Function sol_type 

fn sol_type(
    kind: &FunctionParamType,
    structs: &mut Vec<(FunctionParamType, String)>,
) -> String

The Solidity name of a type. Solidity has no anonymous tuples, so each distinct tuple type is declared once as a struct TupleN with the fields _0, _1, …, which ABI-encodes the same.