Fix #4771: give lsp-find-* precedence over fallbacks

LSP lookup handlers should have the highest precedence, but this wasn't
the case due to a regression in 430d628.

modules/tools/lookup/autoload/lookup.el

@@ -212,11 +212,15 @@ This can be passed nil as its second argument to unset handlers for MODES. e.g.
 
 (defun +lookup-xref-definitions-backend-fn (identifier)
   "Non-interactive wrapper for `xref-find-definitions'"
-  (+lookup--xref-show 'xref-backend-definitions identifier #'xref--show-defs))
+  (condition-case _
+      (+lookup--xref-show 'xref-backend-definitions identifier #'xref--show-defs)
+    (cl-no-applicable-method nil)))
 
 (defun +lookup-xref-references-backend-fn (identifier)
   "Non-interactive wrapper for `xref-find-references'"
-  (+lookup--xref-show 'xref-backend-references identifier #'xref--show-xrefs))
+  (condition-case _
+      (+lookup--xref-show 'xref-backend-references identifier #'xref--show-xrefs)
+    (cl-no-applicable-method nil)))
 
 (defun +lookup-dumb-jump-backend-fn (_identifier)
   "Look up the symbol at point (or selection) with `dumb-jump', which conducts a

modules/tools/lsp/+lsp.el

@@ -58,9 +58,9 @@ about it (it will be logged to *Messages* however).")
         lsp-groovy-server-file        (concat lsp-server-install-dir "groovy-language-server-all.jar"))
 
   (set-popup-rule! "^\\*lsp-help" :size 0.35 :quit t :select t)
-  (set-lookup-handlers! 'lsp-mode :async t
-    ;; NOTE :definitions and :references aren't needed. LSP is integrated into
-    ;;      xref, which the lookup module has first class support for.
+  (set-lookup-handlers! 'lsp-mode
+    :definition #'+lsp-lookup-definition-handler
+    :references #'+lsp-lookup-references-handler
     :documentation #'lsp-describe-thing-at-point
     :implementations #'lsp-find-implementation
     :type-definition #'lsp-find-type-definition)

modules/tools/lsp/autoload/lsp-mode.el

@@ -49,3 +49,25 @@
                  (car workspaces)))
             (lsp-mode +1))
         (setf (lsp--client-priority match) old-priority)))))
+
+;;;###autoload
+(defun +lsp-lookup-definition-handler ()
+  "Find definition of the symbol at point using LSP."
+  (interactive)
+  (when-let (loc (lsp-request "textDocument/definition"
+                              (lsp--text-document-position-params)))
+    (lsp-show-xrefs (lsp--locations-to-xref-items loc) nil nil)
+    t))
+
+;;;###autoload
+(defun +lsp-lookup-references-handler (&optional include-declaration)
+  "Find project-wide references of the symbol at point using LSP."
+  (interactive "P")
+  (when-let
+      (loc (lsp-request "textDocument/references"
+                        (append (lsp--text-document-position-params)
+                                (list
+                                 :context `(:includeDeclaration
+                                            ,(lsp-json-bool include-declaration))))))
+    (lsp-show-xrefs (lsp--locations-to-xref-items loc) nil t)
+    t))