Transient input methods bound to 'C-x \' (bug#44266)

* lisp/international/mule-cmds.el (ctl-x-map): Bind 'C-x \' to
'transient-input-method'.
(input-method-function): New defcustom.
(transient-input-method): New command.

* doc/emacs/mule.texi (Select Input Method): Document transient-input-method.

doc/emacs/mule.texi

@@ -578,6 +578,11 @@ Enable or disable use of the selected input method (@code{toggle-input-method}).
 @item C-x @key{RET} C-\ @var{method} @key{RET}
 Select a new input method for the current buffer (@code{set-input-method}).
 
+@item C-x \ @var{method} @key{RET}
+Temporarily enable the selected transient input method, and
+automatically disable it after inserting a single character
+(@code{transient-input-method}).
+
 @item C-h I @var{method} @key{RET}
 @itemx C-h C-\ @var{method} @key{RET}
 @findex describe-input-method
@@ -675,6 +680,16 @@ character.
 input methods.  The list gives information about each input method,
 including the string that stands for it in the mode line.
 
+@findex transient-input-method
+@kindex C-x \
+  To insert only a single character using a transient input method you
+can first select a transient input method by typing @kbd{C-u C-x \}.
+Then typing @kbd{C-x \} (@code{transient-input-method}) will
+temporarily enable the selected transient input method, and disable it
+automatically after using the activated input method to insert
+a single character.  This is useful to insert a character from input
+methods with rare Unicode characters.
+
 @node Coding Systems
 @section Coding Systems
 @cindex coding systems

etc/NEWS

@@ -132,14 +132,6 @@ Emacs to break lines after more characters than just whitespace
 characters.  In particular, this significantly improves word-wrapping
 for CJK text mixed with Latin text.
 
----
-** New input method 'compose' based on X Multi_key sequences.
-
----
-** Improved language transliteration in Malayalam input methods.
-Added a new Mozhi scheme.  The inapplicable ITRANS scheme is now
-deprecated.  Errors in the Inscript method were corrected.
-
 ---
 ** Rudimentary support for the 'st' terminal emulator.
 Emacs now supports 256 color display on the 'st' terminal emulator.
@@ -629,6 +621,23 @@ recorded for the purpose of 'view-lossage'.
 The menu bar "Help" menu now has a "Show Recent Inputs" item under the
 "Describe" sub-menu.
 
+** Input methods
+
++++
+*** 'C-x \' temporarily enables a transient input method.
+'C-u C-x \' can be used to select a transient input method, e.g.
+'C-u C-x \ compose RET' selects the 'compose' input method.  Then typing
+'C-x \ 1 2' will insert the character '½', and disable the input method
+afterwards.
+
+---
+*** New input method 'compose' based on X Multi_key sequences.
+
+---
+*** Improved language transliteration in Malayalam input methods.
+Added a new Mozhi scheme.  The inapplicable ITRANS scheme is now
+deprecated.  Errors in the Inscript method were corrected.
+
 ** Ispell
 
 +++

lisp/international/mule-cmds.el

@@ -55,6 +55,7 @@
 
 ;; Keep "C-x C-m ..." for mule specific commands.
 (define-key ctl-x-map "\C-m" mule-keymap)
+(define-key ctl-x-map "\\" 'transient-input-method)
 
 (defvar describe-language-environment-map
   (let ((map (make-sparse-keymap "Describe Language Environment")))
@@ -1344,6 +1345,16 @@ This is the input method activated automatically by the command
                  mule-input-method-string)
   :set-after '(current-language-environment))
 
+(defcustom transient-input-method nil
+  "Default transient input method.
+This is the input method activated automatically by the command
+`transient-input-method' (\\[transient-input-method])."
+  :link  '(custom-manual "(emacs)Input Methods")
+  :group 'mule
+  :type '(choice (const nil)
+                 mule-input-method-string)
+  :set-after '(current-language-environment))
+
 (put 'input-method-function 'permanent-local t)
 
 (defvar input-method-history nil
@@ -1519,6 +1530,35 @@ To deactivate it programmatically, use `deactivate-input-method'."
 (defvar toggle-input-method-active nil
   "Non-nil inside `toggle-input-method'.")
 
+(defun transient-input-method (&optional arg interactive)
+  "Enable transient input method for the current buffer."
+  (interactive "P\np")
+  (when (or arg (not transient-input-method))
+    (let* ((default (or (car input-method-history) default-input-method))
+           (input-method
+            (read-input-method-name
+             (if default "Transient input method (default %s): " "Transient input method: ")
+             default t)))
+      (setq transient-input-method input-method)
+      (when interactive
+        (customize-mark-as-set 'transient-input-method))))
+  (let* ((previous-input-method current-input-method)
+         (history input-method-history)
+         (clearfun (make-symbol "clear-transient-input-method"))
+         (exitfun
+          (lambda ()
+            (deactivate-input-method)
+            (when previous-input-method
+              (activate-input-method previous-input-method))
+            (setq input-method-history history)
+            (remove-hook 'input-method-after-insert-chunk-hook clearfun))))
+    (fset clearfun (lambda () (funcall exitfun)))
+    (add-hook 'input-method-after-insert-chunk-hook clearfun)
+    (when previous-input-method
+      (deactivate-input-method))
+    (activate-input-method transient-input-method)
+    exitfun))
+
 (defun toggle-input-method (&optional arg interactive)
   "Enable or disable multilingual text input method for the current buffer.
 Only one input method can be enabled at any time in a given buffer.