Support transient input methods in Isearch mode (bug#44266)

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

* doc/emacs/search.texi (Special Isearch):
Document isearch-transient-input-method.

* lisp/international/isearch-x.el (isearch-transient-input-method):
New function.
(isearch-process-search-multibyte-characters):
Call 'deactivate-transient-input-method' after 'read-string'.

* lisp/international/mule-cmds.el (mule-menu-keymap): Remove
duplicate menu item 'describe-input-method'.  Add new menu item
'activate-transient-input-method'.
(default-transient-input-method): Rename from transient-input-method.
(current-transient-input-method)
(previous-transient-input-method): New buffer-local variables.
(deactivate-input-method): Don't add
current-transient-input-method to input-method-history.
(toggle-input-method): Call deactivate-transient-input-method
when current-transient-input-method is non-nil.
(activate-transient-input-method): Rename from transient-input-method.
(deactivate-transient-input-method): New function with body from
renamed function transient-input-method.

* lisp/isearch.el (isearch-menu-bar-map): Add new menu item
'isearch-transient-input-method'.
(isearch-mode-map): Bind 'C-x \' to isearch-transient-input-method.
(isearch-forward): Add isearch-transient-input-method to docstring.
(isearch-message-prefix): Use shorter string for narrowed buffer.

6 files changed, 90 insertions, 44 deletions

diff --git a/doc/emacs/mule.texi b/doc/emacs/mule.texi
index 2fca4a544ca..200937c9d71 100644
--- a/doc/emacs/mule.texi
+++ b/doc/emacs/mule.texi
@@ -581,7 +581,7 @@ 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 ; it will be
 automatically disabled after inserting a single character
-(@code{transient-input-method}).
+(@code{activate-transient-input-method}).
 
 @item C-h I @var{method} @key{RET}
 @itemx C-h C-\ @var{method} @key{RET}
@@ -680,13 +680,13 @@ 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
+@findex activate-transient-input-method
 @kindex C-x \
   Sometimes it can be convenient to enable an input method
 @dfn{transiently}, for inserting only a single character.  Typing
-@kbd{C-x \} (@code{transient-input-method}) will temporarily enable an
-input method, let you insert a single character using the input method
-rules, and then automatically disable the input method.  If no
+@kbd{C-x \} (@code{activate-transient-input-method}) will temporarily
+enable an input method, let you insert a single character using the input
+method rules, and then automatically disable the input method.  If no
 transient input method was selected yet, @kbd{C-x \} will prompt you
 for an input method; subsequent invocations of this command will
 enable the selected transient input method.  To select a different
diff --git a/doc/emacs/search.texi b/doc/emacs/search.texi
index 91b433f1738..f33c71db229 100644
--- a/doc/emacs/search.texi
+++ b/doc/emacs/search.texi
@@ -419,6 +419,7 @@ character into the search string, similar to the usual
 @kindex C-^ @r{(Incremental Search)}
 @findex isearch-toggle-input-method
 @findex isearch-toggle-specified-input-method
+@findex isearch-transient-input-method
 Use an input method (@pxref{Input Methods}).  If an input method is
 enabled in the current buffer when you start the search, the same
 method will be active in the minibuffer when you type the search
@@ -437,7 +438,10 @@ I-search [@var{im}]:
 @noindent
 where @var{im} is the mnemonic of the active input method.  Any input
 method you enable during incremental search remains enabled in the
-current buffer afterwards.
+current buffer afterwards.  You can temporarily enable a transient
+input method with @kbd{C-x \} (@code{isearch-transient-input-method})
+to insert a single character to the search string and automatically
+disable the input method afterwards.
 @end itemize
 
 @kindex M-s o @r{(Incremental Search)}
diff --git a/etc/NEWS b/etc/NEWS
index ca8f71fe269..1a1cfc3751d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -652,6 +652,8 @@ transient input method (which can be different from the input method
 enabled by 'C-\').  For example, '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 'compose' input method afterwards.
+You can use 'C-x \' in incremental search to insert a single character
+to the search string.
 
