Go to declaration service (#734)

* Go to declaration service

* Removed tests and implementation

packages/langium/src/lsp/declaration-provider.ts

@@ -0,0 +1,21 @@
+/******************************************************************************
+ * Copyright 2022 TypeFox GmbH
+ * This program and the accompanying materials are made available under the
+ * terms of the MIT License, which is available in the project root.
+ ******************************************************************************/
+
+import { CancellationToken, DeclarationParams, LocationLink } from 'vscode-languageserver';
+import { MaybePromise } from '../utils/promise-util';
+import { LangiumDocument } from '../workspace/documents';
+
+/**
+ * Language-specific service for handling go to declaration requests
+ */
+export interface DeclarationProvider {
+    /**
+     * Handle a go to declaration request.
+     * @throws `OperationCancelled` if cancellation is detected during execution
+     * @throws `ResponseError` if an error is detected that should be sent as response to the client
+     */
+    getDeclaration(document: LangiumDocument, params: DeclarationParams, cancelToken?: CancellationToken): MaybePromise<LocationLink[] | undefined>
+}

packages/langium/src/lsp/language-server.ts

@@ -85,6 +85,7 @@ export class DefaultLanguageServer implements LanguageServer {
         const hasRenameProvider = this.hasService(e => e.lsp.RenameProvider);
         const hasCallHierarchyProvider = this.hasService(e => e.lsp.CallHierarchyProvider);
         const codeLensProvider = this.services.lsp.CodeLensProvider;
+        const hasDeclarationProvider = this.hasService(e => e.lsp.DeclarationProvider);
 
         const result: InitializeResult = {
             capabilities: {
@@ -125,7 +126,8 @@ export class DefaultLanguageServer implements LanguageServer {
                     : undefined,
                 codeLensProvider: codeLensProvider
                     ? { resolveProvider: !!codeLensProvider.resolveCodeLens }
-                    : undefined
+                    : undefined,
+                declarationProvider: hasDeclarationProvider
             }
         };
 
@@ -165,6 +167,7 @@ export function startLanguageServer(services: LangiumSharedServices): void {
     addCodeLensHandler(connection, services);
     addDocumentLinkHandler(connection, services);
     addConfigurationChangeHandler(connection, services);
+    addGoToDeclarationHandler(connection, services);
 
     connection.onInitialize(params => {
         return services.lsp.LanguageServer.initialize(params);
@@ -268,6 +271,13 @@ export function addGoToImplementationHandler(connection: Connection, services: L
     ));
 }
 
+export function addGoToDeclarationHandler(connection: Connection, services: LangiumSharedServices) {
+    connection.onDeclaration(createRequestHandler(
+        (services, document, params, cancelToken) => services.lsp.DeclarationProvider?.getDeclaration(document, params, cancelToken),
+        services
+    ));
+}
+
 export function addDocumentHighlightsHandler(connection: Connection, services: LangiumSharedServices): void {
     connection.onDocumentHighlight(createRequestHandler(
         (services, document, params, cancelToken) => services.lsp.DocumentHighlightProvider?.getDocumentHighlight(document, params, cancelToken),

packages/langium/src/services.ts

@@ -53,6 +53,7 @@ import type { ImplementationProvider } from './lsp/implementation-provider';
 import type { CallHierarchyProvider } from './lsp/call-hierarchy-provider';
 import type { DocumentLinkProvider } from './lsp/document-link-provider';
 import type { CodeLensProvider } from './lsp/code-lens-provider';
+import type { DeclarationProvider } from './lsp/declaration-provider';
 
 /**
  * The services generated by `langium-cli` for a single language. These are derived from the
@@ -85,6 +86,7 @@ export type LangiumLspServices = {
     Formatter?: Formatter
     SignatureHelp?: SignatureHelpProvider
     CallHierarchyProvider?: CallHierarchyProvider
+    DeclarationProvider?: DeclarationProvider
 }
 
 /**