Array FromIterator proposal

from_iter.rs

/// This takes at most `N` elements from the iterator.
///
/// # Examples
///
/// ```
/// let mut iter = 0..;
/// let arr: [usize, 10] = iter.by_ref().collect().unwrap();
/// assert_eq!(iter.next(), 10);
/// ```
impl<T, const N: usize> FromIterator<T> for Result<[T; N], FillError<T, N>> {
    /* ... */   
}

/// An error returned by `FromIterator` for arrays if the iterator contained less than `N` elements.
pub struct FillError<T, const N: usize> {
    arr: [MaybeUninit<T>; N]
    len: usize,
}

impl<T, const N: usize> FillError<T, N> {
    /// Returns the how many elements were read from the given iterator.
    pub fn len(&self) -> usize {
        self.len
    }
    
    /// Returns an immutable slice of all initialized elements.
    pub fn as_slice(&self) -> &[T] {
        MaybeUninit::slice_get_ref(&self.arr[0..self.len])
    }
    
    /// Returns a mutable slice of all initialized elements.
    pub fn as_mut_slice(&mut self) -> &mut [T] {
        MaybeUninit::slice_get_mut(&mut self.arr[0..self.len])
    }
    
    /// Tries to initialize the left-over elements using `iter`.
    pub fn fill<I>(self, iter: I) -> Result<[T; N], FillError<T; N>> {
        /* ... */   
    }
}

rust-lang/rust#69985 (comment)