func (t *Type) Fields() []*Field { return t.fields().Slice() }

// setFields sets struct type t's fields to fields.
func (t *Type) setFields(fields []*Field) {
	// If we've calculated the width of t before,
	// then some other type such as a function signature
	// might now have the wrong type.
	// Rather than try to track and invalidate those,
	// enforce that SetFields cannot be called once
	// t's width has been calculated.