helper: split out ExactSizeArr into its own module

src/exactsizearr.rs

@@ -0,0 +1,23 @@
+/// An iterator adaptor (derived from ExactSizedIterator) which has a guaranteed
+/// compile time size, allowing for collection of an iterator into a stack
+/// allocated array.
+pub trait ExactSizedArr<I>: ExactSizeIterator<Item = I> + Sized
+where
+    I: Default,
+{
+    fn into_array<const N: usize>(mut self) -> Result<[Self::Item; N], Self> {
+        if self.len() < N {
+            Err(self)
+        } else {
+            Ok(std::array::from_fn(|_| self.next().unwrap()))
+        }
+    }
+}
+
+/// Default implementation of ExactSizedArr for any ExactSizeIterator.
+impl<T, I> ExactSizedArr<T> for I
+where
+    T: Default + Copy + Clone,
+    I: ExactSizeIterator<Item = T>,
+{
+}