Function: cc-eval-when-compile

cc-eval-when-compile is a macro defined in cc-defs.el.gz.

Signature

(cc-eval-when-compile &rest BODY)

Documentation

Like progn, but evaluates the body at compile time.

The result of the body appears to the compiler as a quoted constant.

This variant works around bugs in eval-when-compile in various
(X)Emacs versions. See cc-defs.el for details.

Source Code

;; Defined in /usr/src/emacs/lisp/progmodes/cc-defs.el.gz
;; The following is used below during compilation.
(eval-and-compile
  (defvar c-inside-eval-when-compile nil)

  (defmacro cc-eval-when-compile (&rest body)
    "Like `progn', but evaluates the body at compile time.
The result of the body appears to the compiler as a quoted constant.

This variant works around bugs in `eval-when-compile' in various
\(X)Emacs versions.  See cc-defs.el for details."
    (declare (indent 0) (debug t))
    (if c-inside-eval-when-compile
	;; XEmacs 21.4.6 has a bug in `eval-when-compile' in that it
	;; evaluates its body at macro expansion time if it's nested
	;; inside another `eval-when-compile'.  So we use a dynamically
	;; bound variable to avoid nesting them.
	`(progn ,@body)

      `(eval-when-compile
	 ;; In all (X)Emacsen so far, `eval-when-compile' byte compiles
	 ;; its contents before evaluating it.  That can cause forms to
	 ;; be compiled in situations they aren't intended to be
	 ;; compiled.
	 ;;
	 ;; Example: It's not possible to defsubst a primitive, e.g. the
	 ;; following will produce an error (in any emacs flavor), since
	 ;; `nthcdr' is a primitive function that's handled specially by
	 ;; the byte compiler and thus can't be redefined:
	 ;;
	 ;;     (defsubst nthcdr (val) val)
	 ;;
	 ;; `defsubst', like `defmacro', needs to be evaluated at
	 ;; compile time, so this will produce an error during byte
	 ;; compilation.
	 ;;
	 ;; CC Mode occasionally needs to do things like this for
	 ;; cross-emacs compatibility.  It therefore uses the following
	 ;; to conditionally do a `defsubst':
	 ;;
	 ;;     (eval-when-compile
	 ;;       (if (not (fboundp 'foo))
	 ;;           (defsubst foo ...)))
	 ;;
	 ;; But `eval-when-compile' byte compiles its contents and
	 ;; _then_ evaluates it (in all current emacs versions, up to
	 ;; and including Emacs 20.6 and XEmacs 21.1 as of this
	 ;; writing).  So this will still produce an error, since the
	 ;; byte compiler will get to the defsubst anyway.  That's
	 ;; arguably a bug because the point with `eval-when-compile' is
	 ;; that it should evaluate rather than compile its contents.
	 ;;
	 ;; We get around it by expanding the body to a quoted
	 ;; constant that we eval.  That otoh introduce a problem in
	 ;; that a returned lambda expression doesn't get byte
	 ;; compiled (even if `function' is used).
	 (eval '(let ((c-inside-eval-when-compile t)) ,@body))))))