Add client capability: localDocs

When the client has this capability, the server can send local
links in response to an open-external-documentation request.

* README.md (rust-analyzer): Mention the new feature.

* eglot-x.el (eglot-x-enable-local-docs-support): New defcustom.
(eglot-client-capabilities, eglot-x-open-external-documentation):
Use it.

2 files changed, 24 insertions, 3 deletions

diff --git a/README.md b/README.md
index 43f2384..bb17535 100644
--- a/README.md
+++ b/README.md
@@ -66,6 +66,7 @@ positions.
   However, emacs' own `backward-sexp`, and `forward-sexp` seem to be
   more useful.
 - [Open External Documentation]: see defun `eglot-x-open-external-documentation`.
+- [Local Documentation]: see variable `eglot-x-enable-local-docs-support`.
 - [Structural Search Replace (SSR)][ssr]: see defun `eglot-x-structural-search-replace`.
 
   The server checks the correctness of the query while you type:
@@ -169,14 +170,15 @@ See the documentation of `eglot-x-enable-ff-related-file-integration`.
 [Analyzer Status]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#analyzer-status
 [Colored diagnostics]: https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#colored-diagnostic-output
 [Expand Macro]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#expand-macro
+[Flycheck commands]: https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#controlling-flycheck
 [Join Lines]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#join-lines
+[Local Documentation]: https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#local-documentation
 [Matching Brace]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#matching-brace
 [Move Item]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#move-item
 [On Enter]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#on-enter
 [Open External Documentation]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#open-external-documentation
 [Related Tests]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#related-tests
 [Reload Workspace]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#reload-workspace
-[Flycheck commands]: https://github.com/rust-lang/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#controlling-flycheck
 [Runnables]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#runnables
 [Server Status]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#server-status
 [Snippet TextEdits]: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/lsp-extensions.md#snippet-textedit
diff --git a/eglot-x.el b/eglot-x.el
index 9684944..f8478a5 100644
--- a/eglot-x.el
+++ b/eglot-x.el
@@ -190,6 +190,15 @@ manages .toml files, or (ii) the rust-analyzer LSP server manages
     :type 'boolean
     :link '(function-link ff-find-related-file)
     :link '(variable-link ff-related-file-alist))
+
+(defcustom eglot-x-enable-local-docs-support t
+  "If non-nil, `eglot-x-open-external-documentation' can receive local links."
+  :type 'boolean
+  :link '(function-link eglot-x-open-external-documentation)
+  :link '(url-link
+          :tag "the definition of the extension (rust-analyzer)"
+          "https://github.com/rust-lang/rust-analyzer/blob/master/\
+docs/dev/lsp-extensions.md#local-documentation"))
 
 ;;; Enable the extensions
 ;;
@@ -331,6 +340,11 @@ connections."
       (when eglot-x-enable-ff-related-file-integration
         (add-hook 'eglot-managed-mode-hook
                   #'eglot-x--configure-ff-related-file-alist))
+      (when eglot-x-enable-local-docs-support
+        (let* ((exp (plist-get capabilities :experimental))
+               (old (if (eq exp eglot--{}) '() exp))
+               (new (plist-put old :localDocs t)))
+          (setq capabilities (plist-put capabilities :experimental new))))
       capabilities)))
 
 (defvar ff-other-file-alist)
@@ -799,7 +813,12 @@ case.")
                               :experimental/externalDocs
                               (eglot--TextDocumentPositionParams))))
     (when res
-      (browse-url res))))
+      (let ((local (plist-get res :local))
+            (web (or (plist-get res :web)
+                     res)))
+        (if (and local (file-exists-p (eglot--uri-to-path local)))
+            (browse-url local)
+          (browse-url web))))))
 
 ;;; Analyzer Status
 
@@ -1594,7 +1613,7 @@ This is in contrast to merely setting it to 0."
 (defun eglot-x--put-in-server (server prop val)
   "Put PROP-VAL into SERVER object just like `plist-put'."
   ;; We can't inherit from eglot-lsp-server and add an additional
-  ;; slot, becasue this should work on any object inheriting from
+  ;; slot, because this should work on any object inheriting from
   ;; eglot-lsp-server.  So let's hide PROP-VAL in the eglot-lsp-server
   ;; object.
   (let ((eglot-x--remove-hidden-info nil))