Function merge 

pub fn merge<T, F: FnMut(&T, &T) -> bool>(
    v: &mut [T],
    scratch: &mut [MaybeUninit<T>],
    mid: usize,
    is_less: &mut F,
)

🔬This is a nightly-only experimental API. (slice_internals)

Merges non-decreasing runs v[..mid] and v[mid..] using scratch as temporary storage, and stores the result into v[..].