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).

doc/misc/eshell.texi

@@ -256,12 +256,40 @@ as an argument will ``spread'' the elements into multiple arguments:
 @end example
 
 @subsection Quoting and escaping
-As with other shells, you can escape special characters and spaces
+As with other shells, you can escape special characters and spaces by
-with by prefixing the character with a backslash (@code{\}), or by
+prefixing the character with a backslash (@samp{\}), or by surrounding
-surrounding the string with apostrophes (@code{''}) or double quotes
+the string with apostrophes (@samp{''}) or double quotes (@samp{""}).
-(@code{""}).  This is needed especially for file names with special
+This is needed especially for file names with special characters like
-characters like pipe (@code{|}), which could be part of remote file
+pipe (@samp{|}), which could be part of remote file names.
-names.
+
+When you escape a character with @samp{\} outside of any quotes, the
+result is the literal character immediately following it.  For
+example, @code{\$10} means the literal string @code{$10}.
+
+Inside of double quotes, most characters have no special meaning.
+However, @samp{\}, @samp{"}, and @samp{$} are still special; to escape
+them, use backslash as above.  Thus, if the value of the variable
+@var{answer} is @code{42}, then @code{"The answer is: \"$answer\""}
+returns the string @code{The answer is: "42"}.  However, when escaping
+characters with no special meaning, the result is the full
+@code{\@var{c}} sequence.  For example, @code{"foo\bar"} means the
+literal string @code{foo\bar}.
+
+Additionally, when escaping a newline, the whole escape sequence is
+removed by the parser.  This lets you continue commands across
+multiple lines:
+
+@example
+~ $ echo "foo\
+bar"
+foobar
+@end example
+
+Inside apostrophes, escaping works differently.  All characters
+between the apostrophes have their literal meaning except @samp{'},
+which ends the quoted string.  To insert a literal apostrophe, you can
+use @samp{''}, so @code{'It''s me'} means the literal string
+@code{It's me}.
 
 When using expansions (@pxref{Expansion}) in an Eshell command, the
 result may potentially be of any data type.  To ensure that the result

lisp/eshell/esh-arg.el

@@ -146,9 +146,10 @@ If POS is nil, the location of point is checked."
 When each function on this hook is called, point will be at the
 current position within the argument list.  The function should either
 return nil, meaning that it did no argument parsing, or it should
-return the result of the parse as a sexp.  It is also responsible for
+return the result of the parse as a sexp.  If the function did do
-moving the point forward to reflect the amount of input text that was
+argument parsing, but the result was nothing at all, it should return
-parsed.
+`eshell-empty-token'.  The function is also responsible for moving the
+point forward to reflect the amount of input text that was parsed.
 
 If the hook determines that it has reached the end of an argument, it
 should call `eshell-finish-arg' to complete processing of the current
@@ -325,13 +326,14 @@ Point is left at the end of the arguments."
 		   (prog1
 		       (char-to-string (char-after))
 		     (forward-char)))))
-	  (if (not eshell-current-argument)
+          (unless (eq result 'eshell-empty-token)
-	      (setq eshell-current-argument result)
+            (if (not eshell-current-argument)
-	    (unless eshell-arg-listified
+                (setq eshell-current-argument result)
-	      (setq eshell-current-argument
+              (unless eshell-arg-listified
-		    (list eshell-current-argument)
+                (setq eshell-current-argument
-		    eshell-arg-listified t))
+                      (list eshell-current-argument)
-	    (nconc eshell-current-argument (list result))))))
+                      eshell-arg-listified t))
+              (nconc eshell-current-argument (list result)))))))
     (when (and outer eshell-current-argument)
       (add-text-properties arg-begin (1+ arg-begin)
 			   '(arg-begin t rear-nonsticky
@@ -375,15 +377,20 @@ after are both returned."
     (when (eshell-looking-at-backslash-return (point))
 	(throw 'eshell-incomplete ?\\))
     (forward-char 2) ; Move one char past the backslash.
-    ;; If the char is in a quote, backslash only has special meaning
+    (if (eq (char-before) ?\n)
-    ;; if it is escaping a special char.
+        ;; Escaped newlines are extra-special: they expand to an empty
-    (if eshell-current-quoted
+        ;; token to allow for continuing Eshell commands across
-        (if (memq (char-before) eshell-special-chars-inside-quoting)
+        ;; multiple lines.
+        'eshell-empty-token
+      ;; If the char is in a quote, backslash only has special meaning
+      ;; if it is escaping a special char.
+      (if eshell-current-quoted
+          (if (memq (char-before) eshell-special-chars-inside-quoting)
+              (list 'eshell-escape-arg (char-to-string (char-before)))
+            (concat "\\" (char-to-string (char-before))))
+        (if (memq (char-before) eshell-special-chars-outside-quoting)
             (list 'eshell-escape-arg (char-to-string (char-before)))
-          (concat "\\" (char-to-string (char-before))))
+          (char-to-string (char-before)))))))
-      (if (memq (char-before) eshell-special-chars-outside-quoting)
-          (list 'eshell-escape-arg (char-to-string (char-before)))
-        (char-to-string (char-before))))))
 
 (defun eshell-parse-literal-quote ()
   "Parse a literally quoted string.  Nothing has special meaning!"

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

test/lisp/eshell/eshell-tests.el

@@ -105,37 +105,6 @@
      (format template "format \"%s\" eshell-in-pipeline-p")
      "nil")))
 
-(ert-deftest eshell-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 eshell-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 eshell-test/escape-nonspecial-quoted ()
-  "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 eshell-test/escape-special-quoted ()
-  "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 eshell-test/command-running-p ()
   "Modeline should show no command running"
   (with-temp-eshell