add regression test for rust-lang#71805

Author: lcnr

src/test/ui/const-generics/type-dependent/issue-71805.rs

@@ -0,0 +1,41 @@
+// run-pass
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+use std::mem::MaybeUninit;
+
+trait CollectSlice<'a>: Iterator {
+    fn inner_array<const N: usize>(&mut self) -> [Self::Item; N];
+
+    fn collect_array<const N: usize>(&mut self) -> [Self::Item; N] {
+        let result = self.inner_array();
+        assert!(self.next().is_none());
+        result
+    }
+}
+
+impl<'a, I: ?Sized> CollectSlice<'a> for I
+where
+    I: Iterator,
+{
+    fn inner_array<const N: usize>(&mut self) -> [Self::Item; N] {
+        let mut result: [MaybeUninit<Self::Item>; N] =
+            unsafe { MaybeUninit::uninit().assume_init() };
+
+        let mut count = 0;
+        for (dest, item) in result.iter_mut().zip(self) {
+            *dest = MaybeUninit::new(item);
+            count += 1;
+        }
+
+        assert_eq!(N, count);
+
+        let temp_ptr: *const [MaybeUninit<Self::Item>; N] = &result;
+        unsafe { std::ptr::read(temp_ptr as *const [Self::Item; N]) }
+    }
+}
+
+fn main() {
+    let mut foos = [0u64; 9].iter().cloned();
+    let _bar: [u64; 9] = foos.collect_array::<9_usize>();
+}