Function is_dyn_method 

pub fn is_dyn_method<'db>(
    interner: DbInterner<'db>,
    _env: ParamEnv<'db>,
    func: FunctionId,
    fn_subst: GenericArgs<'db>,
) -> Option<usize>

Checks if the self parameter of Trait method is the dyn Trait and we should call the method using the vtable.