Vec::from_array now const

Author: tower120

CHANGELOG.md

@@ -12,6 +12,7 @@ of panicking drop implementations.
 - Added `from_bytes_truncating_at_nul` to `CString`
 - Added `CString::{into_bytes, into_bytes_with_nul, into_string}`
 - Added `pop_front_if` and `pop_back_if` to `Deque`
+- Made `Vec::from_array` const.
 
 ## [v0.9.2] 2025-11-12
 

src/len_type.rs

@@ -1,8 +1,17 @@
 use core::{
     fmt::{Debug, Display},
+    mem,
     ops::{Add, AddAssign, Sub, SubAssign},
 };
 
+#[allow(non_camel_case_types)]
+pub enum TypeEnum {
+    u8,
+    u16,
+    u32,
+    usize,
+}
+
 #[cfg(feature = "zeroize")]
 use zeroize::Zeroize;
 
@@ -27,6 +36,8 @@ pub trait Sealed:
     const MAX: Self;
     /// The maximum value of this type, as a `usize`.
     const MAX_USIZE: usize;
+    /// This type as an enum.
+    const TYPE: TypeEnum;
 
     /// The one value of the integer type.
     ///
@@ -58,12 +69,13 @@ pub trait Sealed:
 }
 
 macro_rules! impl_lentype {
-    ($($(#[$meta:meta])* $LenT:ty),*) => {$(
+    ($($(#[$meta:meta])* $LenT:ident),*) => {$(
         $(#[$meta])*
         impl Sealed for $LenT {
             const ZERO: Self = 0;
             const MAX: Self = Self::MAX;
             const MAX_USIZE: usize = Self::MAX as _;
+            const TYPE: TypeEnum = TypeEnum::$LenT;
 
             fn one() -> Self {
                 1
@@ -100,3 +112,69 @@ impl_lentype!(
 pub const fn check_capacity_fits<LenT: LenType, const N: usize>() {
     assert!(LenT::MAX_USIZE >= N, "The capacity is larger than `LenT` can hold, increase the size of `LenT` or reduce the capacity");
 }
+
+/// Const cast from [`usize`] to [`LenType`] with `as`.
+#[inline]
+pub const fn as_len_type<L: LenType>(n: usize) -> L {
+    // SAFETY: transmute is safe since after cast we cast to the same type.
+    unsafe {
+        // ALWAYS compiletime switch.
+        match L::TYPE {
+            // transmute_copy, instead of transmute - because `L`
+            // is a "dependent type".
+            TypeEnum::u8 => mem::transmute_copy(&(n as u8)),
+            TypeEnum::u16 => mem::transmute_copy(&(n as u16)),
+            TypeEnum::u32 => mem::transmute_copy(&(n as u32)),
+            TypeEnum::usize => mem::transmute_copy(&n),
+        }
+    }
+}
+
+/// Checked cast to [`LenType`].
+///
+/// # Panic
+///
+/// Panics if `n` is outside of `L` range.
+#[inline]
+pub const fn to_len_type<L: LenType>(n: usize) -> L {
+    try_to_len_type(n).unwrap()
+}
+
+/// Checked cast to [`LenType`].
+///
+/// Returns `None` if `n` is outside of `L` range.
+#[inline]
+pub const fn try_to_len_type<L: LenType>(n: usize) -> Option<L> {
+    if n > L::MAX_USIZE {
+        return None;
+    }
+    Some(as_len_type(n))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_len_cast() {
+        // 1. Check constness
+        const {
+            assert!(to_len_type::<u8>(150) == 150);
+            assert!(to_len_type::<u16>(15_000) == 15_000);
+            assert!(to_len_type::<u32>(1_500_000) == 1_500_000);
+            assert!(to_len_type::<usize>(usize::MAX) == usize::MAX);
+        }
+        // 2. Check correctness
+        fn check<T: LenType>() {
+            const COUNT: usize = 100;
+            for i in 0..COUNT {
+                let n = i * (T::MAX_USIZE / COUNT);
+                assert_eq!(to_len_type::<T>(n).into_usize(), n);
+            }
+        }
+        check::<u8>();
+        check::<u16>();
+        check::<u32>();
+        check::<usize>();
+    }
+}

src/vec/mod.rs

@@ -6,7 +6,7 @@ use core::{
     fmt, hash,
     iter::FusedIterator,
     marker::PhantomData,
-    mem::{self, ManuallyDrop, MaybeUninit},
+    mem::{ManuallyDrop, MaybeUninit},
     ops::{self, Range, RangeBounds},
     ptr::{self, NonNull},
     slice,
@@ -16,7 +16,7 @@ use core::{
 use zeroize::Zeroize;
 
 use crate::{
-    len_type::{check_capacity_fits, LenType},
+    len_type::{check_capacity_fits, to_len_type, LenType},
     CapacityError,
 };
 
@@ -355,7 +355,7 @@ impl<T, LenT: LenType, const N: usize> Vec<T, N, LenT> {
     ///
     /// If the length of the provided array is greater than the capacity of the
     /// vector a compile-time error will be produced.
-    pub fn from_array<const M: usize>(src: [T; M]) -> Self {
+    pub const fn from_array<const M: usize>(src: [T; M]) -> Self {
         const {
             assert!(N >= M);
         }
@@ -364,26 +364,24 @@ impl<T, LenT: LenType, const N: usize> Vec<T, N, LenT> {
         // any Drop code for T.
         let src = ManuallyDrop::new(src);
 
-        if N == M {
-            Self {
-                phantom: PhantomData,
-                len: LenT::from_usize(N),
-                // NOTE(unsafe) ManuallyDrop<[T; M]> and [MaybeUninit<T>; N]
-                // have the same layout when N == M.
-                buffer: unsafe { mem::transmute_copy(&src) },
-            }
-        } else {
-            let mut v = Self::new();
+        let len: LenT = to_len_type(M);
 
-            for (src_elem, dst_elem) in src.iter().zip(v.buffer.buffer.iter_mut()) {
-                // NOTE(unsafe) src element is not going to drop as src itself
-                // is wrapped in a ManuallyDrop.
-                dst_elem.write(unsafe { ptr::read(src_elem) });
-            }
+        let mut v = Self::new();
 
-            unsafe { v.set_len(M) };
-            v
-        }
+        // MaybeUninit::deref is non-const, so we just cast to it's internal value.
+        let src_ptr: *const T = ptr::from_ref(&src).cast();
+
+        // Cast from buffer's [MaybeUninit<T>] to [T].
+        let dst_ptr: *mut T = v.buffer.buffer.as_mut_ptr().cast();
+
+        // SAFETY: Move/copy data from src to v's internal buffer.
+        // * Using src_ptr as `*const T` is safe since src's ManuallyDrop<[T; M]> is transparent.
+        // * Using dst_ptr as `*mut T` is safe since v's [MaybeUninit<T>; N] is an array of
+        //   transparent types.
+        unsafe { ptr::copy_nonoverlapping(src_ptr, dst_ptr, M) };
+        v.len = len;
+
+        v
     }
 
     /// Returns the contents of the vector as an array of length `M` if the length
@@ -466,6 +464,8 @@ impl<T, LenT: LenType, S: VecStorage<T> + ?Sized> VecInner<T, LenT, S> {
     /// [`mem::forget`], for example), the vector may have lost and leaked
     /// elements arbitrarily, including elements outside the range.
     ///
+    /// [`mem::forget`]: core::mem::forget
+    ///
     /// # Examples
     ///
     /// ```