Treat escaped newlines in Eshell as the empty string

This fixes a regression introduced during Emacs 29's development.

* lisp/eshell/esh-arg.el (eshell-parse-argument): Handle
'eshell-empty-token' as the result of an argument-parsing hook.
(eshell-parse-argument-hook): Document 'eshell-empty-token'.
(eshell-parse-backslash): Return 'eshell-empty-token' when
encountering an escaped newline.

* test/lisp/eshell/eshell-tests.el (eshell-test/escape-nonspecial)
(eshell-test/escape-nonspecial-unicode)
(eshell-test/escape-nonspecial-quoted)
(eshell-test/escape-special-quoted): Move from here...

* test/lisp/eshell/esh-arg-tests.el (esh-arg-test/escape/nonspecial)
(esh-arg-test/escape/nonspecial-unicode)
(esh-arg-test/escape-quoted/nonspecial)
(esh-arg-test/escape-quoted/special): ... to here.
(esh-arg-test/escape/special, esh-arg-test/escape/newline)
(esh-arg-test/escape-quoted/newline): New tests.

* doc/misc/eshell.texi (Arguments): Explain escaping logic in more
detail (bug#59622).

1 files changed, 105 insertions, 0 deletions

diff --git a/test/lisp/eshell/esh-arg-tests.el b/test/lisp/eshell/esh-arg-tests.el
new file mode 100644
index 00000000000..77f9404d4c7
--- /dev/null
+++ b/test/lisp/eshell/esh-arg-tests.el
@@ -0,0 +1,105 @@
+;;; esh-arg-tests.el --- esh-arg test suite  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+
+;; 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:
+
+;; Tests for Eshell's argument handling.
+
+;;; Code:
+
+(require 'ert)
+(require 'esh-mode)
+(require 'eshell)
+
+(require 'eshell-tests-helpers
+         (expand-file-name "eshell-tests-helpers"
+                           (file-name-directory (or load-file-name
+                                                    default-directory))))
+
+(defvar eshell-test-value nil)
+
+;;; Tests:
+
+(ert-deftest esh-arg-test/escape/nonspecial ()
+  "Test that \"\\c\" and \"c\" are equivalent when \"c\" is not a
+special character."
+  (with-temp-eshell
+   (eshell-match-command-output "echo he\\llo"
+                                "hello\n")))
+
+(ert-deftest esh-arg-test/escape/nonspecial-unicode ()
+  "Test that \"\\c\" and \"c\" are equivalent when \"c\" is a
+unicode character (unicode characters are nonspecial by
+definition)."
+  (with-temp-eshell
+   (eshell-match-command-output "echo Vid\\éos"
+                                "Vidéos\n")))
+
+(ert-deftest esh-arg-test/escape/special ()
+  "Test that the backslash is not preserved for escaped special
+chars."
+  (with-temp-eshell
+   (eshell-match-command-output "echo he\\\\llo"
+                                ;; Backslashes are doubled for regexp.
+                                "he\\\\llo\n")))
+
+(ert-deftest esh-arg-test/escape/newline ()
+  "Test that an escaped newline is equivalent to the empty string.
+When newlines are *nonspecial*, an escaped newline should be
+treated as just a newline."
+  (with-temp-eshell
+   (eshell-match-command-output "echo hi\\\nthere"
+                                "hithere\n")))
+
+(ert-deftest esh-arg-test/escape/newline-conditional ()
+  "Test invocation of an if/else statement using line continuations."
+  (let ((eshell-test-value t))
+    (eshell-command-result-equal
+     "if $eshell-test-value \\\n{echo yes} \\\n{echo no}"
+     "yes"))
+  (let ((eshell-test-value nil))
+    (eshell-command-result-equal
+     "if $eshell-test-value \\\n{echo yes} \\\n{echo no}"
+     "no")))
+
+(ert-deftest esh-arg-test/escape-quoted/nonspecial ()
+  "Test that the backslash is preserved for escaped nonspecial
+chars."
+  (with-temp-eshell
+   (eshell-match-command-output "echo \"h\\i\""
+                                ;; Backslashes are doubled for regexp.
+                                "h\\\\i\n")))
+
+(ert-deftest esh-arg-test/escape-quoted/special ()
+  "Test that the backslash is not preserved for escaped special
+chars."
+  (with-temp-eshell
+   (eshell-match-command-output "echo \"\\\"hi\\\\\""
+                                ;; Backslashes are doubled for regexp.
+                                "\\\"hi\\\\\n")))
+
+(ert-deftest esh-arg-test/escape-quoted/newline ()
+  "Test that an escaped newline is equivalent to the empty string.
+When newlines are *nonspecial*, an escaped newline should be
+treated literally, as a backslash and a newline."
+  (with-temp-eshell
+   (eshell-match-command-output "echo \"hi\\\nthere\""
+                                "hithere\n")))
+
+;; esh-arg-tests.el ends here