This package provides two minor modes which automatically recompile Emacs Lisp source files. Together these modes guarantee that Emacs never loads outdated byte code files.

auto-compile-on-save-mode re-compiles source files when they are being saved and auto-compile-on-load-mode does so before they are being loaded (by advising load and require). Both modes only ever re-compile a source file when the respective byte code file already exists but is outdated. Otherwise they do not compile the source file.

Even when using auto-compile-on-save-mode it can happen that some source file is newer than the respective byte code file, which is a problem because by default Emacs loads the byte code file even when the respective source file has been modified more recently.

Setting load-prefer-newer to t prevents outdated byte code files from being loaded. However this does not cause re-compilation of the source file, to actually do that auto-compile-on-load-mode is still required.

To reduce the risk of loading outdated byte-code files, you should set load-prefer-newer and enable auto-compile-on-load-mode as early as possible, in early-init.el. If you additionally enable auto-compile-on-save-mode, it is best to do that in the early init file as well.

So if you use package.el, I recommend something like this:

;;; early-init.el -*- no-byte-compile: t -*-
(setq load-prefer-newer t)
(package-initialize)
(require 'auto-compile)
(auto-compile-on-load-mode)
(auto-compile-on-save-mode)

Or when installing manually or using an esoteric package manager:

;;; early-init.el -*- no-byte-compile: t -*-
(setq load-prefer-newer t)
(setq package-enable-at-startup nil)
(add-to-list 'load-path "/path/to/auto-compile")
(require 'auto-compile)
(auto-compile-on-load-mode)
(auto-compile-on-save-mode)

By putting these settings in the early init file, you ensure that all other personal init files and user installed packages benefit from the extra protection. Except for the early init file itself of course, but thanks to the -*- no-byte-compile: t -*- that does not matter.

As an additional safety-net you might want to consider enabling load-prefer-newer in a system-wide init file, so that it also takes effect when using emacs -q. (Or even to patch Emacs, to enable load-prefer-newer by default, so that it is enabled even when using emacs -Q.)

Take note of the compile warnings and fix them.

To permanently or temporarily toggle automatic compilation of some source file use the command toggle-auto-compile. Since the modes only ever update byte code files, toggling automatic compilation is done simply by either creating the byte code file or by removing it. toggle-auto-compile can also toggle automatic compilation of multiple files at once; see its doc-string for more information.

Constantly having the *Compile-Log* buffer pop up when a file is being saved can quickly become annoying. Obviously the first thing you should do to about that is to actually fix outstanding issues.

Once you have done that you might also want to keep that buffer from being automatically displayed and instead only show the number of compile warnings for the current file in the mode-line.

(setq auto-compile-display-buffer nil)
(setq auto-compile-mode-line-counter t)

To display the buffer use M-x auto-compile-display-log or click on the counter in the mode-line.

Using auto-compile-inhibit-compile-hook it is possible to inhibit automatic compilation under certain circumstances; e.g., when HEAD is detached inside a Git repository (useful during rebase sessions).