Fix documentation related to 'command-switch-alist'.

While there, add a unit test to verify the behavior.

* doc/lispref/os.texi (Command-Line Arguments): Fix documentation: the
option string in 'command-switch-alist' does include leading hyphens.
Also mention that 'command-switch-alist' parsing ignores equals signs
in options.

* test/lisp/startup-tests.el
(startup-tests/command-switch-alist): New unit test.

doc/lispref/os.texi

@@ -613,7 +613,7 @@ The elements of the @code{command-switch-alist} look like this:
 @end example
 
 The @sc{car}, @var{option}, is a string, the name of a command-line
-option (not including the initial hyphen).  The @var{handler-function}
+option (including the initial hyphen).  The @var{handler-function}
 is called to handle @var{option}, and receives the option name as its
 sole argument.
 
@@ -623,6 +623,14 @@ remaining command-line arguments in the variable
 @code{command-line-args-left} (see below).  (The entire list of
 command-line arguments is in @code{command-line-args}.)
 
+Note that the handling of @code{command-switch-alist} doesn't treat
+equals signs in @var{option} specially.  That is, if there's an option
+like @code{--name=value} on the command line, then only a
+@code{command-switch-alist} member whose @code{car} is literally
+@code{--name=value} will match this option.  If you want to parse such
+options, you need to use @code{command-line-functions} instead (see
+below).
+
 The command-line arguments are parsed by the @code{command-line-1}
 function in the @file{startup.el} file.  See also @ref{Emacs
 Invocation, , Command Line Arguments for Emacs Invocation, emacs, The

test/lisp/startup-tests.el

@@ -0,0 +1,47 @@
+;;; startup-tests.el --- unit tests for startup.el   -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2020  Free Software Foundation, Inc.
+
+;; Author: Philipp Stephani <phst@google.com>
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Unit tests for startup.el.
+
+;;; Code:
+
+(ert-deftest startup-tests/command-switch-alist ()
+  (let* ((foo-args ()) (bar-args ())
+         (command-switch-alist
+          (list (cons "--foo"
+                      (lambda (arg)
+                        (ert-info ("Processing argument --foo")
+                          (push arg foo-args)
+                          (should (equal command-line-args-left
+                                         '("value" "--bar=value")))
+                          (pop command-line-args-left))))
+                (cons "--bar=value"
+                      (lambda (arg)
+                        (ert-info ("Processing argument --bar")
+                          (push arg bar-args)
+                          (should-not command-line-args-left)))))))
+    (command-line-1 '("--foo" "value" "--bar=value"))
+    (should (equal foo-args '("--foo")))
+    (should (equal bar-args '("--bar=value")))))
+
+;;; startup-tests.el ends here