Fix const fns that were not const and add const ArrayVec::from

src/lib.rs

@@ -6,6 +6,7 @@
 #![feature(const_mut_refs)]
 #![feature(const_option)]
 #![feature(const_refs_to_cell)]
+#![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_split_at_mut)]
 #![feature(const_trait_impl)]
 #![feature(effects)]
@@ -23,7 +24,6 @@ use std::fmt::{Debug, Display};
 use std::hash::Hash;
 use std::mem::MaybeUninit;
 use std::ops::{Deref, DerefMut};
-use std::slice::SliceIndex;
 
 #[derive(Copy)]
 pub struct ArrayVec<T, const N: usize> {
@@ -36,6 +36,32 @@ pub struct ArrayVec<T, const N: usize> {
 //     pub const EMPTY: ArrayVec<T, 0> = Self::new();
 // }
 
+#[inline(always)]
+const unsafe fn slice_unchecked<T>(slice: &[T], offset: usize, len: Option<usize>) -> &[T] {
+    std::slice::from_raw_parts(
+        slice.as_ptr().add(offset),
+        match len {
+            Some(t) => t,
+            None => slice.len() - offset,
+        },
+    )
+}
+
+#[inline(always)]
+const unsafe fn slice_unchecked_mut<T>(
+    slice: &mut [T],
+    offset: usize,
+    len: Option<usize>,
+) -> &mut [T] {
+    std::slice::from_raw_parts_mut(
+        slice.as_mut_ptr().add(offset),
+        match len {
+            Some(t) => t,
+            None => slice.len() - offset,
+        },
+    )
+}
+
 impl<T, const N: usize> ArrayVec<T, N> {
     #[inline]
     pub const fn new() -> Self {
@@ -48,14 +74,19 @@ impl<T, const N: usize> ArrayVec<T, N> {
     #[inline]
     pub const fn as_slice(&self) -> &[T] {
         unsafe {
-            MaybeUninit::slice_assume_init_ref(&*(0..self.len).get_unchecked(self.data.as_slice()))
+            MaybeUninit::slice_assume_init_ref(slice_unchecked(&self.data, 0, Some(self.len)))
         }
     }
 
     #[inline]
     pub const fn as_slice_mut(&mut self) -> &mut [T] {
-        let slice: &mut [MaybeUninit<T>] = &mut self.data;
-        unsafe { MaybeUninit::slice_assume_init_mut(&mut *(0..self.len).get_unchecked_mut(slice)) }
+        unsafe {
+            MaybeUninit::slice_assume_init_mut(slice_unchecked_mut(
+                &mut self.data,
+                0,
+                Some(self.len),
+            ))
+        }
     }
 
     #[inline]
@@ -85,13 +116,12 @@ impl<T, const N: usize> ArrayVec<T, N> {
 
     #[inline(always)]
     pub const fn unused(&self) -> &[MaybeUninit<T>] {
-        unsafe { &*(self.len..).get_unchecked(self.data.as_slice()) }
+        unsafe { slice_unchecked(&self.data, self.len, None) }
     }
 
     #[inline(always)]
     pub const fn unused_mut(&mut self) -> &mut [MaybeUninit<T>] {
-        let slice: &mut [MaybeUninit<T>] = &mut self.data;
-        unsafe { &mut *(self.len..).get_unchecked_mut(slice) }
+        unsafe { slice_unchecked_mut(&mut self.data, self.len, None) }
     }
 
     /// # Safety
@@ -121,6 +151,14 @@ impl<T, const N: usize> ArrayVec<T, N> {
             self.set_len(old_len + slice.len());
         }
     }
+
+    #[inline]
+    pub const fn from(value: [T; N]) -> Self {
+        Self {
+            len: value.len(),
+            data: MaybeUninit::transpose(MaybeUninit::new(value)),
+        }
+    }
 }
 
 impl<T: Clone, const N: usize> Clone for ArrayVec<T, N> {
@@ -352,10 +390,7 @@ impl<const N: usize> Display for ArrayStr<N> {
 impl<T, const N: usize> From<[T; N]> for ArrayVec<T, N> {
     #[inline]
     fn from(value: [T; N]) -> Self {
-        Self {
-            len: value.len(),
-            data: MaybeUninit::transpose(MaybeUninit::new(value)),
-        }
+        Self::from(value)
     }
 }
 
@@ -488,3 +523,12 @@ impl<'a, const N: usize> TryFrom<char> for ArrayStr<N> {
 //         ArrayStr(self.0.concat(rhs.0))
 //     }
 // }
+
+#[cfg(test)]
+mod tests {
+    use crate::ArrayVec;
+
+    const _FOO: ArrayVec<(), 4> = ArrayVec::from([(), (), (), ()]);
+    const _FOO_LEN: usize = _FOO.as_slice().len();
+    const _FOO_UNUSED: usize = _FOO.unused().len();
+}