Eglot: add new command 'eglot-momentary-inlay-hints'

* doc/misc/eglot.texi (Eglot Commands)
(Customization Variables): Advertise eglot-momentary-inlay-hints.

* etc/EGLOT-NEWS: Advertise new command.

* lisp/progmodes/eglot.el (eglot--momentary-hints-data): New variable.
(eglot-momentary-inlay-hints): New command.

3 files changed, 65 insertions, 0 deletions

diff --git a/doc/misc/eglot.texi b/doc/misc/eglot.texi
index 579c568f264..8483881c52a 100644
--- a/doc/misc/eglot.texi
+++ b/doc/misc/eglot.texi
@@ -766,6 +766,22 @@ serve hints about positional parameter names in function calls and a
 variable's automatically deduced type.  Inlay hints help the user not
 have to remember these things by heart.
 
+@cindex momentary inlay hints
+@item eglot-momentary-inlay-hints
+When bound to a single key in @code{eglot-mode-map}
+(@pxref{Customization Variables}), this will arrange for inlay hints to
+be displayed as long as the key is held down, and then hidden shortly
+after it is released.  The best way to set it up is something like this:
+
+@lisp
+(define-key eglot-mode-map [f7] 'eglot-momentary-inlay-hints)
+@end lisp
+
+@noindent
+Note that Emacs doesn't support binding to \"key up\" events, so this
+command offers an approximation by estimating your system keyboard delay
+and repeat rate.
+
 @cindex semantic tokens
 @item M-x eglot-semantic-tokens-mode
 This command toggles LSP @dfn{semantic tokens} fontification on and off
@@ -978,6 +994,7 @@ For example:
   (define-key eglot-mode-map (kbd "C-c o") 'eglot-code-action-organize-imports)
   (define-key eglot-mode-map (kbd "C-c h") 'eldoc)
   (define-key eglot-mode-map (kbd "<f6>") 'xref-find-definitions)
+  (define-key eglot-mode-map (kbd "<f7>") 'eglot-momentary-inlay-hints)
 @end lisp
 
 @cindex progress
diff --git a/etc/EGLOT-NEWS b/etc/EGLOT-NEWS
index 3fe87d77690..a6425ea4234 100644
--- a/etc/EGLOT-NEWS
+++ b/etc/EGLOT-NEWS
@@ -49,6 +49,15 @@ requests diagnostics explicitly rather than relying on sporadic
 server is known to support the "pull" variant exclusively, while the
 'ty' server is known to support it alongside "push".
 
+** New command 'eglot-momentary-inlay-hints'
+
+When bound to a single key in 'eglot-mode-map' this will arrange for
+inlay hints to be displayed as long as the key is held down, and then
+hidden shortly after it is released.  Emacs doesn't support binding to
+\"key up\" events, but this function offers an approximation.  It relies
+on measuring your keyboard initial delay and repeat rate, and may not be
+100% accurate.
+
 ** Support for watching files outside the project (bug#79809)
 
 Eglot now supports and advertises the 'relativePatternSupport'
diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index f95451b35af..b22ed2b9c57 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -4882,6 +4882,45 @@ If NOERROR, return predicate, else erroring function."
          (jit-lock-unregister #'eglot--update-hints)
          (eglot--delete-overlays 'eglot--inlay-hint))))
 
+(defvar eglot--momentary-hints-data (list nil nil nil 0 nil))
+
+(defun eglot-momentary-inlay-hints ()
+  "Display inlay hints while holding down a key.
+Emacs doesn't support binding to \"key up\" events, but this function
+offers an approximation.  When bound to a key it will arrange for inlay
+hints to be displayed as long as the key is held down, and then hidden
+shortly after it is released.  This relies on measuring your keyboard
+initial delay and repeat rate, and may not be 100% accurate."
+  (interactive)
+  (when eglot-inlay-hints-mode
+    (eglot-inlay-hints-mode -1))
+  (cl-symbol-macrolet
+      ((timer (nth 0 eglot--momentary-hints-data))
+       (initial-delay (nth 1 eglot--momentary-hints-data))
+       (repeat-delay (nth 2 eglot--momentary-hints-data))
+       (calls (nth 3 eglot--momentary-hints-data))
+       (last-call-time (nth 4 eglot--momentary-hints-data)))
+    (cl-incf calls)
+    (cl-flet ((runit (delay)
+                (setf timer
+                      (run-at-time (+ 0.1 delay)
+                                   nil (lambda ()
+                                         (dolist (o (overlays-in (point-min) (point-max)))
+                                           (when (overlay-get o 'eglot--inlay-hint)
+                                             (delete-overlay o)))
+                                         (setf timer nil calls 0)))
+                      last-call-time (float-time))))
+      (cond ((timerp timer)
+             (when (and (not initial-delay) (= calls 2))
+               (setf initial-delay (- (float-time) last-call-time)))
+             (when (and (not repeat-delay) (= calls 3))
+               (setf repeat-delay (- (float-time) last-call-time)))
+             (cancel-timer timer)
+             (runit (or repeat-delay 0.5)))
+          (t
+           (eglot--update-hints-1 (window-start) (window-end))
+           (runit (or initial-delay 1.0)))))))
+
 
 ;;; Semantic tokens
 (defmacro eglot--semtok-define-things ()