Use Box<[usize]> instead of Vec<usize> in Permutations

Author: ronnodas

src/lazy_buffer.rs

@@ -1,5 +1,7 @@
+use alloc::boxed::Box;
 use alloc::vec::Vec;
 use core::borrow::BorrowMut;
+use core::ops::Deref as _;
 use std::iter::Fuse;
 use std::ops::Index;
 
@@ -148,6 +150,30 @@ impl ArrayOrVecHelper for Vec<usize> {
     }
 }
 
+impl ArrayOrVecHelper for Box<[usize]> {
+    type Item<T> = Vec<T>;
+    type Length = usize;
+
+    fn extract_item<I: Iterator>(&self, pool: &LazyBuffer<I>) -> Self::Item<I::Item>
+    where
+        I::Item: Clone,
+    {
+        pool.get_at(self)
+    }
+
+    fn item_from_fn<T, F: Fn(usize) -> T>(len: Self::Length, f: F) -> Self::Item<T> {
+        (0..len).map(f).collect()
+    }
+
+    fn len(&self) -> Self::Length {
+        self.deref().len()
+    }
+
+    fn from_fn<F: Fn(usize) -> usize>(k: Self::Length, f: F) -> Self {
+        (0..k).map(f).collect()
+    }
+}
+
 impl<const K: usize> ArrayOrVecHelper for [usize; K] {
     type Item<T> = [T; K];
     type Length = ConstUsize<K>;

src/permutations.rs

@@ -1,5 +1,4 @@
 use alloc::boxed::Box;
-use alloc::vec::Vec;
 use std::fmt;
 use std::iter::FusedIterator;
 
@@ -17,7 +16,7 @@ pub struct PermutationsGeneric<I: Iterator, Idx: ArrayOrVecHelper> {
 ///
 /// See [`.permutations()`](crate::Itertools::permutations) for
 /// more information.
-pub type Permutations<I> = PermutationsGeneric<I, Vec<usize>>;
+pub type Permutations<I> = PermutationsGeneric<I, Box<[usize]>>;
 
 impl<I, Idx> Clone for PermutationsGeneric<I, Idx>
 where