feat(ignored-observable): Add rule.

package.json

@@ -59,7 +59,7 @@
     "test": "yarn run lint && yarn run test:build && yarn run test:mocha && yarn run test:tslint-v5 && yarn run test:tslint-v6 && yarn run test:tslint-v6-compat",
     "test:build": "yarn run test:clean && tsc -p tsconfig.json",
     "test:clean": "rimraf build",
-    "test:debug": "tslint --test ./test/v6/fixtures/issues/87/tslint.json",
+    "test:debug": "tslint --test ./test/v6/fixtures/no-ignored-observable/default/tslint.json",
     "test:issues": "yarn run test:clean && tsc -p tsconfig.json && tslint --test ./test/v6/fixtures/issues/**/tslint.json",
     "test:mocha": "mocha build/**/*-spec.js",
     "test:tslint-v5": "yarn --cwd ./test/v5 install && yarn --cwd ./test/v5 upgrade && tslint --test ./test/v5/fixtures/**/tslint.json",

source/rules/rxjsNoIgnoredObservableRule.ts

@@ -0,0 +1,51 @@
+/**
+ * @license Use of this source code is governed by an MIT-style license that
+ * can be found in the LICENSE file at https://github.com/cartant/rxjs-tslint-rules
+ */
+/*tslint:disable:no-use-before-declare*/
+
+import { tsquery } from "@phenomnomnominal/tsquery";
+import * as Lint from "tslint";
+import * as ts from "typescript";
+import { couldBeType } from "../support/util";
+
+export class Rule extends Lint.Rules.TypedRule {
+    public static metadata: Lint.IRuleMetadata = {
+        description: "Disallows the ignoring of observables returned by functions.",
+        options: null,
+        optionsDescription: "Not configurable.",
+        requiresTypeInfo: true,
+        ruleName: "rxjs-no-ignored-observable",
+        type: "functionality",
+        typescriptOnly: true
+    };
+
+    public static FAILURE_STRING = "Ignoring a returned Observable is forbidden";
+
+    public applyWithProgram(
+        sourceFile: ts.SourceFile,
+        program: ts.Program
+    ): Lint.RuleFailure[] {
+
+        const failures: Lint.RuleFailure[] = [];
+        const typeChecker = program.getTypeChecker();
+
+        const callExpressions = tsquery(
+            sourceFile,
+            `ExpressionStatement > CallExpression`
+        );
+        callExpressions.forEach(callExpression => {
+            const type = typeChecker.getTypeAtLocation(callExpression);
+            if (couldBeType(type, "Observable")) {
+                failures.push(new Lint.RuleFailure(
+                    sourceFile,
+                    callExpression.getStart(),
+                    callExpression.getStart() + callExpression.getWidth(),
+                    Rule.FAILURE_STRING,
+                    this.ruleName
+                ));
+            }
+        });
+        return failures;
+    }
+}

test/v6/fixtures/no-ignored-observable/default/fixture.ts.lint

@@ -0,0 +1,23 @@
+import { Observable, of } from "rxjs";
+
+function functionSource() {
+    return of(42);
+}
+
+const arrowSource = () => of(42);
+
+function sink(source: Observable<number>) {
+}
+
+const a = functionSource();
+const b = arrowSource();
+sink(functionSource());
+sink(arrowSource());
+
+functionSource();
+~~~~~~~~~~~~~~~~                [no-ignored-observable]
+
+arrowSource();
+~~~~~~~~~~~~~                   [no-ignored-observable]
+
+[no-ignored-observable]: Ignoring a returned Observable is forbidden

test/v6/fixtures/no-ignored-observable/default/tsconfig.json

@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "baseUrl": ".",
+    "lib": ["es2015"],
+    "noEmit": true,
+    "paths": {
+      "rxjs": ["../../node_modules/rxjs"]
+    },
+    "skipLibCheck": true,
+    "target": "es5"
+  },
+  "include": ["fixture.ts"]
+}

test/v6/fixtures/no-ignored-observable/default/tslint.json

@@ -0,0 +1,8 @@
+{
+  "defaultSeverity": "error",
+  "jsRules": {},
+  "rules": {
+    "rxjs-no-ignored-observable": { "severity": "error" }
+  },
+  "rulesDirectory": "../../../../../build/rules"
+}