bug#61066: [PATCH] Add inlay hint support to eglot

[Dimitri Belopopsky <dimitri@belopopsky.com>]

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Hello,

I've been working on adding support for inlay hints inside eglot using
overlays.
Here is a working patch, but I'm still missing a few things:

- I can't figure out a way to show the hints on a document without causing
lags or timeouts from the lsp server
- I'm currently updating the hints by sending the whole file each time (to
make sure all hints get updated correctly). I'm not sure on how to make
this more efficient (or if it even necessary).

On the implementation side:
- implemented with overlays as a minor model, enabled by default
- shows all hints supported by the protocol
- there is a customisation to disable the minor mode if the user doesn't
want the feature

I'd love to get a few points to finish this patch, and of course any ideas
and feedbacks are welcome!

Kind regards,

Dimitri Belopopsky

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From 848fa16eb6257ce454c1810f81be8ed8ed140164 Mon Sep 17 00:00:00 2001
From: Dimitri Belopopsky <dimitri@belopopsky.com>
Date: Wed, 25 Jan 2023 23:24:13 +0100
Subject: [PATCH] Add inlay hints to eglot

---
 lisp/progmodes/eglot.el | 88 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 87 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 8ce1a8b7baf..2d947c377a4 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -404,6 +404,11 @@ eglot-withhold-process-id
 (when (assoc 'flex completion-styles-alist)
   (add-to-list 'completion-category-defaults '(eglot (styles flex basic))))
 
+(defcustom eglot-inlay-hints t
+  "If non-nil, activate inlay hints inside the buffer."
+  :type 'boolean)
+
+
 \f
 ;;; Constants
 ;;;
@@ -1624,7 +1629,8 @@ eglot-ignored-server-capabilities
           (const :tag "Highlight links in document" :documentLinkProvider)
           (const :tag "Decorate color references" :colorProvider)
           (const :tag "Fold regions of buffer" :foldingRangeProvider)
-          (const :tag "Execute custom commands" :executeCommandProvider)))
+           (const :tag "Execute custom commands" :executeCommandProvider)
+           (const :tag "Inlay Hint" :inlayHintProvider)))
 
 (defun eglot--server-capable (&rest feats)
   "Determine if current server is capable of FEATS."
@@ -1845,6 +1851,9 @@ eglot--maybe-activate-editing-mode
     (when (and buffer-file-name (eglot-current-server))
       (setq eglot--diagnostics nil)
       (eglot--managed-mode)
+      (unless (not eglot-inlay-hints)
+        (eglot-inlay-mode)
+        )
       (eglot--signal-textDocument/didOpen))))
 
 (add-hook 'find-file-hook 'eglot--maybe-activate-editing-mode)
@@ -3453,6 +3462,83 @@ eglot-list-connections
       (revert-buffer)
       (pop-to-buffer (current-buffer)))))
 
+(defface eglot-inlay-hint
+  '((t (:height 0.8 :inherit shadow)))
+  "Face used for inlay hint overlays.")
+
+(define-minor-mode eglot-inlay-mode
+  "Mode for displaying inlay hint."
+  :lighter " inlay"
+  (cond
+    (eglot--managed-mode
+      (add-hook 'after-save-hook 'eglot--update-hints nil t))
+    (t
+      (remove-hook 'after-save-hook 'eglot--update-hints t))
+    )
+  ;; Note: the public hook runs before the internal eglot--managed-mode-hook.
+  (run-hooks 'eglot-managed-mode-hook)
+  )
+
+(defun eglot--update-hints()
+  "Refresh inlay hints from LSP server."
+  (unless (eglot--server-capable :inlayHintProvider)
+    (error "Server does not support inlay hint."))
+  (mapc #'delete-overlay (overlays-in (point-min) (point-max)))
+  (let ((read-only-p buffer-read-only)
+	       overlays)
+	    (let ((lens-table (make-hash-table)))
+	      ;; Get the inlay hint objects.
+	      (mapc (lambda (inlayHint)
+		            (when (eglot--server-capable
+			                       :inlayHintProvider :resolveProvider)
+		              (setq inlayHint
+			              (jsonrpc-request (eglot--current-server-or-lose)
+					            :inlayHint/resolve inlayHint)))
+		            (let ((line (thread-first inlayHint
+					                    (plist-get :position)
+					                    (plist-get :line))))
+		              (puthash line
+			              (append (gethash line lens-table) (list inlayHint))
+			              lens-table)))
+		      (jsonrpc-request
+		        (eglot--current-server-or-lose)
+		        :textDocument/inlayHint
+            (list :textDocument (eglot--TextDocumentIdentifier) :range (list :start (list :line 0 :character 0) :end (list :line (count-lines (point-min) (point-max)) :character 0)))
+		        :deferred t))
+
+	      ;; Make overlays for them.
+	      (maphash
+	        (lambda (line values)
+            (cl-loop for value in values
+              do
+              (cl-loop for val being the elements of
+                (if (stringp (plist-get value :label))
+                  (list (plist-get value :label))
+                  (plist-get value :label))
+                do
+	              (eglot--widening
+	                (let ((c (plist-get (plist-get value :position) :character))
+		                     (text (propertize
+                                 (concat
+                                   (if (plist-get value :paddingLeft) " " "")
+                                   (if (stringp val) val (plist-get val :value))
+                                   (if (plist-get value :paddingRight) " " ""))
+                           'mouse-face 'highlight))
+                         )
+	              (goto-char (point-min))
+	              (forward-line line)
+	              (eglot-move-to-column c)
+	              (let ((ov (make-overlay (point) (point))))
+		              (push ov overlays)
+		              (overlay-put ov 'eglot-inlay-hint values)
+		              (overlay-put ov 'before-string (propertize text 'face 'eglot-inlay-hint))
+                  )))
+                )))
+	        lens-table)
+        )
+    ))
+
+
 \f
 ;;; Hacks
 ;;;
-- 
2.34.1