Implement quoting the local part of a remote file name

* doc/emacs/files.texi (Quoted File Names):
* etc/NEWS: Mention quoting the local part of a remote file name.

* lisp/net/tramp.el (tramp-dissect-file-name): Check with
`tramp-tramp-file-p'.
(tramp-quoted-name-p, tramp-quote-name, tramp-unquote-name):
New defsubst.
(tramp-handle-substitute-in-file-name)
(tramp-handle-make-auto-save-file-name): Handle quoted files.
(tramp-shell-quote-argument): Unquote argument.

* lisp/net/tramp-sh.el (tramp-sh-handle-file-truename):
Handle quoted files.

* test/lisp/net/tramp-tests.el (tramp--test-expensive-test): New defvar.
(tramp--test-make-temp-name): New argument QUOTED.
(tramp-test01-file-name-syntax)
(tramp-test02-file-name-dissect)
(tramp-test04-substitute-in-file-name)
(tramp-test05-expand-file-name, tramp-test07-file-exists-p)
(tramp-test08-file-local-copy)
(tramp-test09-insert-file-contents)
(tramp-test10-write-region, tramp-test11-copy-file)
(tramp-test12-rename-file, tramp-test13-make-directory)
(tramp-test14-delete-directory, tramp-test15-copy-directory)
(tramp-test16-directory-files)
(tramp-test17-insert-directory)
(tramp-test18-file-attributes)
(tramp-test19-directory-files-and-attributes)
(tramp-test20-file-modes, tramp-test21-file-links)
(tramp-test22-file-times, tramp-test23-visited-file-modtime)
(tramp-test24-file-name-completion, tramp-test25-load)
(tramp-test26-process-file, tramp-test27-start-file-process)
(tramp-test28-shell-command, tramp-test30-vc-registered)
(tramp-test31-make-auto-save-file-name)
(tramp--test-check-files)
(tramp-test35-asynchronous-requests): Test also quoted file names.
(tramp--test-shell-command-to-string-asynchronously): Rename.
(tramp-test29-environment-variables): Use it.

doc/emacs/files.texi

@@ -1870,6 +1870,11 @@ prevent it from being treated as a remote file name.  Thus, if you have
 a directory named @file{/foo:} and a file named @file{bar} in it, you
 can refer to that file in Emacs as @samp{/:/foo:/bar}.
 
+  If you want to quote only special characters in the local part of a
+remote file name, you can quote just the local part.
+@samp{/baz:/:/foo:/bar} refers to the file @file{bar} of directory
+@file{/foo:} on the host @file{baz}.
+
   @samp{/:} can also prevent @samp{~} from being treated as a special
 character for a user's home directory.  For example, @file{/:/tmp/~hack}
 refers to a file whose name is @file{~hack} in directory @file{/tmp}.

etc/NEWS

@@ -268,6 +268,12 @@ variable of this kind to swap modifiers in Emacs.
 ---
 ** New input methods: 'cyrillic-tuvan', 'polish-prefix'.
 
++++
+** File name quoting by adding the prefix "/:" is now possible for the
+local part of a remote file name.  Thus, if you have a directory named
+"/~" on the remote host "foo", you can prevent it from being
+substituted by a home directory by writing it as "/foo:/:/~/file".
+
 
 * Editing Changes in Emacs 26.1
 

lisp/net/tramp-sh.el

@@ -1146,7 +1146,9 @@ target of the symlink differ."
      (tramp-make-tramp-file-name
       method user host
       (with-tramp-file-property v localname "file-truename"
-	(let ((result nil))			; result steps in reverse order
+	(let ((result nil)			; result steps in reverse order
+	      (quoted (tramp-quoted-name-p localname))
+	      (localname (tramp-unquote-name localname)))
 	  (tramp-message v 4 "Finding true name for `%s'" filename)
 	  (cond
 	   ;; Use GNU readlink --canonicalize-missing where available.
@@ -1241,6 +1243,7 @@ target of the symlink differ."
 		(when (string= "" result)
 		  (setq result "/")))))
 
+	  (when quoted (setq result (tramp-quote-name result)))
 	  (tramp-message v 4 "True name of `%s' is `%s'" localname result)
 	  result))))
 

lisp/net/tramp.el

@@ -1239,9 +1239,9 @@ localname (file name on remote host) and hop.  If NODEFAULT is
 non-nil, the file name parts are not expanded to their default
 values."
   (save-match-data
+    (unless (tramp-tramp-file-p name)
+      (tramp-compat-user-error nil "Not a Tramp file name: \"%s\"" name))
     (let ((match (string-match (nth 0 tramp-file-name-structure) name)))
-      (unless match
-	(tramp-compat-user-error nil "Not a Tramp file name: \"%s\"" name))
       (let ((method    (match-string (nth 1 tramp-file-name-structure) name))
 	    (user      (match-string (nth 2 tramp-file-name-structure) name))
 	    (host      (match-string (nth 3 tramp-file-name-structure) name))
@@ -1679,6 +1679,27 @@ FILE must be a local file name on a connection identified via VEC."
 (font-lock-add-keywords
  'emacs-lisp-mode '("\\<with-tramp-connection-property\\>"))
 
