Function matvec 

pub fn matvec(
    operator: &(impl LinearOperator + ?Sized),
    input: &[Complex64],
    output: &mut [Complex64],
    coefficient: Complex64,
    overwrite: bool,
) -> Result<()>

Shared scaled matrix-vector adapter corresponding to QuSpin’s low-level matvec helper. When overwrite is false the result is accumulated into the existing output buffer.