Integrate with improved throws helper

Show instructions on how to use `t.throws()` with the test results,
without writing them to stderr. Detect the improper usage even if user
code swallows the error, meaning that tests will definitely fail.

Assume that errors thrown, or emitted from an observable, or if the
returned promise was rejected, that that error is due to the improper
usage of `t.throws()`.

Assume that if a test has a pending throws assertion, and an error leaks
as an uncaught exception or an unhandled rejection, the error was
thrown due to the pending throws assertion. Attribute it to the test.

lib/reporters/improper-usage-messages.js

@@ -0,0 +1,21 @@
+'use strict';
+const chalk = require('chalk');
+
+exports.forError = error => {
+	if (!error.improperUsage) {
+		return null;
+	}
+
+	const assertion = error.assertion;
+	if (assertion !== 'throws' || !assertion === 'notThrows') {
+		return null;
+	}
+
+	return `Try wrapping the first argument to \`t.${assertion}()\` in a function:
+
+  ${chalk.cyan(`t.${assertion}(() => { `)}${chalk.grey('/* your code here */')}${chalk.cyan(' })')}
+
+Visit the following URL for more details:
+
+  ${chalk.blue.underline('https://github.com/avajs/ava#throwsfunctionpromise-error-message')}`;
+};

lib/reporters/mini.js

@@ -12,6 +12,7 @@ const formatAssertError = require('../format-assert-error');
 const extractStack = require('../extract-stack');
 const codeExcerpt = require('../code-excerpt');
 const colors = require('../colors');
+const improperUsageMessages = require('./improper-usage-messages');
 
 // TODO(@jamestalamge): This should be fixed in log-update and ansi-escapes once we are confident it's a good solution.
 const CSI = '\u001B[';
@@ -200,6 +201,11 @@ class MiniReporter {
 					if (formatted) {
 						status += '\n' + indentString(formatted, 2);
 					}
+
+					const message = improperUsageMessages.forError(test.error);
+					if (message) {
+						status += '\n' + indentString(message, 2) + '\n';
+					}
 				}
 
 				if (test.error.stack) {

lib/reporters/verbose.js

@@ -8,6 +8,7 @@ const formatAssertError = require('../format-assert-error');
 const extractStack = require('../extract-stack');
 const codeExcerpt = require('../code-excerpt');
 const colors = require('../colors');
+const improperUsageMessages = require('./improper-usage-messages');
 
 class VerboseReporter {
 	constructor(options) {
@@ -120,6 +121,11 @@ class VerboseReporter {
 					if (formatted) {
 						output += '\n' + indentString(formatted, 2);
 					}
+
+					const message = improperUsageMessages.forError(test.error);
+					if (message) {
+						output += '\n' + indentString(message, 2) + '\n';
+					}
 				}
 
 				if (test.error.stack) {

lib/runner.js

@@ -148,6 +148,9 @@ class Runner extends EventEmitter {
 
 		return Promise.resolve(this.tests.build().run()).then(this._buildStats);
 	}
+	attributeLeakedError(err) {
+		return this.tests.attributeLeakedError(err);
+	}
 }
 
 optionChain(chainableMethods, function (opts, args) {

lib/test-collection.js

@@ -28,6 +28,8 @@ class TestCollection extends EventEmitter {
 			afterEachAlways: []
 		};
 
+		this.pendingTestInstances = new Set();
+
 		this._emitTestResult = this._emitTestResult.bind(this);
 	}
 	add(test) {
@@ -100,8 +102,9 @@ class TestCollection extends EventEmitter {
 			}
 		};
 	}
-	_emitTestResult(test) {
-		this.emit('test', test);
+	_emitTestResult(result) {
+		this.pendingTestInstances.delete(result.result);
+		this.emit('test', result);
 	}
 	_buildHooks(hooks, testTitle, context) {
 		return hooks.map(hook => {
@@ -125,28 +128,32 @@ class TestCollection extends EventEmitter {
 			contextRef = null;
 		}
 
