presto-main/src/main/java/io/prestosql/type/InternalTypeManager.java/InternalTypeManager/getCoercion
class InternalTypeManager:    @Override
    public MethodHandle getCoercion(Type fromType, Type toType)
    {
        ResolvedFunction function = metadata.getCoercion(fromType, toType);
        return metadata.getScalarFunctionImplementation(function).getMethodHandle();
    }