// run-pass // compile-flags:-Zmir-opt-level=3 use std::mem::MaybeUninit; const N: usize = 2; trait CollectArray<A>: Iterator<Item = A> { fn inner_array(&mut self) -> [A; N]; fn collect_array(&mut self) -> [A; N] { let result = self.inner_array(); assert!(self.next().is_none()); result } } impl<A, I: ?Sized> CollectArray<A> for I where I: Iterator<Item = A>, { fn inner_array(&mut self) -> [A; N] { let mut result: [MaybeUninit<A>; N] = unsafe { MaybeUninit::uninit().assume_init() }; for (dest, item) in result.iter_mut().zip(self) { *dest = MaybeUninit::new(item); } let temp_ptr: *const [MaybeUninit<A>; N] = &result; unsafe { std::ptr::read(temp_ptr as *const [A; N]) } } } fn main() { assert_eq!( [[1, 2], [3, 4]] .iter() .map(|row| row.iter().collect_array()) .collect_array(), [[&1, &2], [&3, &4]] ); }