-		return new Test({
+		const test = new Test({
 			contextRef,
 			failWithoutAssertions: false,
 			fn: hook.fn,
 			metadata: hook.metadata,
 			onResult: this._emitTestResult,
 			title
 		});
+		this.pendingTestInstances.add(test);
+		return test;
 	}
 	_buildTest(test, contextRef) {
 		if (!contextRef) {
 			contextRef = null;
 		}
 
-		return new Test({
+		test = new Test({
 			contextRef,
 			failWithoutAssertions: this.failWithoutAssertions,
 			fn: test.fn,
 			metadata: test.metadata,
 			onResult: this._emitTestResult,
 			title: test.title
 		});
+		this.pendingTestInstances.add(test);
+		return test;
 	}
 	_buildTestWithHooks(test) {
 		if (test.metadata.skipped || test.metadata.todo) {
@@ -183,6 +190,14 @@ class TestCollection extends EventEmitter {
 		}
 		return finalTests;
 	}
+	attributeLeakedError(err) {
+		for (const test of this.pendingTestInstances) {
+			if (test.attributeLeakedError(err)) {
+				return true;
+			}
+		}
+		return false;
+	}
 }
 
 module.exports = TestCollection;

lib/test-worker.js

@@ -24,7 +24,6 @@ const isObj = require('is-obj');
 const adapter = require('./process-adapter');
 const globals = require('./globals');
 const serializeError = require('./serialize-error');
-const throwsHelper = require('./throws-helper');
 
 const opts = adapter.opts;
 const testPath = opts.file;
@@ -54,10 +53,25 @@ if (!runner) {
 	adapter.send('no-tests', {avaRequired: false});
 }
 
-process.on('unhandledRejection', throwsHelper);
+function attributeLeakedError(err) {
+	if (!runner) {
+		return false;
+	}
+
+	return runner.attributeLeakedError(err);
+}
+
+const attributedRejections = new Set();
+process.on('unhandledRejection', (reason, promise) => {
+	if (attributeLeakedError(reason)) {
+		attributedRejections.add(promise);
+	}
+});
 
 process.on('uncaughtException', exception => {
-	throwsHelper(exception);
+	if (attributeLeakedError(exception)) {
+		return;
+	}
 
 	let serialized;
 	try {
@@ -88,7 +102,8 @@ process.on('ava-teardown', () => {
 	}
 	tearingDown = true;
 
-	let rejections = currentlyUnhandled();
+	let rejections = currentlyUnhandled()
+		.filter(rejection => !attributedRejections.has(rejection.promise));
 
 	if (rejections.length > 0) {
 		rejections = rejections.map(rejection => {

lib/test.js

@@ -8,7 +8,6 @@ const plur = require('plur');
 const assert = require('./assert');
 const formatAssertError = require('./format-assert-error');
 const globals = require('./globals');
-const throwsHelper = require('./throws-helper');
 
 class SkipApi {
 	constructor(test) {
@@ -60,6 +59,13 @@ class ExecutionContext {
 
 		contextRef.context = context;
 	}
+
+	_throwsArgStart(assertion, file, line) {
+		this._test.trackThrows({assertion, file, line});
+	}
+	_throwsArgEnd() {
+		this._test.trackThrows(null);
+	}
 }
 Object.defineProperty(ExecutionContext.prototype, 'context', {enumerable: true});
 
@@ -101,9 +107,11 @@ class Test {
 		this.calledEnd = false;
 		this.duration = null;
 		this.endCallbackFinisher = null;
+		this.finishDueToAttributedError = null;
 		this.finishDueToInactivity = null;
 		this.finishing = false;
 		this.pendingAssertions = [];
+		this.pendingThrowsAssertion = null;
 		this.planCount = null;
 		this.startedAt = 0;
 	}
@@ -204,6 +212,60 @@ class Test {
 		}
 	}
 
