3 files changed, 62 insertions, 5 deletions

diff --git a/editors/code/src/commands/expand_macro.ts b/editors/code/src/commands/expand_macro.ts
new file mode 100644
index 000000000..bf1923190
--- /dev/null
+++ b/editors/code/src/commands/expand_macro.ts
@@ -0,0 +1,45 @@
+import * as vscode from 'vscode';
+import { Position, TextDocumentIdentifier } from 'vscode-languageclient';
+import { Server } from '../server';
+
+type ExpandMacroResult = [string, string]
+
+function code_format([name, text]: [string, string]): vscode.MarkdownString {
+    const markdown = new vscode.MarkdownString(`#### Recursive expansion of ${name}! macro`);
+    markdown.appendCodeblock(text, 'rust');    
+    return markdown;
+}
+
+export class ExpandMacroHoverProvider implements vscode.HoverProvider {
+    public provideHover(
+        document: vscode.TextDocument,
+        position: vscode.Position,
+        token: vscode.CancellationToken,
+    ): Thenable<vscode.Hover | null> | null {
+        async function handle() {
+            const request: MacroExpandParams = {
+                textDocument: { uri: document.uri.toString() },
+                position,
+            };
+            const result = await Server.client.sendRequest<ExpandMacroResult>(
+                'rust-analyzer/expandMacro',
+                request
+            );
+            if (result != null) {
+                const formated = code_format(result);
+                return new vscode.Hover(formated);
+            }
+
+            return null;
+        };
+
+        return handle();
+    }
+}
+
+
+interface MacroExpandParams {
+    textDocument: TextDocumentIdentifier;
+    position: Position;
+}
+
diff --git a/editors/code/src/commands/index.ts b/editors/code/src/commands/index.ts
index c194bd2ea..2ade6d331 100644
--- a/editors/code/src/commands/index.ts
+++ b/editors/code/src/commands/index.ts
@@ -1,5 +1,6 @@
 import * as analyzerStatus from './analyzer_status';
 import * as applySourceChange from './apply_source_change';
+import * as expandMacro from './expand_macro';
 import * as inlayHints from './inlay_hints';
 import * as joinLines from './join_lines';
 import * as matchingBrace from './matching_brace';
@@ -11,6 +12,7 @@ import * as syntaxTree from './syntaxTree';
 export {
     analyzerStatus,
     applySourceChange,
+    expandMacro,
     joinLines,
     matchingBrace,
     parentModule,
diff --git a/editors/code/src/extension.ts b/editors/code/src/extension.ts
index c06928d12..1dfa6046f 100644
--- a/editors/code/src/extension.ts
+++ b/editors/code/src/extension.ts
@@ -3,6 +3,7 @@ import * as lc from 'vscode-languageclient';
 
 import * as commands from './commands';
 import { CargoWatchProvider } from './commands/cargo_watch';
+import { ExpandMacroHoverProvider } from './commands/expand_macro'
 import { HintsUpdater } from './commands/inlay_hints';
 import {
     interactivelyStartCargoWatch,
@@ -91,11 +92,11 @@ export function activate(context: vscode.ExtensionContext) {
     const allNotifications: Iterable<
         [string, lc.GenericNotificationHandler]
     > = [
-        [
-            'rust-analyzer/publishDecorations',
-            notifications.publishDecorations.handle
-        ]
-    ];
+            [
+                'rust-analyzer/publishDecorations',
+                notifications.publishDecorations.handle
+            ]
+        ];
     const syntaxTreeContentProvider = new SyntaxTreeContentProvider();
 
     // The events below are plain old javascript events, triggered and handled by vscode
@@ -121,6 +122,15 @@ export function activate(context: vscode.ExtensionContext) {
         context.subscriptions
     );
 
+    const expandMacroContentProvider = new ExpandMacroHoverProvider();
+
+    disposeOnDeactivation(
+        vscode.languages.registerHoverProvider(
+            'rust',
+            expandMacroContentProvider
+        )
+    );
+
     const startServer = () => Server.start(allNotifications);
     const reloadCommand = () => reloadServer(startServer);