Fix size check when composed array is too long

Ensure the union fields of ArrayConcatComposed have matching sizes by
updating the size validations in the macros concat_array and
split_array.

Previously, the size check in concat_arrays were only catching cases
where the inferred result type was shorter than the sum of input array
lengths. It failed to detect longer result arrays, since the size of
Self matches its biggest union field.
For example, this compiled without error:
let c: [u32; 6] = concat_arrays!([1, 2, 3], [4, 5]);
This led to arrays containing undefined values.
The same applied for split_array when the input array is longer than
the sum of the output array lengths. While this did not lead to
undefined values, the exceeding elements would not be dropped.

Author: jackscan

src/lib.rs

@@ -9,6 +9,21 @@ macro_rules! concat_arrays_size {
 }
 
 /// Concatenates provided arrays.
+///
+/// The macro verifies that the combined length of the input arrays matches the length of the result array.
+/// The examples below will fail to compile as the input arrays are either too short or too long.
+/// ```compile_fail
+/// # #[macro_use] extern crate array_concat;
+/// # fn main() {
+/// let c: [u32; 6] = concat_arrays!([1, 2, 3], [4, 5]); // too short
+/// # }
+/// ```
+/// ```compile_fail
+/// # #[macro_use] extern crate array_concat;
+/// # fn main() {
+/// let c: [u32; 6] = concat_arrays!([1, 2, 3], [4, 5, 6, 7]); // too long
+/// # }
+/// ```
 #[macro_export]
 macro_rules! concat_arrays {
     ($( $array:expr ),*) => ({
@@ -39,7 +54,7 @@ macro_rules! concat_arrays {
         }
 
         impl<T, A, B, const N: usize> ArrayConcatComposed<T, A, B, N> {
-            const PANIC: bool = $crate::_const_assert_same_size::<[T; N], Self>();
+            const PANIC: bool = $crate::_const_assert_same_size::<[T; N], ArrayConcatDecomposed::<T, A, B>>();
 
             #[inline(always)]
             const fn have_same_size(&self) -> bool {
@@ -89,6 +104,21 @@ macro_rules! flatten_split {
 }
 
 /// Split the provided array into the specified sizes.
+///
+/// The macro verifies that the length of the input array matches the combined length of the result arrays.
+/// The examples below will fail to compile as the input array is either too short or too long.
+/// ```compile_fail
+/// # #[macro_use] extern crate array_concat;
+/// # fn main() {
+/// let (left, right): ([u32; 3], [u32; 4]) = split_array!([1, 2, 3, 4, 5, 6], 3, 4); // too short
+/// # }
+/// ```
+/// ```compile_fail
+/// # #[macro_use] extern crate array_concat;
+/// # fn main() {
+/// let (left, right): ([u32; 3], [u32; 2]) = split_array!([1, 2, 3, 4, 5, 6], 3, 2); // too long
+/// # }
+/// ```
 #[macro_export]
 macro_rules! split_array {
     ($array:expr, $size:expr) => ($array);
@@ -141,7 +171,7 @@ macro_rules! split_array {
                 }
 
                 impl<T, A, B, const N: usize> ArrayConcatComposed<T, A, B, N> {
-                    const PANIC: bool = $crate::_const_assert_same_size::<[T; N], Self>();
+                    const PANIC: bool = $crate::_const_assert_same_size::<[T; N], ArrayConcatDecomposed::<T, A, B>>();
 
                     #[inline(always)]
                     const fn have_same_size(&self) -> bool {