Function parse_read_with_capacity 

pub fn parse_read_with_capacity<R: Read, V: Visitor>(
    reader: R,
    visitor: &mut V,
    capacity: usize,
) -> Result<(), ReadError<V::Error>>

Like parse_read, but with a caller-specified buffer capacity.

capacity is clamped to a minimum of 64 bytes (one SIMD block).