That's a feature of the Lisp IDE. File mode lines described language, the syntax (which Lisp dialect), the Package, the numeric base, the encoding, ... Exampl

That's a feature of the Lisp IDE. File mode lines described language, the syntax (which Lisp dialect), the Package, the numeric base, the encoding, ... Example: ;;; -*- Mode: lisp; Syntax: ANSI-Common-Lisp; Package: http; Base: 10 -*- But to implement compiler directives in Common Lisp, this is not needed. That can already be done with macros and reader macros. If you for example use a macro in source code, the compiler will need to expand it. That allows arbitrary code execution during compilation.