[Fix] no-cycle: add ignoreExternal option

Fixes #1647

docs/rules/no-cycle.md

@@ -55,6 +55,26 @@ import { b } from './dep-b.js' // not reported as the cycle is at depth 2
 This is not necessarily recommended, but available as a cost/benefit tradeoff mechanism
 for reducing total project lint time, if needed.
 
+#### `ignoreExternal`
+
+An `ignoreExternal` option is available to prevent the cycle detection to expand to external modules:
+
+```js
+/*eslint import/no-cycle: [2, { ignoreExternal: true }]*/
+
+// dep-a.js
+import 'module-b/dep-b.js'
+
+export function a() { /* ... */ }
+```
+
+```js
+// node_modules/module-b/dep-b.js
+import { a } from './dep-a.js' // not reported as this module is external
+```
+
+Its value is `false` by default, but can be set to `true` for reducing total project lint time, if needed.
+
 ## When Not To Use It
 
 This rule is comparatively computationally expensive. If you are pressed for lint
@@ -65,5 +85,8 @@ this rule enabled.
 
 - [Original inspiring issue](https://github.com/benmosher/eslint-plugin-import/issues/941)
 - Rule to detect that module imports itself: [`no-self-import`]
+- [`import/external-module-folders`] setting
 
 [`no-self-import`]: ./no-self-import.md
+
+[`import/external-module-folders`]: ../../README.md#importexternal-module-folders

src/rules/no-cycle.js

@@ -4,6 +4,7 @@
  */
 
 import Exports from '../ExportMap'
+import { isExternalModule } from '../core/importType'
 import moduleVisitor, { makeOptionsSchema } from 'eslint-module-utils/moduleVisitor'
 import docsUrl from '../docsUrl'
 
@@ -18,6 +19,11 @@ module.exports = {
         type: 'integer',
         minimum: 1,
       },
+      ignoreExternal:{
+        description: 'ignore external modules',
+        type: 'boolean',
+        default: false,
+      },
     })],
   },
 
@@ -27,8 +33,13 @@ module.exports = {
 
     const options = context.options[0] || {}
     const maxDepth = options.maxDepth || Infinity
+    const ignoreModule = (name) => options.ignoreExternal && isExternalModule(name)
 
     function checkSourceValue(sourceNode, importer) {
+      if (ignoreModule(sourceNode.value)) {
+        return // ignore external modules
+      }
+
       const imported = Exports.get(sourceNode.value, context)
 
       if (importer.importKind === 'type') {
@@ -54,6 +65,7 @@ module.exports = {
         for (let [path, { getter, source }] of m.imports) {
           if (path === myPath) return true
           if (traversed.has(path)) continue
+          if (ignoreModule(source.value)) continue
           if (route.length + 1 < maxDepth) {
             untraversed.push({
               mget: getter,

tests/files/cycles/external-depth-two.js

@@ -0,0 +1,2 @@
+import { foo } from "cycles/external/depth-one"
+export { foo }

tests/files/cycles/external/depth-one.js

@@ -0,0 +1,2 @@
+import foo from "../depth-zero"
+export { foo }

tests/src/rules/no-cycle.js

@@ -40,6 +40,22 @@ ruleTester.run('no-cycle', rule, {
       code: 'import { foo, bar } from "./depth-two"',
       options: [{ maxDepth: 1 }],
     }),
+    test({
+      code: 'import { foo } from "cycles/external/depth-one"',
+      options: [{ ignoreExternal: true }],
+      settings: {
+        'import/resolver': 'webpack',
+        'import/external-module-folders': ['external'],
+      },
+    }),
+    test({
+      code: 'import { foo } from "./external-depth-two"',
+      options: [{ ignoreExternal: true }],
+      settings: {
+        'import/resolver': 'webpack',
+        'import/external-module-folders': ['external'],
+      },
+    }),
     test({
       code: 'import("./depth-two").then(function({ foo }){})',
       options: [{ maxDepth: 1 }],
@@ -63,6 +79,22 @@ ruleTester.run('no-cycle', rule, {
       code: 'import { foo } from "./depth-one"',
       errors: [error(`Dependency cycle detected.`)],
     }),
+    test({
+      code: 'import { foo } from "cycles/external/depth-one"',
+      errors: [error(`Dependency cycle detected.`)],
+      settings: {
+        'import/resolver': 'webpack',
+        'import/external-module-folders': ['external'],
+      },
+    }),
+    test({
+      code: 'import { foo } from "./external-depth-two"',
+      errors: [error(`Dependency cycle via cycles/external/depth-one:1`)],
+      settings: {
+        'import/resolver': 'webpack',
+        'import/external-module-folders': ['external'],
+      },
+    }),
     test({
       code: 'import { foo } from "./depth-one"',
       options: [{ maxDepth: 1 }],