Use 3.6 Goto Impl only when the RLS version supports it

Author: Xanewok

src/extension.ts

@@ -204,14 +204,19 @@ class ClientWorkspace {
                         return;
                     }
                     await this.lc.onReady();
+                    // Prior to https://github.com/rust-lang-nursery/rls/pull/936 we used a custom
+                    // LSP message - if the implementation provider is specified this means we can use the 3.6 one.
+                    const useLSPRequest = this.lc.initializeResult &&
+                        this.lc.initializeResult.capabilities.implementationProvider === true;
+                    const request = useLSPRequest ? 'textDocument/implementation' : 'rustDocument/implementations';
 
                     const params =
                         this.lc
                             .code2ProtocolConverter
                             .asTextDocumentPositionParams(textEditor.document, textEditor.selection.active);
                     let locations: Location[];
                     try {
-                        locations = await this.lc.sendRequest<Location[]>('textDocument/implementation', params);
+                        locations = await this.lc.sendRequest<Location[]>(request, params);
                     } catch (reason) {
                         window.showWarningMessage('find implementations failed: ' + reason);
                         return;