     using the nightly `const_fn` feature.
+//!   - Unstable and requires nightly.
+//!
 //! ## Rust Version
 //!
 //! This version of arrayvec requires Rust 1.36 or later.
 //!
 #![doc(html_root_url="https://docs.rs/arrayvec/0.4/")]
 #![cfg_attr(not(feature="std"), no_std)]
+#![cfg_attr(feature="unstable-const-fn", feature(const_fn))]
 
 #[cfg(feature="serde")]
 extern crate serde;
@@ -106,9 +113,17 @@ impl<A: Array> ArrayVec<A> {
     /// assert_eq!(&array[..], &[1, 2]);
     /// assert_eq!(array.capacity(), 16);
     /// ```
+    #[cfg(not(feature="unstable-const-fn"))]
     pub fn new() -> ArrayVec<A> {
         unsafe {
-            ArrayVec { xs: MaybeUninit::uninitialized(), len: Index::from(0) }
+            ArrayVec { xs: MaybeUninit::uninitialized(), len: Index::ZERO }
+        }
+    }
+
+    #[cfg(feature="unstable-const-fn")]
+    pub const fn new() -> ArrayVec<A> {
+        unsafe {
+            ArrayVec { xs: MaybeUninit::uninitialized(), len: Index::ZERO }
         }
     }
 

src/maybe_uninit.rs

@@ -16,7 +16,7 @@ impl<T> Clone for MaybeUninit<T>
 
 impl<T> MaybeUninit<T> {
     /// Create a new MaybeUninit with uninitialized interior
-    pub unsafe fn uninitialized() -> Self {
+    pub const unsafe fn uninitialized() -> Self {
         MaybeUninit { inner: StdMaybeUninit::uninit() }
     }