feat(eslint-plugin): allow explicit variable type with arrow functions

Fixes typescript-eslint#149

Author: gilbsgilbs

Diff for: packages/eslint-plugin/src/rules/explicit-function-return-type.ts

@@ -65,6 +65,17 @@ export default util.createRule<Options, MessageIds>({
       );
     }
 
+    /**
+     * Checks if the parent of a function expression has a type annotation.
+     * @param parent The parent of a function expression node
+     */
+    function hasTypeAnnotation(parent: TSESTree.Node): boolean {
+      return (
+        parent.type === AST_NODE_TYPES.VariableDeclarator &&
+        'typeAnnotation' in parent.id
+      );
+    }
+
     /**
      * Checks if a function declaration/expression has a return type.
      * @param node The node representing a function.
@@ -80,6 +91,7 @@ export default util.createRule<Options, MessageIds>({
         node.parent &&
         !isConstructor(node.parent) &&
         !isSetter(node.parent) &&
+        !hasTypeAnnotation(node.parent) &&
         util.isTypeScriptFile(context.getFilename())
       ) {
         context.report({

Diff for: packages/eslint-plugin/tests/rules/explicit-function-return-type.test.ts

@@ -32,6 +32,12 @@ var arrowFn = (): string => 'test';
     {
       filename: 'test.ts',
       code: `
+var arrowFn: Foo = () => 'test';
+            `
+    },
+    {
+      filename: 'test.ts',
+      code: `
 class Test {
   constructor() {}
   get prop(): number {