* lisp/eshell: Make backslash a no-op in front of normal chars

Fixes: debbugs:8531

* lisp/eshell/esh-arg.el (eshell-parse-argument-hook): Update comment.
(eshell-parse-backslash): Return escaped character after backslash
if it is special.  Otherwise, if the backslash is not in a quoted
string, ignore the backslash and return the character after; if
the backslash is in a quoted string, return the backslash and the
character after.

* test/automated/eshell.el (eshell-test/escape-nonspecial)
(eshell-test/escape-nonspecial-unicode)
(eshell-test/escape-nonspecial-quoted)
(eshell-test/escape-special-quoted): Add tests for new
`eshell-parse-backslash' behavior.

lisp/eshell/esh-arg.el

@@ -89,7 +89,7 @@ yield the values intended."
 	  (goto-char (match-end 0))
 	  (eshell-finish-arg)))))
 
-   ;; backslash before a special character means escape it
+   ;; parse backslash and the character after
    'eshell-parse-backslash
 
    ;; text beginning with ' is a literally quoted
@@ -305,34 +305,27 @@ If the character is itself a backslash, it needs no escaping."
 	  (string ?\\ char)))))
 
 (defun eshell-parse-backslash ()
-  "Parse a single backslash (\) character, which might mean escape.
-It only means escape if the character immediately following is a
-special character that is not itself a backslash."
+  "Parse a single backslash (\\) character and the character after.
+If the character after the backslash is special, always ignore
+the backslash and return the escaped character.
+
+Otherwise, if the backslash is not in quoted string, the
+backslash is ignored and the character after is returned.  If the
+backslash is in a quoted string, the backslash and the character
+after are both returned."
   (when (eq (char-after) ?\\)
-    (if (eshell-looking-at-backslash-return (point))
-	(throw 'eshell-incomplete ?\\)
-      (if (and (not (eq (char-after (1+ (point))) ?\\))
-	       (if eshell-current-quoted
-		   (memq (char-after (1+ (point)))
-			 eshell-special-chars-inside-quoting)
-		 (memq (char-after (1+ (point)))
-		       eshell-special-chars-outside-quoting)))
-	  (progn
-	    (forward-char 2)
-	    (list 'eshell-escape-arg
-		  (char-to-string (char-before))))
-	;; allow \\<RET> to mean a literal "\" character followed by a
-	;; normal return, rather than a backslash followed by a line
-	;; continuation (i.e., "\\ + \n" rather than "\ + \\n").  This
-	;; is necessary because backslashes in Eshell are not special
-	;; unless they either precede something special, or precede a
-	;; backslash that precedes something special.  (Mainly this is
-	;; done to make using backslash on Windows systems more
-	;; natural-feeling).
-	(if (eshell-looking-at-backslash-return (1+ (point)))
-	    (forward-char))
-	(forward-char)
-	"\\"))))
+    (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 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)))
+        (char-to-string (char-before))))))
 
 (defun eshell-parse-literal-quote ()
   "Parse a literally quoted string.  Nothing has special meaning!"

test/automated/eshell.el

@@ -166,6 +166,37 @@ e.g. \"{(+ 1 2)} 3\" => 3"
    (eshell-command-result-p "+ 1 2; + $_ 4"
                              "3\n6\n")))
 
+(ert-deftest eshell-test/escape-nonspecial ()
+  "Test that \"\\c\" and \"c\" are equivalent when \"c\" is not a
+special character."
+  (with-temp-eshell
+   (eshell-command-result-p "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-command-result-p "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-command-result-p "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-command-result-p "echo \"h\\\\i\""
+                            ;; Backslashes are doubled for regexp.
+                            "h\\\\i\n")))
+
 (ert-deftest eshell-test/command-running-p ()
   "Modeline should show no command running"
   (with-temp-eshell