FIX: Add new_const() for const construction and revert new to old version

The new() function is significantly faster, it optimizes better at the
moment/in current rust. For this reason, provide a const fn constructor,
but not as the default `new`.

Author: bluss

src/array_string.rs

@@ -59,8 +59,27 @@ impl<const CAP: usize> ArrayString<CAP>
     /// assert_eq!(&string[..], "foo");
     /// assert_eq!(string.capacity(), 16);
     /// ```
-    pub const fn new() -> ArrayString<CAP> {
+    pub fn new() -> ArrayString<CAP> {
         assert_capacity_limit!(CAP);
+        unsafe {
+            ArrayString { xs: MaybeUninit::uninit().assume_init(), len: 0 }
+        }
+    }
+
+    /// Create a new empty `ArrayString` (const fn).
+    ///
+    /// Capacity is inferred from the type parameter.
+    ///
+    /// ```
+    /// use arrayvec::ArrayString;
+    ///
+    /// let mut string = ArrayString::<16>::new();
+    /// string.push_str("foo");
+    /// assert_eq!(&string[..], "foo");
+    /// assert_eq!(string.capacity(), 16);
+    /// ```
+    pub const fn new_const() -> ArrayString<CAP> {
+        assert_capacity_limit_const!(CAP);
         ArrayString { xs: MakeMaybeUninit::ARRAY, len: 0 }
     }
 

src/arrayvec.rs

@@ -77,8 +77,28 @@ impl<T, const CAP: usize> ArrayVec<T, CAP> {
     /// assert_eq!(&array[..], &[1, 2]);
     /// assert_eq!(array.capacity(), 16);
     /// ```
-    pub const fn new() -> ArrayVec<T, CAP> {
+    pub fn new() -> ArrayVec<T, CAP> {
         assert_capacity_limit!(CAP);
+        unsafe {
+            ArrayVec { xs: MaybeUninit::uninit().assume_init(), len: 0 }
+        }
+    }
+
+    /// Create a new empty `ArrayVec` (const fn).
+    ///
+    /// The maximum capacity is given by the generic parameter `CAP`.
+    ///
+    /// ```
+    /// use arrayvec::ArrayVec;
+    ///
+    /// let mut array = ArrayVec::<_, 16>::new();
+    /// array.push(1);
+    /// array.push(2);
+    /// assert_eq!(&array[..], &[1, 2]);
+    /// assert_eq!(array.capacity(), 16);
+    /// ```
+    pub const fn new_const() -> ArrayVec<T, CAP> {
+        assert_capacity_limit_const!(CAP);
         ArrayVec { xs: MakeMaybeUninit::ARRAY, len: 0 }
     }
 

src/lib.rs

@@ -31,6 +31,16 @@ extern crate core as std;
 pub(crate) type LenUint = u32;
 
 macro_rules! assert_capacity_limit {
+    ($cap:expr) => {
+        if std::mem::size_of::<usize>() > std::mem::size_of::<LenUint>() {
+            if $cap > LenUint::MAX as usize {
+                panic!("ArrayVec: largest supported capacity is u32::MAX")
+            }
+        }
+    }
+}
+
+macro_rules! assert_capacity_limit_const {
     ($cap:expr) => {
         if std::mem::size_of::<usize>() > std::mem::size_of::<LenUint>() {
             if $cap > LenUint::MAX as usize {

tests/tests.rs

@@ -718,19 +718,29 @@ fn allow_max_capacity_arrayvec_type() {
     let _v: ArrayVec<(), {usize::MAX}>;
 }
 
-#[should_panic(expected="index out of bounds")]
+#[should_panic(expected="largest supported capacity")]
 #[test]
 fn deny_max_capacity_arrayvec_value() {
     if mem::size_of::<usize>() <= mem::size_of::<u32>() {
-        panic!("This test does not work on this platform. 'index out of bounds'");
+        panic!("This test does not work on this platform. 'largest supported capacity'");
     }
     // this type is allowed to be used (but can't be constructed)
     let _v: ArrayVec<(), {usize::MAX}> = ArrayVec::new();
 }
 
+#[should_panic(expected="index out of bounds")]
+#[test]
+fn deny_max_capacity_arrayvec_value_const() {
+    if mem::size_of::<usize>() <= mem::size_of::<u32>() {
+        panic!("This test does not work on this platform. 'index out of bounds'");
+    }
+    // this type is allowed to be used (but can't be constructed)
+    let _v: ArrayVec<(), {usize::MAX}> = ArrayVec::new_const();
+}
+
 #[test]
 fn test_arrayvec_const_constructible() {
-    const OF_U8: ArrayVec<Vec<u8>, 10> = ArrayVec::new();
+    const OF_U8: ArrayVec<Vec<u8>, 10> = ArrayVec::new_const();
 
     let mut var = OF_U8;
     assert!(var.is_empty());
@@ -742,7 +752,7 @@ fn test_arrayvec_const_constructible() {
 
 #[test]
 fn test_arraystring_const_constructible() {
-    const AS: ArrayString<10> = ArrayString::new();
+    const AS: ArrayString<10> = ArrayString::new_const();
 
     let mut var = AS;
     assert!(var.is_empty());