Make c-ts-mode indent tests side-effect-free

Running indent tests changes the global value of
c-ts-mode-indent-style.  That's not good.  This change fixes that.

I also refactored the indent style functions a bit.

* lisp/progmodes/c-ts-mode.el:
(c-ts-mode--prompt-for-style): New function.
(c-ts-mode-set-local-style): New function.
(c-ts-mode-set-style): Use c-ts-mode--prompt-for-style.  Use
derived-mode-p when testing for major mode.  Remove check of current
buffer's major mode since it doesn't matter.

* test/lisp/progmodes/c-ts-mode-resources/indent-bsd.erts:
* test/lisp/progmodes/c-ts-mode-resources/indent.erts: Use
c-ts-mode-set-local-style to set the indent style locally.

lisp/progmodes/c-ts-mode.el

@@ -100,12 +100,11 @@ the value of SYM in `c-ts-mode' and `c++-ts-mode' buffers to VAL."
                   (setq-local treesit-simple-indent-rules
                               (treesit--indent-rules-optimize
                                (c-ts-mode--get-indent-style
-                                (if (eq major-mode 'c-ts-mode) 'c 'cpp))))))
+                                (if (derived-mode-p 'c-ts-mode) 'c 'cpp))))))
               res)
       (let ((buffer (car buffers)))
         (with-current-buffer buffer
-          ;; FIXME: Should we use `derived-mode-p' here?
-          (if (or (eq major-mode 'c-ts-mode) (eq major-mode 'c++-ts-mode))
+          (if (derived-mode-p 'c-ts-mode 'c++-ts-mode)
               (loop (append res (list buffer)) (cdr buffers))
             (loop res (cdr buffers))))))))
 
@@ -134,24 +133,33 @@ MODE is either `c' or `cpp'."
            (alist-get c-ts-mode-indent-style (c-ts-mode--indent-styles mode)))))
     `((,mode ,@style))))
 
-(defun c-ts-mode-set-style ()
-  "Set the indent style of C/C++ modes globally.
+(defun c-ts-mode--prompt-for-style ()
+  "Prompt for a indent style and return the symbol for it."
+  (let ((mode (if (derived-mode-p 'c-ts-mode) 'c 'c++)))
+    (intern
+     (completing-read
+      "Style: "
+      (mapcar #'car (c-ts-mode--indent-styles mode))
+      nil t nil nil "gnu"))))
+
+(defun c-ts-mode-set-style (style)
+  "Set the indent style of C/C++ modes globally to STYLE.
 
 This changes the current indent style of every C/C++ buffer and
 the default C/C++ indent style in this Emacs session."
-  (interactive)
-  ;; FIXME: Should we use `derived-mode-p' here?
-  (or (eq major-mode 'c-ts-mode) (eq major-mode 'c++-ts-mode)
-      (error "Buffer %s is not a c-ts-mode (c-ts-mode-set-style)"
-             (buffer-name)))
-  (c-ts-mode--indent-style-setter
-   'c-ts-mode-indent-style
-   ;; NOTE: We can probably use the interactive form for this.
-   (intern
-    (completing-read
-     "Select style: "
-     (mapcar #'car (c-ts-mode--indent-styles (if (eq major-mode 'c-ts-mode) 'c 'cpp)))
-     nil t nil nil "gnu"))))
+  (interactive (list (c-ts-mode--prompt-for-style)))
+  (c-ts-mode--indent-style-setter 'c-ts-mode-indent-style style))
+
+(defun c-ts-mode-set-local-style (style)
+  "Set the C/C++ indent style of the current buffer to STYLE."
+  (interactive (list (c-ts-mode--prompt-for-style)))
+  (if (not (derived-mode-p 'c-ts-mode 'c++-ts-mode))
+      (user-error "The current buffer is not in `c-ts-mode' nor `c++-ts-mode'")
+    (setq-local c-ts-mode-indent-style style)
+    (setq treesit-simple-indent-rules
+          (treesit--indent-rules-optimize
+           (c-ts-mode--get-indent-style
+            (if (derived-mode-p 'c-ts-mode) 'c 'cpp))))))
 
 ;;; Syntax table
 

test/lisp/progmodes/c-ts-mode-resources/indent-bsd.erts

@@ -1,9 +1,9 @@
 Code:
   (lambda ()
-    (setq indent-tabs-mode nil)
-    (setq c-ts-mode-indent-offset 2)
-    (setq c-ts-mode-indent-style 'bsd)
     (c-ts-mode)
+    (setq-local indent-tabs-mode nil)
+    (setq-local c-ts-mode-indent-offset 2)
+    (c-ts-mode-set-local-style 'bsd)
     (indent-region (point-min) (point-max)))
 
 Point-Char: |

test/lisp/progmodes/c-ts-mode-resources/indent.erts

@@ -1,9 +1,9 @@
 Code:
   (lambda ()
-    (setq indent-tabs-mode nil)
-    (setq c-ts-mode-indent-offset 2)
-    (setq c-ts-mode-indent-style 'gnu)
     (c-ts-mode)
+    (setq-local indent-tabs-mode nil)
+    (setq-local c-ts-mode-indent-offset 2)
+    (c-ts-mode-set-local-style 'gnu)
     (indent-region (point-min) (point-max)))
 
 Point-Char: |
@@ -219,10 +219,10 @@ line 2
 
 Code:
   (lambda ()
-    (setq indent-tabs-mode nil)
-    (setq c-ts-mode-indent-offset 8)
-    (setq c-ts-mode-indent-style 'linux)
     (c-ts-mode)
+    (setq-local indent-tabs-mode nil)
+    (setq-local c-ts-mode-indent-offset 8)
+    (c-ts-mode-set-local-style 'linux)
     (indent-region (point-min) (point-max)))
 
 Name: Labels (Linux Style)