Add const assert to MapWindows

Checks if const generic N is not set to 0, otherwise it prevents
compilation.

library/core/src/iter/adapters/map_windows.rs

@@ -49,9 +49,12 @@ struct Buffer<T, const N: usize> {
 }
 
 impl<I: Iterator, F, const N: usize> MapWindows<I, F, N> {
-    pub(in crate::iter) fn new(iter: I, f: F) -> Self {
+    const N_NOT_ZERO: () =
         assert!(N != 0, "array in `Iterator::map_windows` must contain more than 0 elements");
 
+    pub(in crate::iter) fn new(iter: I, f: F) -> Self {
+        let _ = Self::N_NOT_ZERO;
+
         // Only ZST arrays' length can be so large.
         if mem::size_of::<I::Item>() == 0 {
             assert!(

library/core/src/iter/traits/iterator.rs

@@ -1625,12 +1625,9 @@ pub trait Iterator {
     /// [`slice::windows()`]: slice::windows
     /// [`FusedIterator`]: crate::iter::FusedIterator
     ///
-    /// # Panics
-    ///
-    /// Panics if `N` is 0. This check will most probably get changed to a
-    /// compile time error before this method gets stabilized.
+    /// If `N` is 0, then the code will not compile.
     ///
-    /// ```should_panic
+    /// ```compile_fail
     /// #![feature(iter_map_windows)]
     ///
     /// let iter = std::iter::repeat(0).map_windows(|&[]| ());