5 files changed, 150 insertions, 1 deletions

diff --git a/editors/code/package.json b/editors/code/package.json
index fd30c7946..060a3a247 100644
--- a/editors/code/package.json
+++ b/editors/code/package.json
@@ -238,6 +238,11 @@
                     "type": "number",
                     "default": null,
                     "description": "Number of syntax trees rust-analyzer keeps in memory"
+                },
+                "rust-analyzer.displayInlayHints": {
+                    "type": "boolean",
+                    "default": true,
+                    "description": "Display additional type information in the editor"
                 }
             }
         },
@@ -444,6 +449,15 @@
                     "light": "#000000",
                     "highContrast": "#FFFFFF"
                 }
+            },
+            {
+                "id": "ralsp.inlayHint",
+                "description": "Color for inlay hints",
+                "defaults": {
+                    "dark": "#A0A0A0F0",
+                    "light": "#747474",
+                    "highContrast": "#BEBEBE"
+                }
             }
         ]
     }
diff --git a/editors/code/src/commands/index.ts b/editors/code/src/commands/index.ts
index 194658497..c194bd2ea 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 inlayHints from './inlay_hints';
 import * as joinLines from './join_lines';
 import * as matchingBrace from './matching_brace';
 import * as onEnter from './on_enter';
@@ -15,5 +16,6 @@ export {
     parentModule,
     runnables,
     syntaxTree,
-    onEnter
+    onEnter,
+    inlayHints
 };
diff --git a/editors/code/src/commands/inlay_hints.ts b/editors/code/src/commands/inlay_hints.ts
new file mode 100644
index 000000000..8154af8dc
--- /dev/null
+++ b/editors/code/src/commands/inlay_hints.ts
@@ -0,0 +1,104 @@
+import * as vscode from 'vscode';
+import { Range, TextDocumentChangeEvent, TextEditor } from 'vscode';
+import { TextDocumentIdentifier } from 'vscode-languageclient';
+import { Server } from '../server';
+
+interface InlayHintsParams {
+    textDocument: TextDocumentIdentifier;
+}
+
+interface InlayHint {
+    range: Range;
+    kind: string;
+    label: string;
+}
+
+const typeHintDecorationType = vscode.window.createTextEditorDecorationType({
+    after: {
+        color: new vscode.ThemeColor('ralsp.inlayHint')
+    }
+});
+
+export class HintsUpdater {
+    private displayHints = true;
+
+    public async loadHints(
+        editor: vscode.TextEditor | undefined
+    ): Promise<void> {
+        if (
+            this.displayHints &&
+            editor !== undefined &&
+            this.isRustDocument(editor.document)
+        ) {
+            await this.updateDecorationsFromServer(
+                editor.document.uri.toString(),
+                editor
+            );
+        }
+    }
+
+    public async toggleHintsDisplay(displayHints: boolean): Promise<void> {
+        if (this.displayHints !== displayHints) {
+            this.displayHints = displayHints;
+
+            if (displayHints) {
+                return this.updateHints();
+            } else {
+                const editor = vscode.window.activeTextEditor;
+                if (editor != null) {
+                    return editor.setDecorations(typeHintDecorationType, []);
+                }
+            }
+        }
+    }
+
+    public async updateHints(cause?: TextDocumentChangeEvent): Promise<void> {
+        if (!this.displayHints) {
+            return;
+        }
+        const editor = vscode.window.activeTextEditor;
+        if (editor == null) {
+            return;
+        }
+        const document = cause == null ? editor.document : cause.document;
+        if (!this.isRustDocument(document)) {
+            return;
+        }
+
+        return await this.updateDecorationsFromServer(
+            document.uri.toString(),
+            editor
+        );
+    }
+
+    private isRustDocument(document: vscode.TextDocument): boolean {
+        return document && document.languageId === 'rust';
+    }
+
+    private async updateDecorationsFromServer(
+        documentUri: string,
+        editor: TextEditor
+    ): Promise<void> {
+        const newHints = (await this.queryHints(documentUri)) || [];
+        const newDecorations = newHints.map(hint => ({
+            range: hint.range,
+            renderOptions: { after: { contentText: `: ${hint.label}` } }
+        }));
+        return editor.setDecorations(typeHintDecorationType, newDecorations);
+    }
+
+    private async queryHints(documentUri: string): Promise<InlayHint[] | null> {
+        const request: InlayHintsParams = {
+            textDocument: { uri: documentUri }
+        };
+        const client = Server.client;
+        return client
+            .onReady()
+            .then(() =>
+                client.sendRequest<InlayHint[] | null>(
+                    'rust-analyzer/inlayHints',
+                    request
+                )
+            );
+    }
+}
diff --git a/editors/code/src/config.ts b/editors/code/src/config.ts
index 3f1b482e3..4d58a1a93 100644
--- a/editors/code/src/config.ts
+++ b/editors/code/src/config.ts
@@ -21,6 +21,7 @@ export class Config {
     public raLspServerPath = RA_LSP_DEBUG || 'ra_lsp_server';
     public showWorkspaceLoadedNotification = true;
     public lruCapacity: null | number = null;
+    public displayInlayHints = true;
     public cargoWatchOptions: CargoWatchOptions = {
         enableOnStartup: 'ask',
         trace: 'off',
@@ -123,5 +124,9 @@ export class Config {
         if (config.has('lruCapacity')) {
             this.lruCapacity = config.get('lruCapacity') as number;
         }
+
+        if (config.has('displayInlayHints')) {
+            this.displayInlayHints = config.get('displayInlayHints') as boolean;
+        }
     }
 }
diff --git a/editors/code/src/extension.ts b/editors/code/src/extension.ts
index c8c3004a7..c6efc2e7e 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 { HintsUpdater } from './commands/inlay_hints';
 import {
     interactivelyStartCargoWatch,
     startCargoWatch
@@ -147,6 +148,29 @@ export function activate(context: vscode.ExtensionContext) {
 
     // Start the language server, finally!
     startServer();
+
+    if (Server.config.displayInlayHints) {
+        const hintsUpdater = new HintsUpdater();
+        hintsUpdater.loadHints(vscode.window.activeTextEditor).then(() => {
+            disposeOnDeactivation(
+                vscode.window.onDidChangeActiveTextEditor(editor =>
+                    hintsUpdater.loadHints(editor)
+                )
+            );
+            disposeOnDeactivation(
+                vscode.workspace.onDidChangeTextDocument(e =>
+                    hintsUpdater.updateHints(e)
+                )
+            );
+            disposeOnDeactivation(
+                vscode.workspace.onDidChangeConfiguration(_ =>
+                    hintsUpdater.toggleHintsDisplay(
+                        Server.config.displayInlayHints
+                    )
+                )
+            );
+        });
+    }
 }
 
 export function deactivate(): Thenable<void> {