lsp-rust: support rust-analyzer.showReference lens

Add a rust-analyzer specific :action-handler for the showReference lens,
which leverages `lsp-show-xrefs'. Refactor: eliminate
`lsp-execute-command', because it caused the :title to be lost, which is
needed to distinguish between the "references" or "implementations"
variants of that lens.

Defining a new `lsp-interface' to destructure
"rust-analyzer.showReference" was impossible, as the former doesn't
support destructuring lists (the :arguments? paramter is a special,
3-element list.).

Author: nbfalcon

clients/lsp-rust.el

@@ -290,7 +290,7 @@ PARAMS progress report notification data."
         (lsp-workspace-status nil workspace)
       (lsp-workspace-status (format "%s - %s" title (or message? "")) workspace))))
 
-(cl-defmethod lsp-execute-command (_server (_command (eql rls.run)) params)
+(lsp-defun lsp-rust--rls-run ((&Command :arguments? params))
   (-let* (((&rls:Cmd :env :binary :args :cwd) (lsp-seq-first params))
           (default-directory (or cwd (lsp-workspace-root) default-directory) ))
     (compile
@@ -585,6 +585,13 @@ The command should include `--message=format=json` or similar option."
 (lsp-defun lsp-rust--analyzer-run-single ((&Command :arguments?))
   (lsp-rust-analyzer-run (lsp-seq-first arguments?)))
 
+;; `lsp-interface' is unsuitable for ShowReference destructuring, as that code
+;; action yields an array of three elements, not a map.
+(lsp-defun lsp-rust--analyzer-show-references
+  ((&Command :title :arguments? [_uri _filepos references]))
+  (lsp-show-xrefs (lsp--locations-to-xref-items references) nil
+                  (s-contains-p "reference" title)))
+
 (lsp-register-client
  (make-lsp-client
   :new-connection (lsp-stdio-connection
@@ -598,7 +605,8 @@ The command should include `--message=format=json` or similar option."
   :priority (if (eq lsp-rust-server 'rust-analyzer) 1 -1)
   :initialization-options 'lsp-rust-analyzer--make-init-options
   :notification-handlers (ht<-alist lsp-rust-notification-handlers)
-  :action-handlers (ht ("rust-analyzer.runSingle" #'lsp-rust--analyzer-run-single))
+  :action-handlers (ht ("rust-analyzer.runSingle" #'lsp-rust--analyzer-run-single)
+                       ("rust-analyzer.showReferences" #'lsp-rust--analyzer-show-references))
   :library-folders-fn (lambda (_workspace) lsp-rust-library-directories)
   :after-open-fn (lambda ()
                    (when lsp-rust-analyzer-server-display-inlay-hints

lsp-lens.el

@@ -154,9 +154,7 @@ BUFFER-MODIFIED? determines whether the buffer is modified or not."
         (lsp:command-command command?)
       (lambda ()
         (interactive)
-        (lsp-execute-command server-id
-                             (intern (lsp:command-command command?))
-                             (lsp:command-arguments? command?))))))
+        (lsp--execute-command command?)))))
 
 (defun lsp-lens--display (lenses)
   "Show LENSES."

lsp-mode.el

@@ -1085,9 +1085,6 @@ calling `remove-overlays'.")
 
 (defvar-local lsp--virtual-buffer-point-max nil)
 
-(cl-defgeneric lsp-execute-command (server command arguments)
-  "Ask SERVER to execute COMMAND with ARGUMENTS.")
-
 (defun lsp-elt (sequence n)
   "Return Nth element of SEQUENCE or nil if N is out of range."
   (cond
@@ -5772,11 +5769,6 @@ REFERENCES? t when METHOD returns references."
 
 (defalias 'lsp-feature? 'lsp--find-workspaces-for)
 
-(cl-defmethod lsp-execute-command (_server command arguments)
-  "Dispatch COMMAND execution."
-  (lsp--execute-command (lsp-make-command :command (symbol-name command)
-                                          :arguments? arguments)))
-
 (defun lsp--send-execute-command (command &optional args)
   "Create and send a 'workspace/executeCommand' message having command COMMAND and optional ARGS."
   (let ((params (if args