 ---
 *** New input method 'compose' based on X Multi_key sequences.
diff --git a/lisp/international/isearch-x.el b/lisp/international/isearch-x.el
index d77234ec77b..f50f86a035f 100644
--- a/lisp/international/isearch-x.el
+++ b/lisp/international/isearch-x.el
@@ -51,6 +51,17 @@
   (setq input-method-function nil)
   (isearch-update))
 
+;;;###autoload
+(defun isearch-transient-input-method ()
+  "Activate transient input method in interactive search."
+  (interactive)
+  (let ((overriding-terminal-local-map nil))
+    (activate-transient-input-method))
+  (setq isearch-input-method-function input-method-function
+	isearch-input-method-local-p t)
+  (setq input-method-function nil)
+  (isearch-update))
+
 (defvar isearch-minibuffer-local-map
   (let ((map (copy-keymap minibuffer-local-map)))
     (define-key map [with-keyboard-coding] 'isearch-with-keyboard-coding)
@@ -117,6 +128,7 @@
 			  (cons last-char unread-command-events))
 		    ;; Inherit current-input-method in a minibuffer.
 		    str (read-string prompt isearch-message 'junk-hist nil t))
+	      (deactivate-transient-input-method)
 	      (if (or (not str) (< (length str) (length isearch-message)))
 		  ;; All inputs were deleted while the input method
 		  ;; was working.
diff --git a/lisp/international/mule-cmds.el b/lisp/international/mule-cmds.el
index dc435d9b170..439843aaf8e 100644
--- a/lisp/international/mule-cmds.el
+++ b/lisp/international/mule-cmds.el
@@ -55,7 +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)
+(define-key ctl-x-map "\\" 'activate-transient-input-method)
 
 (defvar describe-language-environment-map
   (let ((map (make-sparse-keymap "Describe Language Environment")))
@@ -140,8 +140,8 @@
       `(menu-item "Set Coding Systems" ,set-coding-system-map))
 
     (bindings--define-key map [separator-input-method] menu-bar-separator)
-    (bindings--define-key map [describe-input-method]
-      '(menu-item "Describe Input Method"  describe-input-method))
+    (bindings--define-key map [activate-transient-input-method]
+      '(menu-item "Transient Input Method" activate-transient-input-method))
     (bindings--define-key map [set-input-method]
       '(menu-item "Select Input Method..." set-input-method))
     (bindings--define-key map [toggle-input-method]
@@ -1345,10 +1345,10 @@ This is the input method activated automatically by the command
                  mule-input-method-string)
   :set-after '(current-language-environment))
 
-(defcustom transient-input-method nil
+(defcustom default-transient-input-method nil
   "Default transient input method.
 This is the input method activated by the command
-`transient-input-method' (\\[transient-input-method])."
+`activate-transient-input-method' (\\[activate-transient-input-method])."
   :link  '(custom-manual "(emacs)Input Methods")
   :group 'mule
   :type '(choice (const nil)
@@ -1356,6 +1356,18 @@ This is the input method activated by the command
   :set-after '(current-language-environment)
   :version "28.1")
 
+(defvar current-transient-input-method nil
+  "The current input method temporarily enabled by `activate-transient-input-method'.
+If nil, that means no transient input method is active now.")
+(make-variable-buffer-local 'current-transient-input-method)
+(put 'current-transient-input-method 'permanent-local t)
+
+(defvar previous-transient-input-method nil
+  "The input method that was active before enabling the transient input method.
+If nil, that means no previous input method was active.")
+(make-variable-buffer-local 'previous-transient-input-method)
+(put 'previous-transient-input-method 'permanent-local t)
+
 (put 'input-method-function 'permanent-local t)
 
 (defvar input-method-history nil
@@ -1490,7 +1502,8 @@ If INPUT-METHOD is nil, deactivate any current input method."
 (defun deactivate-input-method ()
   "Turn off the current input method."
   (when current-input-method
-    (add-to-history 'input-method-history current-input-method)
+    (unless current-transient-input-method
+      (add-to-history 'input-method-history current-input-method))
     (unwind-protect
 	(progn
 	  (setq input-method-function nil
@@ -1531,36 +1544,6 @@ 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 a transient input method for the current buffer.
-If `transient-input-method' was not yet defined, prompt for it."
-  (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.
@@ -1582,7 +1565,9 @@ which marks the variable `default-input-method' as set for Custom buffers."
   (if toggle-input-method-active
       (error "Recursive use of `toggle-input-method'"))
   (if (and current-input-method (not arg))
-      (deactivate-input-method)
+      (if current-transient-input-method
+          (deactivate-transient-input-method)
+        (deactivate-input-method))
     (let ((toggle-input-method-active t)
 	  (default (or (car input-method-history) default-input-method)))
       (if (and arg default (equal current-input-method default)
@@ -1601,6 +1586,42 @@ which marks the variable `default-input-method' as set for Custom buffers."
 	  (when interactive
 	    (customize-mark-as-set 'default-input-method)))))))
 
