bug#61412: [PATCH v2] Add inlay hints to eglot

[Chinmay Dalal <dalal.chinmay.0101@gmail.com>]

This uses different faces based on the "kind" of the hint:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#inlayHintKind

---
 lisp/progmodes/eglot.el | 85 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 84 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 6caf589..101644f 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -404,6 +404,10 @@ This can be useful when using docker to run a language server.")
 (when (assoc 'flex completion-styles-alist)
   (add-to-list 'completion-category-defaults '(eglot (styles flex basic))))
 
+(defcustom eglot-inlay-hints t
+  "If non-nil, enable inlay hints."
+  :type 'boolean)
+
 \f
 ;;; Constants
 ;;;
@@ -1624,7 +1628,8 @@ under cursor."
           (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 hints" :inlayHintProvider)))
 
 (defun eglot--server-capable (&rest feats)
   "Determine if current server is capable of FEATS."
@@ -1845,6 +1850,8 @@ If it is activated, also signal textDocument/didOpen."
     (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)
@@ -3456,6 +3463,82 @@ If NOERROR, return predicate, else erroring function."
       (revert-buffer)
       (pop-to-buffer (current-buffer)))))
 
+(defface eglot-inlay-hint-face
+  '((t (:height 0.8 :inherit shadow)))
+  "Face used for inlay hint overlays.")
+
+(defface eglot-type-hint-face
+  '((t (:inherit eglot-inlay-hint)))
+  "Face used for type hints.")
+
+(defface eglot-parameter-hint-face
+  '((t (:inherit eglot-inlay-hint)))
+  "Face used for parameter hints.")
+
+(define-minor-mode eglot-inlay-mode
+  "Mode for displaying inlay hints."
+  :lighter " inlay"
+  (if eglot-inlay-mode
+      (progn
+        (add-hook 'after-save-hook 'eglot--update-hints 0 t)
+        (eglot--update-hints))
+      (progn
+        (remove-hook 'after-save-hook 'eglot--update-hints t)
+        (eglot--remove-hints))))
+
+(defun eglot--inlay-handler (buffer hints)
+  "Apply vector of inlay hints HINTS on buffer BUFFER."
+  (seq-doseq (hint hints)
+    (let* ((position (plist-get hint :position))
+           (line (plist-get position :line))
+           (character (plist-get position :character))
+           (kind (plist-get hint :kind))
+           (face (cond ((eq kind 1)
+                        'eglot-type-hint-face)
+                       ((eq kind 2)
+                        'eglot-parameter-hint-face)
+                       (t
+                        'eglot-inlay-hint-face)))
+           (label (plist-get hint :label)))
+      (when (stringp label)
+        (with-current-buffer buffer
+          (eglot--widening
+            (goto-char (point-min))
+            (forward-line line)
+            (eglot-move-to-column character)
+            (let ((overlay (make-overlay (point) (point))))
+              (overlay-put overlay 'before-string (propertize
+                                                    (concat (if (plist-get hint :paddingLeft) " " "")
+                                                            label
+                                                            (if (plist-get hint :paddingRight) " " ""))
+                                                    'face face))
+              (overlay-put overlay 'is-eglot-inlay-hint t))))))))
+
+(defun eglot--remove-hints ()
+  "Remove inlay hints from the buffer."
+  (remove-overlays nil nil 'is-eglot-inlay-hint t))
+
+(defun eglot--update-hints ()
+  "Request inlay hints for the current buffer and apply them."
+  (unless (eglot--server-capable :inlayHintProvider)
+    (eglot--error "This LSP server isn't an :inlayHintProvider"))
+  ;; Remove existing hints
+  (eglot--remove-hints)
+  (let ((buffer (current-buffer)))
+    (jsonrpc-async-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)))
+      :success-fn (lambda (hints)
+                    (eglot--inlay-handler buffer hints))
+      :deferred t)))
+
 \f
 ;;; Hacks
 ;;;
-- 
2.39.1