Improve ergonomics of Eglot's inlay hints

Instead of deleting inlay hints instantly as soon as the affected
changes, make hint overlays span one character (instead of being
length 0).  Give the overlays an "evaporate" property.

Given an inlay hints at position POS, its attached to [POS, POS+1] if
it's kind=1 (usually type hints) and [POS-1, POS] otherwise.  For
kind=1 hints, the 'cursor position of the first such overlay is also
tweaked, so that's it's less akward to edit around it.

* lisp/progmodes/eglot.el (eglot--before-change): Don't delete hints
  here.
  (eglot--update-hints-1): Rework.

1 files changed, 23 insertions, 14 deletions

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 1a7c46c36c6..ccae764c130 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -2356,7 +2356,6 @@ THINGS are either registrations or unregisterations (sic)."
 
 (defun eglot--before-change (beg end)
   "Hook onto `before-change-functions' with BEG and END."
-  (remove-overlays beg end 'eglot--overlay t)
   (when (listp eglot--recent-changes)
     ;; Records BEG and END, crucially convert them into LSP
     ;; (line/char) positions before that information is lost (because
@@ -3612,31 +3611,41 @@ If NOERROR, return predicate, else erroring function."
           (eglot--lambda ((InlayHint) position kind label paddingLeft paddingRight)
             (goto-char (eglot--lsp-position-to-point position))
             (when (or (> (point) to) (< (point) from)) (cl-return))
-            (let ((left-pad (and paddingLeft
-                                 (not (eq paddingLeft :json-false))
-                                 (not (memq (char-before) '(32 9))) " "))
-                  (right-pad (and paddingRight
-                                  (not (eq paddingRight :json-false))
-                                  (not (memq (char-after) '(32 9))) " ")))
-              (cl-flet
-                  ((do-it (text lpad rpad)
-                     (let ((ov (make-overlay (point) (point))))
-                       (overlay-put ov 'before-string
+            (let* ((left-pad (and paddingLeft
+                                  (not (eq paddingLeft :json-false))
+                                  (not (memq (char-before) '(32 9))) " "))
+                   (right-pad (and paddingRight
+                                   (not (eq paddingRight :json-false))
+                                   (not (memq (char-after) '(32 9))) " "))
+                   (peg-after-p (eql kind 1)))
+              (cl-labels
+                  ((make-ov ()
+                     (if peg-after-p
+                         (make-overlay (point) (1+ (point)) nil t)
+                       (make-overlay (1- (point)) (point) nil nil nil)))
+                   (do-it (label lpad rpad firstp)
+                     (let* ((tweak-cursor-p (and firstp peg-after-p))
+                            (ov (make-ov))
+                            (text (concat lpad label rpad)))
+                       (when tweak-cursor-p (put-text-property 0 1 'cursor 1 text))
+                       (overlay-put ov (if peg-after-p 'before-string 'after-string)
                                     (propertize
-                                     (concat lpad text rpad)
+                                     text
                                      'face (pcase kind
                                              (1 'eglot-type-hint-face)
                                              (2 'eglot-parameter-hint-face)
                                              (_ 'eglot-inlay-hint-face))))
                        (overlay-put ov 'eglot--inlay-hint t)
+                       (overlay-put ov 'evaporate t)
                        (overlay-put ov 'eglot--overlay t))))
-                (if (stringp label) (do-it label left-pad right-pad)
+                (if (stringp label) (do-it label left-pad right-pad t)
                   (cl-loop
                    for i from 0 for ldetail across label
                    do (eglot--dbind ((InlayHintLabelPart) value) ldetail
                         (do-it value
                                (and (zerop i) left-pad)
-                               (and (= i (1- (length label))) right-pad))))))))))
+                               (and (= i (1- (length label))) right-pad)
+                               (zerop i))))))))))
     (jsonrpc-async-request
      (eglot--current-server-or-lose)
      :textDocument/inlayHint