Add basic markdown link completions

For #140602

Only normal links for now. Will add reference links later. Should support the forms:

- `[](dir/file.md)`
- `[](./dir/file.md)`
- `[](/root-dir/file.md)`
- `[](#header)`
- `[](./dir/file.md#header)`

extensions/markdown-language-features/src/test/pathCompletion.test.ts

@@ -0,0 +1,112 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as assert from 'assert';
+import 'mocha';
+import * as vscode from 'vscode';
+import { PathCompletionProvider } from '../features/pathCompletions';
+import { createNewMarkdownEngine } from './engine';
+import { InMemoryDocument } from './inMemoryDocument';
+import { CURSOR, getCursorPositions, joinLines, noopToken } from './util';
+
+
+function workspaceFile(...segments: string[]): vscode.Uri {
+	return vscode.Uri.joinPath(vscode.workspace.workspaceFolders![0].uri, ...segments);
+}
+
+function getCompletionsAtCursor(resource: vscode.Uri, fileContents: string) {
+	const doc = new InMemoryDocument(resource, fileContents);
+	const provider = new PathCompletionProvider(createNewMarkdownEngine());
+	const cursorPositions = getCursorPositions(fileContents, doc);
+	return provider.provideCompletionItems(doc, cursorPositions[0], noopToken, {
+		triggerCharacter: undefined,
+		triggerKind: vscode.CompletionTriggerKind.Invoke,
+	});
+}
+
+
+suite('markdown.PathCompletionProvider', () => {
+
+	setup(async () => {
+		// These tests assume that the markdown completion provider is already registered
+		await vscode.extensions.getExtension('vscode.markdown-language-features')!.activate();
+	});
+
+	test('Should not return anything when triggered in empty doc', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('new.md'), `${CURSOR}`);
+		assert.strictEqual(completions.length, 0);
+	});
+
+	test('Should return anchor completions', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('new.md'), joinLines(
+			`[](#${CURSOR}`,
+			``,
+			`# A b C`,
+			`# x y Z`,
+		));
+
+		assert.strictEqual(completions.length, 2);
+		assert.ok(completions.some(x => x.label === '#a-b-c'), 'Has a-b-c anchor completion');
+		assert.ok(completions.some(x => x.label === '#x-y-z'), 'Has x-y-z anchor completion');
+	});
+
+	test('Should not return suggestions for http links', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('new.md'), joinLines(
+			`[](http:${CURSOR}`,
+			``,
+			`# http`,
+			`# http:`,
+			`# https:`,
+		));
+
+		assert.strictEqual(completions.length, 0);
+	});
+
+	test('Should return relative path suggestions', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('new.md'), joinLines(
+			`[](${CURSOR}`,
+			``,
+			`# A b C`,
+		));
+
+		assert.ok(completions.some(x => x.label === 'a.md'), 'Has a.md file completion');
+		assert.ok(completions.some(x => x.label === 'b.md'), 'Has b.md file completion');
+		assert.ok(completions.some(x => x.label === 'sub/'), 'Has sub folder completion');
+	});
+
+	test('Should return relative path suggestions using ./', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('new.md'), joinLines(
+			`[](./${CURSOR}`,
+			``,
+			`# A b C`,
+		));
+
+		assert.ok(completions.some(x => x.label === 'a.md'), 'Has a.md file completion');
+		assert.ok(completions.some(x => x.label === 'b.md'), 'Has b.md file completion');
+		assert.ok(completions.some(x => x.label === 'sub/'), 'Has sub folder completion');
+	});
+
+	test('Should return absolute path suggestions using /', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('sub', 'new.md'), joinLines(
+			`[](/${CURSOR}`,
+			``,
+			`# A b C`,
+		));
+
+		assert.ok(completions.some(x => x.label === 'a.md'), 'Has a.md file completion');
+		assert.ok(completions.some(x => x.label === 'b.md'), 'Has b.md file completion');
+		assert.ok(completions.some(x => x.label === 'sub/'), 'Has sub folder completion');
+		assert.ok(!completions.some(x => x.label === 'c.md'), 'Should not have c.md from sub folder');
+	});
+
+	test('Should return anchor suggestions in other file', async () => {
+		const completions = await getCompletionsAtCursor(workspaceFile('sub', 'new.md'), joinLines(
+			`[](/b.md#${CURSOR}`,
+		));
+
+		assert.ok(completions.some(x => x.label === '#b'), 'Has #b header completion');
+		assert.ok(completions.some(x => x.label === '#header1'), 'Has #header1 header completion');
+	});
+});