Function write_to_spare_capacity_of_vec 

unsafe fn write_to_spare_capacity_of_vec<T>(
    output: &mut Vec<u8>,
    writer: impl FnOnce(&mut [MaybeUninit<u8>]) -> (usize, T),
) -> T

Allows writer to write data into the spare capacity of the output vector. This will not reallocate the vector provided or attempt to grow it, so space for the output must be reserved by the caller before calling this function.

writer needs to return the number of bytes written (and can also return another arbitrary return value).

Safety:

The length returned by the writer must be equal to actual number of bytes written to the uninitialized slice passed in and initialized.