Automatically initialize the Waffle Chai matchers (#456)

packages/buidler-waffle/README.md

@@ -7,7 +7,11 @@
 
 ## What
 
-You can use this plugin to build smart contract tests using Waffle in Buidler, taking advantage of both.
+You can use this plugin to build smart contract tests using Waffle in Buidler,
+taking advantage of both.
+
+This plugin adds a Waffle-compatible provider to the Buidler Runtime Environment,
+and automatically initializes the [Waffle Chai matchers](https://ethereum-waffle.readthedocs.io/en/latest/matchers.html).
 
 ## Installation
 
@@ -45,7 +49,7 @@ Once installed, you can build your tests just like in Waffle. The only differenc
 instead of `createMockProvider()`.
 
 Note that by default, Buidler save its compilation output into `artifacts/` instead of `build/`. You can either use
-that directory in your tests, or [customize your Buidler config](https://buidler.dev/config/#path-configuration).  
+that directory in your tests, or [customize your Buidler config](https://buidler.dev/config/#path-configuration).
 
 ## TypeScript support
 

packages/buidler-waffle/src/index.ts

@@ -1,7 +1,24 @@
 import { extendEnvironment, usePlugin } from "@nomiclabs/buidler/config";
 import { lazyObject } from "@nomiclabs/buidler/plugins";
 
+function initializeWaffleMatchers() {
+  const wafflePath = require.resolve("ethereum-waffle");
+  const waffleChaiPath = require.resolve("@ethereum-waffle/chai", {
+    paths: [wafflePath]
+  });
+  const { waffleChai } = require(waffleChaiPath);
+
+  try {
+    const chai = require("chai");
+    chai.use(waffleChai);
+  } catch (error) {
+    // If chai isn't installed we just don't initialize the matchers
+  }
+}
+
 export default function() {
+  initializeWaffleMatchers();
+
   extendEnvironment(bre => {
     // We can't actually implement a MockProvider because of its private
     // properties, so we cast it here 😢

packages/buidler-waffle/test/buidler-project/test/tests.js

@@ -0,0 +1,12 @@
+describe("Internal test suite of buidler-waffle's test project", function() {
+  it("Should have waffle assertions loaded", function() {
+    const chai = require("chai");
+    if (!("revertedWith" in chai.Assertion.prototype)) {
+      throw new Error("Failed to load it");
+    }
+  });
+
+  it("Should fail", function() {
+    throw new Error("Failed on purpose");
+  });
+});

packages/buidler-waffle/test/index.ts

@@ -89,4 +89,14 @@ describe("Waffle plugin plugin", function() {
       });
     });
   });
+
+  describe("Test environment initialization", function() {
+    useEnvironment(path.join(__dirname, "buidler-project"));
+
+    it("Should load the Waffle chai matchers", async function() {
+      await this.env.run("test", { testFiles: [] });
+      assert.equal(process.exitCode, 1);
+      process.exitCode = 0;
+    });
+  });
 });