From e4fb95fa18072cedb021a82f7aa0e79fa6ca387a Mon Sep 17 00:00:00 2001
From: Stefan Monnier <monnier@iro.umontreal.ca>
Date: Sat, 7 Mar 2020 23:28:12 -0500
Subject: [PATCH] * lisp/emacs-lisp/bytecomp.el: Drop warning for loading into
 Emacs<23

Stash the major version of the compiling Emacs such that the loading
Emacs can later detect when loading a file compiled by a too-new Emacs.

(byte-compile-fix-header): Remove.
(byte-compile-from-buffer): Don't call it any more.
(byte-compile-insert-header): Stash the emacs-major-version in it.
Don't leave space for `byte-compile-fix-header`.
---
 lisp/emacs-lisp/bytecomp.el | 68 ++++++++-----------------------------
 1 file changed, 15 insertions(+), 53 deletions(-)

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 63348456a15..4f01918bdb9 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -2140,50 +2140,9 @@ With argument ARG, insert value in current buffer after the form."
 	;; Make warnings about unresolved functions
 	;; give the end of the file as their position.
 	(setq byte-compile-last-position (point-max))
-	(byte-compile-warn-about-unresolved-functions))
-      ;; Fix up the header at the front of the output
-      ;; if the buffer contains multibyte characters.
-      (and byte-compile-current-file
-	   (with-current-buffer byte-compile--outbuffer
-	     (byte-compile-fix-header byte-compile-current-file))))
+	(byte-compile-warn-about-unresolved-functions)))
      byte-compile--outbuffer)))
 
-(defun byte-compile-fix-header (_filename)
-  "If the current buffer has any multibyte characters, insert a version test."
-  (when (< (point-max) (position-bytes (point-max)))
-    (goto-char (point-min))
-    ;; Find the comment that describes the version condition.
-    (when (search-forward "\n;;; This file does not contain utf-8" nil t)
-      (narrow-to-region (line-beginning-position) (point-max))
-      ;; Find the first line of ballast semicolons.
-      (search-forward ";;;;;;;;;;")
-      (beginning-of-line)
-      (narrow-to-region (point-min) (point))
-      (let ((old-header-end (point))
-	    (minimum-version "23")
-	    delta)
-        (delete-region (point-min) (point-max))
-        (insert
-         ";;; This file contains utf-8 non-ASCII characters,\n"
-         ";;; and so cannot be loaded into Emacs 22 or earlier.\n"
-         ;; Have to check if emacs-version is bound so that this works
-         ;; in files loaded early in loadup.el.
-         "(and (boundp 'emacs-version)\n"
-         ;; If there is a name at the end of emacs-version,
-         ;; don't try to check the version number.
-         "     (< (aref emacs-version (1- (length emacs-version))) ?A)\n"
-         (format "     (string-lessp emacs-version \"%s\")\n" minimum-version)
-         ;; Because the header must fit in a fixed width, we cannot
-         ;; insert arbitrary-length file names (Bug#11585).
-         "     (error \"`%s' was compiled for "
-         (format "Emacs %s or later\" #$))\n\n" minimum-version))
-        ;; Now compensate for any change in size, to make sure all
-        ;; positions in the file remain valid.
-        (setq delta (- (point-max) old-header-end))
-        (goto-char (point-max))
-        (widen)
-        (delete-char delta)))))
-
 (defun byte-compile-insert-header (_filename outbuffer)
   "Insert a header at the start of OUTBUFFER.
 Call from the source buffer."
@@ -2201,7 +2160,19 @@ Call from the source buffer."
       ;; 0	string		;ELC		GNU Emacs Lisp compiled file,
       ;; >4	byte		x		version %d
       (insert
-       ";ELC" 23 "\000\000\000\n"
+       ";ELC"
+       (let ((version
+              (if (zerop emacs-minor-version)
+                  ;; Let's allow silently loading into Emacs-27
+                  ;; files compiled with Emacs-28.0.NN since the two can
+                  ;; be almost identical (e.g. right after cutting the
+                  ;; release branch) and people running the development
+                  ;; branch can be presumed to know that it's risky anyway.
+                  (1- emacs-major-version) emacs-major-version)))
+         ;; Make sure the version is a plain byte that doesn't end the comment!
+         (cl-assert (and (> version 13) (< version 128)))
+         version)
+       "\000\000\000\n"
        ";;; Compiled\n"
        ";;; in Emacs version " emacs-version "\n"
        ";;; with"
@@ -2213,16 +2184,7 @@ Call from the source buffer."
        ".\n"
        (if dynamic ";;; Function definitions are lazy-loaded.\n"
 	 "")
-       "\n"
-       ;; Note that byte-compile-fix-header may change this.
-       ";;; This file does not contain utf-8 non-ASCII characters,\n"
-       ";;; and so can be loaded in Emacs versions earlier than 23.\n\n"
-       ;; Insert semicolons as ballast, so that byte-compile-fix-header
-       ;; can delete them so as to keep the buffer positions
-       ;; constant for the actual compiled code.
-       ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;\n"
-       ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;\n"
-       ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;\n\n"))))
+       "\n\n"))))
 
 (defun byte-compile-output-file-form (form)
   ;; Write the given form to the output buffer, being careful of docstrings
-- 
2.39.2