+(defun activate-transient-input-method (&optional arg interactive)
+  "Select and enable a transient input method for the current buffer.
+If `default-transient-input-method' was not yet defined, prompt for it."
+  (interactive "P\np")
+  (when (or arg (not default-transient-input-method))
+    (let* ((default (or (car input-method-history) default-input-method))
+           (input-method
+            (read-input-method-name
+             (format-prompt "Transient input method" default)
+             default t)))
+      (setq default-transient-input-method input-method)
+      (when interactive
+        (customize-mark-as-set 'default-transient-input-method))))
+  (let* ((clearfun (make-symbol "clear-transient-input-method"))
+         (exitfun
+          (lambda ()
+            (deactivate-transient-input-method)
+            (remove-hook 'input-method-after-insert-chunk-hook clearfun))))
+    (fset clearfun (lambda () (funcall exitfun)))
+    (add-hook 'input-method-after-insert-chunk-hook clearfun)
+    (setq previous-transient-input-method current-input-method)
+    (when previous-transient-input-method
+      (deactivate-input-method))
+    (activate-input-method default-transient-input-method)
+    (setq current-transient-input-method default-transient-input-method)
+    exitfun))
+
+(defun deactivate-transient-input-method ()
+  "Disable currently active transient input method for the current buffer."
+  (when current-transient-input-method
+    (deactivate-input-method)
+    (when previous-transient-input-method
+      (activate-input-method previous-transient-input-method)
+      (setq previous-transient-input-method nil))
+    (setq current-transient-input-method nil)))
+
 (autoload 'help-buffer "help-mode")
 
 (defun describe-input-method (input-method)
diff --git a/lisp/isearch.el b/lisp/isearch.el
index 245bf452b1f..4fba4370d98 100644
--- a/lisp/isearch.el
+++ b/lisp/isearch.el
@@ -565,6 +565,10 @@ This is like `describe-bindings', but displays only Isearch keys."
                   :help "Highlight all matches for current search string"))
     (define-key map [isearch-search-replace-separator]
       '(menu-item "--"))
+    (define-key map [isearch-transient-input-method]
+      '(menu-item "Turn on transient input method"
+                  isearch-transient-input-method
+                  :help "Turn on transient input method for search"))
     (define-key map [isearch-toggle-specified-input-method]
       '(menu-item "Turn on specific input method"
                   isearch-toggle-specified-input-method
@@ -747,6 +751,7 @@ This is like `describe-bindings', but displays only Isearch keys."
     ;; For searching multilingual text.
     (define-key map "\C-\\" 'isearch-toggle-input-method)
     (define-key map "\C-^" 'isearch-toggle-specified-input-method)
+    (define-key map "\C-x\\" 'isearch-transient-input-method)
 
     ;; People expect to be able to paste with the mouse.
     (define-key map [mouse-2] #'isearch-mouse-2)
@@ -1078,6 +1083,8 @@ To use a different input method for searching, type \
 \\[isearch-toggle-specified-input-method],
 and specify an input method you want to use.
 
+To activate a transient input method, type \\[isearch-transient-input-method].
+
 The above keys, bound in `isearch-mode-map', are often controlled by
  options; do \\[apropos] on search-.* to find them.
 Other control and meta characters terminate the search
@@ -3263,7 +3270,7 @@ the word mode."
 		       "over")
 		   (if isearch-wrapped "wrapped ")
                    (if (and (not isearch-success) (buffer-narrowed-p) widen-automatically)
-                       "narrowed-buffer " "")
+                       "narrowed " "")
                    (if (and (not isearch-success) (not isearch-case-fold-search))
                        "case-sensitive ")
                    (let ((prefix ""))