auto-compile

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never loads outdated byte code files.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already exists but is outdated.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Setting `load-prefer-newer' to t prevents outdated byte code files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.

Setup
-----

To reduce the risk of loading outdated byte-code files, you should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".)

Usage
-----

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 docstring for more information.

Customization
-------------

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 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).