error_(r.Pos(), "must call C.%s", fixGo(r.Name.Go))
		}
	}
	return expr
}

// gofmtPos returns the gofmt-formatted string for an AST node,
// with a comment setting the position before the node.
func gofmtPos(n ast.Expr, pos token.Pos) string {
	s := gofmt(n)
	p := fset.Position(pos)
	if p.Column == 0 {
		return s
	}
	return fmt.Sprintf("/*line :%d:%d*/%s", p.Line, p.Column, s)
}