Added no-floating-promises rule

Promises must be stored or passed to a function.

The idea here is that floating (unhandled) Promises can cause unexpected
behavior. For example, in tests, they might resolve at unexpected times
(like after another test following their parent test has started).

Fixes https://github.com/Microsoft/tslint-microsoft-contrib#174 (PRing
here because I'm not sure they support the type checker).

src/rules/noFloatingPromisesRule.ts

@@ -0,0 +1,77 @@
+/**
+ * @license
+ * Copyright 2016 Palantir Technologies, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as ts from "typescript";
+
+import * as Lint from "../lint";
+
+export class Rule extends Lint.Rules.TypedRule {
+    /* tslint:disable:object-literal-sort-keys */
+    public static metadata: Lint.IRuleMetadata = {
+        ruleName: "no-floating-promises",
+        description: "Promises returned by functions must be handled appropriately.",
+        optionsDescription: "Not configurable.",
+        options: null,
+        optionExamples: ["true"],
+        type: "functionality",
+        requiresTypeInfo: true,
+    };
+    /* tslint:enable:object-literal-sort-keys */
+
+    public static FAILURE_STRING = "Promises must be handled appropriately";
+
+    public applyWithProgram(sourceFile: ts.SourceFile, program: ts.Program): Lint.RuleFailure[] {
+        return this.applyWithWalker(new NoFloatingPromisesWalker(sourceFile, this.getOptions(), program));
+    }
+}
+
+class NoFloatingPromisesWalker extends Lint.ProgramAwareRuleWalker {
+    private static allowedParentKinds = {
+        [ts.SyntaxKind.CallExpression]: true,
+        [ts.SyntaxKind.VariableDeclaration]: true
+    };
+
+    public visitCallExpression(node: ts.CallExpression): void {
+        this.checkCallExpression(node);
+        super.visitCallExpression(node);
+    }
+
+    private checkCallExpression(node: ts.CallExpression) {
+        if (this.kindCanContainPromise(node.parent.kind)) {
+            return;
+        }
+
+        const typeChecker = this.getTypeChecker();
+        const type = typeChecker.getTypeAtLocation(node);
+
+        if (this.symbolIsPromise(type.symbol)) {
+            this.addFailure(this.createFailure(node.getStart(), node.getWidth(), Rule.FAILURE_STRING));
+        }
+    }
+
+    private symbolIsPromise(symbol: ts.Symbol) {
+        if (!symbol) {
+            return false;
+        }
+
+        return symbol.name === "Promise";
+    }
+
+    private kindCanContainPromise(kind: ts.SyntaxKind) {
+        return !!NoFloatingPromisesWalker.allowedParentKinds[kind];
+    }
+}

test/rules/no-floating-promises/test.ts.lint

@@ -0,0 +1,58 @@
+class Promise { }
+
+function returnsPromiseFunction() {
+    return new Promise();
+}
+
+const returnsPromiseVariable = () => new Promise();
+
+class ReturnsPromiseClass {
+    returnsPromiseMemberFunction() {
+        return new Promise();
+    }
+
+    returnsPromiseMemberVariable = () => new Promise();
+}
+
+let a = returnsPromiseFunction();
+let b = returnsPromiseVariable();
+let c = new ReturnsPromiseClass().returnsPromiseMemberFunction();
+let d = new ReturnsPromiseClass().returnsPromiseMemberVariable();
+
+returnsPromiseFunction();
+~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+
+returnsPromiseVariable();
+~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+
+new ReturnsPromiseClass().returnsPromiseMemberFunction();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+new ReturnsPromiseClass().returnsPromiseMemberVariable();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+
+(function () {
+    let a = returnsPromiseFunction();
+
+    returnsPromiseFunction();
+    ~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+})();
+
+(() => {
+    let a = returnsPromiseFunction();
+
+    returnsPromiseFunction();
+    ~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+})();
+
+[].push(returnsPromiseFunction());
+
+while (returnsPromiseFunction());
+       ~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+
+for (let i = 0; i < returnsPromiseFunction(); i= 1);
+                    ~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+
+for (let i in returnsPromiseFunction());
+              ~~~~~~~~~~~~~~~~~~~~~~~~       [0]
+
+[0]: Promises must be handled appropriately

test/rules/no-floating-promises/tslint.json

@@ -0,0 +1,8 @@
+{
+  "linterOptions": {
+    "typeCheck": true
+  },
+  "rules": {
+    "no-floating-promises": true
+  }
+}