+	trackThrows(pending) {
+		this.pendingThrowsAssertion = pending;
+	}
+
+	detectImproperThrows(err) {
+		if (!this.pendingThrowsAssertion) {
+			return false;
+		}
+
+		const pending = this.pendingThrowsAssertion;
+		this.pendingThrowsAssertion = null;
+
+		const values = [];
+		if (err) {
+			values.push(formatAssertError.formatWithLabel(`The following error was thrown, possibly before \`t.${pending.assertion}()\` could be called:`, err));
+		}
+
+		this.saveFirstError(new assert.AssertionError({
+			assertion: pending.assertion,
+			fixedSource: {file: pending.file, line: pending.line},
+			improperUsage: true,
+			message: `Improper usage of \`t.${pending.assertion}()\` detected`,
+			stack: err instanceof Error && err.stack,
+			values
+		}));
+		return true;
+	}
+
+	waitForPendingThrowsAssertion() {
+		return new Promise(resolve => {
+			this.finishDueToAttributedError = () => {
+				resolve(this.finishPromised());
+			};
+
+			this.finishDueToInactivity = () => {
+				this.detectImproperThrows();
+				resolve(this.finishPromised());
+			};
+
+			// Wait up to a second to see if an error can be attributed to the
+			// pending assertion.
+			globals.setTimeout(() => this.finishDueToInactivity(), 1000).unref();
+		});
+	}
+
+	attributeLeakedError(err) {
+		if (!this.detectImproperThrows(err)) {
+			return false;
+		}
+
+		this.finishDueToAttributedError();
+		return true;
+	}
+
 	callFn() {
 		try {
 			return {
@@ -223,13 +285,13 @@ class Test {
 
 		const result = this.callFn();
 		if (!result.ok) {
-			throwsHelper(result.error);
-
-			this.saveFirstError(new assert.AssertionError({
-				message: 'Error thrown in test',
-				stack: result.error instanceof Error && result.error.stack,
-				values: [formatAssertError.formatWithLabel('Error:', result.error)]
-			}));
+			if (!this.detectImproperThrows(result.error)) {
+				this.saveFirstError(new assert.AssertionError({
+					message: 'Error thrown in test',
+					stack: result.error instanceof Error && result.error.stack,
+					values: [formatAssertError.formatWithLabel('Error:', result.error)]
+				}));
+			}
 			return this.finish();
 		}
 
@@ -260,6 +322,10 @@ class Test {
 					resolve(this.finishPromised());
 				};
 
+				this.finishDueToAttributedError = () => {
+					resolve(this.finishPromised());
+				};
+
 				this.finishDueToInactivity = () => {
 					this.saveFirstError(new Error('`t.end()` was never called'));
 					resolve(this.finishPromised());
@@ -269,6 +335,10 @@ class Test {
 
 		if (promise) {
 			return new Promise(resolve => {
+				this.finishDueToAttributedError = () => {
+					resolve(this.finishPromised());
+				};
+
 				this.finishDueToInactivity = () => {
 					const err = returnedObservable ?
 						new Error('Observable returned by test never completed') :
@@ -279,13 +349,13 @@ class Test {
 
 				promise
 					.catch(err => {
-						throwsHelper(err);
-
-						this.saveFirstError(new assert.AssertionError({
-							message: 'Rejected promise returned by test',
-							stack: err instanceof Error && err.stack,
-							values: [formatAssertError.formatWithLabel('Rejection reason:', err)]
-						}));
+						if (!this.detectImproperThrows(err)) {
+							this.saveFirstError(new assert.AssertionError({
+								message: 'Rejected promise returned by test',
+								stack: err instanceof Error && err.stack,
+								values: [formatAssertError.formatWithLabel('Rejection reason:', err)]
+							}));
+						}
 					})
 					.then(() => resolve(this.finishPromised()));
 			});
@@ -296,6 +366,11 @@ class Test {
 
 	finish() {
 		this.finishing = true;
+
+		if (!this.assertError && this.pendingThrowsAssertion) {
+			return this.waitForPendingThrowsAssertion();
+		}
+
 		this.verifyPlan();
 		this.verifyAssertions();