Make "eager macro-expansion" warning into an error

* doc/lispref/loading.texi (How Programs Do Loading): Update
documentation.

* lisp/emacs-lisp/macroexp.el:
(internal-macroexpand-for-load): We've been warning about eager
macro expansion for many years, so finally change that into an
error (bug#18154).

doc/lispref/loading.texi

@@ -149,10 +149,9 @@ up the execution of uncompiled code.  Sometimes, this macro expansion
 cannot be done, owing to a cyclic dependency.  In the simplest
 example of this, the file you are loading refers to a macro defined
 in another file, and that file in turn requires the file you are
-loading.  This is generally harmless.  Emacs prints a warning
+loading.  Emacs will issue an error about
 (@samp{Eager macro-expansion skipped due to cycle@dots{}})
-giving details of the problem, but it still loads the file, just
-leaving the macro unexpanded for now.  You may wish to restructure
+giving details of the problem.  You have to restructure
 your code so that this does not happen.  Loading a compiled file does
 not cause macroexpansion, because this should already have happened
 during compilation.  @xref{Compiling Macros}.

etc/NEWS

@@ -159,6 +159,9 @@ of 'user-emacs-directory'.
 
 * Incompatible changes in Emacs 29.1
 
++++
+** Warning about "eager macro-expansion failure" is changed into an error.
+
 ---
 ** Previously, the X reverseVideo value at startup was heeded for all frames.
 This meant that if you had a reverseVideo resource on the initial

lisp/emacs-lisp/macroexp.el

@@ -796,7 +796,7 @@ test of free variables in the following ways:
         (if (eq (car-safe (car bt)) 'macroexpand-all) (setq bt (cdr bt)))
         (if macroexp--debug-eager
             (debug 'eager-macroexp-cycle)
-          (message "Warning: Eager macro-expansion skipped due to cycle:\n  %s"
+          (error "Warning: Eager macro-expansion skipped due to cycle:\n  %s"
                  (mapconcat #'prin1-to-string (nreverse bt) " => ")))
         (push 'skip macroexp--pending-eager-loads)
         form))
@@ -811,7 +811,7 @@ test of free variables in the following ways:
          ;; Hopefully this shouldn't happen thanks to the cycle detection,
          ;; but in case it does happen, let's catch the error and give the
          ;; code a chance to macro-expand later.
-         (message "Eager macro-expansion failure: %S" err)
+         (error "Eager macro-expansion failure: %S" err)
          form))))))
 
 ;; ¡¡¡ Big Ugly Hack !!!