* lisp/cus-start.el: Make 'treesit-extra-load-path' customizable.

* lisp/treesit.el (treesit-auto-install-grammar): Add the value 'ask-dir'.
(treesit-ensure-installed): When 'treesit-auto-install-grammar' is
'ask' or 'ask-dir', ask for the directory to install the grammar library
using the first writable directory of 'treesit-extra-load-path' as default.
Also add the provided directory to 'treesit-extra-load-path' afterwards.

* src/treesit.c (treesit-extra-load-path): In the docstring
mention the fact that the first directory is special (bug#79862).

etc/NEWS

@@ -924,6 +924,11 @@ It controls the automatic installation of tree-sitter grammar libraries
 needed for tree-sitter based modes, if these grammar libraries are not
 available when such modes are turned on.
 
+*** 'treesit-extra-load-path' now is a customizable user option.
+The first directory in the list is used as the default directory
+to install the language grammar when 'treesit-auto-install-grammar'
+is 'ask' or 'ask-dir'.
+
 *** 'treesit-language-source-alist' supports keywords.
 The language and URL are mandatory, but remaining data can use keywords:
 '(json "https://github.com/tree-sitter/tree-sitter-json" :commit "4d770d3")'.

lisp/cus-start.el

@@ -621,6 +621,9 @@ Leaving \"Default\" unchecked is equivalent with specifying a default of
                (const :tag "Default" nil)
                (const :tag "Silent" ignore)
                function))
+             ;; treesit.c
+             (treesit-extra-load-path
+              treesit (repeat (directory :format "%v")))
 	     ;; undo.c
 	     (undo-limit undo integer "27.1")
 	     (undo-strong-limit undo integer "27.1")

lisp/treesit.el

@@ -5461,8 +5461,12 @@ The copied query files are queries/highlights.scm."
   "Whether to install tree-sitter language grammar libraries when needed.
 This controls whether Emacs will install missing grammar libraries
 when they are needed by some tree-sitter based mode.
-If `ask', ask for confirmation before installing the required grammar library.
 If `always', install the grammar library without asking.
+If `ask', ask for confirmation before installing the required grammar library.
+If `ask-dir', ask for confirmation and also for a directory name where
+to install the grammar library.  The directory name is also asked when
+the value is `ask' and `treesit-extra-load-path' is customized to a list
+of directories.
 If nil or `never' or anything else, don't install the grammar library
 even while visiting a file in the mode that requires such grammar; this
 might display a warning and/or fail to turn on the mode."
@@ -5470,7 +5474,9 @@ might display a warning and/or fail to turn on the mode."
                  (const :tag "Always automatically install grammar libraries"
                         always)
                  (const :tag "Ask whether to install missing grammar libraries"
-                        ask))
+                        ask)
+                 (const :tag "Ask where to install missing grammar libraries"
+                        ask-dir))
   :version "31.1")
 
 (defun treesit-ensure-installed (lang)
@@ -5479,14 +5485,28 @@ The option `treesit-auto-install-grammar' defines whether to install
 the grammar library if it's unavailable."
   (when (treesit-available-p)
     (or (treesit-ready-p lang t)
-        (when (or (eq treesit-auto-install-grammar 'always)
-                  (and (eq treesit-auto-install-grammar 'ask)
-                       (y-or-n-p (format "\
-Tree-sitter grammar for `%s' is missing; install it?"
-                                         lang))))
-          (treesit-install-language-grammar lang)
-          ;; Check that the grammar was installed successfully
-          (treesit-ready-p lang)))))
+        (let (out-dir)
+          (when (or (eq treesit-auto-install-grammar 'always)
+                    (and (memq treesit-auto-install-grammar '(ask ask-dir))
+                         (y-or-n-p (format "\
+Tree-sitter grammar for `%s' is missing; install it?" lang))
+                         (or (and (eq treesit-auto-install-grammar 'ask)
+                                  ;; Still ask dir for customized path
+                                  (null treesit-extra-load-path))
+                             (let ((default-out-dir
+                                    (or (seq-find #'file-writable-p
+                                                  treesit-extra-load-path)
+                                        (locate-user-emacs-file "tree-sitter"))))
+                               (setq out-dir (read-directory-name
+                                              (format-prompt "\
+Install grammar for `%s' to" nil lang)
+                                              default-out-dir
+                                              treesit-extra-load-path t))
+                               (add-to-list 'treesit-extra-load-path out-dir)
+                               t))))
+            (treesit-install-language-grammar lang out-dir)
+            ;; Check that the grammar was installed successfully
+            (treesit-ready-p lang))))))
 
 ;;; Treesit enabled modes
 

src/treesit.c

@@ -5273,7 +5273,9 @@ The value should be a list of directories.
 When trying to load a tree-sitter language definition,
 Emacs first looks in the directories mentioned in this variable,
 then in the `tree-sitter' subdirectory of `user-emacs-directory', and
-then in the system default locations for dynamic libraries, in that order.  */);
+then in the system default locations for dynamic libraries, in that order.
+The first writeable directory in the list is special: it's used as the
+default directory when automatically installing the language grammar.  */);
   Vtreesit_extra_load_path = Qnil;
 
   DEFVAR_LISP ("treesit-thing-settings",