+(defsubst tramp-quoted-name-p (name)
+  "Whether NAME is quoted with prefix \"/:\".
+If NAME is a remote file name, check the local part of NAME."
+  (string-match "^/:" (or (file-remote-p name 'localname) name)))
+
+(defsubst tramp-quote-name (name)
+  "Add the quotation prefix \"/:\" to file NAME.
+If NAME is a remote file name, the local part of NAME is quoted."
+  (concat (file-remote-p name) "/:" (or (file-remote-p name 'localname) name)))
+
+(defsubst tramp-unquote-name (name)
+  "Remove quotation prefix \"/:\" from file NAME.
+If NAME is a remote file name, the local part of NAME is unquoted."
+  (save-match-data
+    (let ((localname (or (file-remote-p name 'localname) name)))
+      (when (tramp-quoted-name-p localname)
+	(setq
+	 localname
+	 (replace-match (if (= (length localname) 2) "/" "") nil t localname)))
+      (concat (file-remote-p name) localname))))
+
 (defun tramp-drop-volume-letter (name)
   "Cut off unnecessary drive letter from file NAME.
 The functions `tramp-*-handle-expand-file-name' call `expand-file-name'
@@ -3323,20 +3344,23 @@ User is always nil."
 (defun tramp-handle-substitute-in-file-name (filename)
   "Like `substitute-in-file-name' for Tramp files.
 \"//\" and \"/~\" substitute only in the local filename part."
-  ;; First, we must replace environment variables.
-  (setq filename (tramp-replace-environment-variables filename))
-  (with-parsed-tramp-file-name filename nil
-    ;; Ignore in LOCALNAME everything before "//" or "/~".
-    (when (and (stringp localname) (string-match ".+?/\\(/\\|~\\)" localname))
-      (setq filename
-	    (concat (file-remote-p filename)
-		    (replace-match "\\1" nil nil localname)))
-      ;; "/m:h:~" does not work for completion.  We use "/m:h:~/".
-      (when (string-match "~$" filename)
-	(setq filename (concat filename "/"))))
-    ;; We do not want to replace environment variables, again.
-    (let (process-environment)
-      (tramp-run-real-handler 'substitute-in-file-name (list filename)))))
+  ;; Check, whether the local part is a quoted file name.
+  (if (tramp-quoted-name-p filename)
+      filename
+    ;; First, we must replace environment variables.
+    (setq filename (tramp-replace-environment-variables filename))
+    (with-parsed-tramp-file-name filename nil
+      ;; Ignore in LOCALNAME everything before "//" or "/~".
+      (when (and (stringp localname) (string-match ".+?/\\(/\\|~\\)" localname))
+	(setq filename
+	      (concat (file-remote-p filename)
+		      (replace-match "\\1" nil nil localname)))
+	;; "/m:h:~" does not work for completion.  We use "/m:h:~/".
+	(when (string-match "~$" filename)
+	  (setq filename (concat filename "/"))))
+      ;; We do not want to replace environment variables, again.
+      (let (process-environment)
+	(tramp-run-real-handler 'substitute-in-file-name (list filename))))))
 
 (defun tramp-handle-set-visited-file-modtime (&optional time-list)
   "Like `set-visited-file-modtime' for Tramp files."
@@ -4080,7 +4104,7 @@ this file, if that variable is non-nil."
 	       ("|" . "__")
 	       ("[" . "_l")
 	       ("]" . "_r"))
-	     (buffer-file-name))
+	     (tramp-unquote-name (buffer-file-name)))
 	    tramp-auto-save-directory))))
     ;; Run plain `make-auto-save-file-name'.
     (tramp-run-real-handler 'make-auto-save-file-name nil)))
@@ -4318,7 +4342,7 @@ T1 and T2 are time values (as returned by `current-time' for example)."
 Only works for Bourne-like shells."
   (let ((system-type 'not-windows))
     (save-match-data
-      (let ((result (shell-quote-argument s))
+      (let ((result (shell-quote-argument (tramp-unquote-name s)))
 	    (nl (regexp-quote (format "\\%s" tramp-rsh-end-of-line))))
 	(when (and (>= (length result) 2)
 		   (string= (substring result 0 2) "\\~"))