Add no-ui-in-app rule

.changeset/clean-chicken-tan.md

@@ -0,0 +1,6 @@
+---
+'@feature-sliced/steiger-plugin': minor
+'steiger': minor
+---
+
+Add no-ui-in-app rule

README.md

@@ -69,6 +69,7 @@ Currently, Steiger is not extendable with more rules, though that will change in
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-reserved-folder-names/README.md"><code>no-reserved-folder-names</code></a></td> <td>Forbid subfolders in segments that have the same name as other conventional segments.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-segmentless-slices/README.md"><code>no-segmentless-slices</code></a></td> <td>Forbid slices that don't have any segments.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-segments-on-sliced-layers/README.md"><code>no-segments-on-sliced-layers</code></a></td> <td>Forbid segments (like ui, lib, api ...) that appear directly in sliced layer folders (entities, features, ...)</td> </tr>
+  <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-ui-in-app/README.md"><code>no-ui-in-app</code></a></td> <td>Forbid having the <code>ui</code> segment on the App layer.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/public-api/README.md"><code>public-api</code></a></td> <td>Require slices (and segments on sliceless layers like Shared) to have a public API definition.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/repetitive-naming/README.md"><code>repetitive-naming</code></a></td> <td>Ensure that all entities are named consistently in terms of pluralization.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/segments-by-purpose/README.md"><code>segments-by-purpose</code></a></td> <td>Discourage the use of segment names that group code by its essence, and instead encourage grouping by purpose</td> </tr>

packages/steiger-plugin-fsd/src/index.ts

@@ -10,6 +10,7 @@ import noPublicApiSidestep from './no-public-api-sidestep/index.js'
 import noReservedFolderNames from './no-reserved-folder-names/index.js'
 import noSegmentlessSlices from './no-segmentless-slices/index.js'
 import noSegmentsOnSlicedLayers from './no-segments-on-sliced-layers/index.js'
+import noUiInApp from './no-ui-in-app/index.js'
 import publicApi from './public-api/index.js'
 import repetitiveNaming from './repetitive-naming/index.js'
 import segmentsByPurpose from './segments-by-purpose/index.js'
@@ -28,6 +29,7 @@ const allRules: Array<Rule> = [
   noReservedFolderNames,
   noSegmentlessSlices,
   noSegmentsOnSlicedLayers,
+  noUiInApp,
   publicApi,
   repetitiveNaming,
   segmentsByPurpose,

packages/steiger-plugin-fsd/src/no-ui-in-app/README.md

@@ -0,0 +1,43 @@
+# `no-ui-in-app`
+
+Forbid having <code>ui</code> segment in <code>app</code> layer.
+
+Examples of project structures that pass this rule:
+
+```
+📂 shared
+  📂 ui
+    📄 index.ts
+📂 pages
+  📂 home
+    📂 ui
+    📄 index.ts
+📂 app
+  📂 providers
+    📄 index.ts
+```
+
+Examples of project structures that fail this rule:
+
+```
+📂 shared
+  📂 ui
+    📄 index.ts
+📂 pages
+  📂 home
+    📂 ui
+    📄 index.ts
+📂 app
+  📂 providers
+    📄 index.ts
+  📂 ui  // ❌
+    📄 index.ts
+```
+
+## Rationale
+
+It's uncommon to define the `ui` segment on the App layer. The App layer is typically used to combine the application into a single entry point. The UI of your application should already be created on the layers below to avoid mixing up responsibilities. Therefore, the `ui` segment on the App layer is typically a mistake.
+
+For example, context providers are components, but they are not UI. Global styles are technically UI, but they aren't scoped to that segment, so the name `ui` might be a misdirection.
+
+As one possible exception, the `ui` segment can be used on the App layer if the entire application consists of only one page and there is no reason to define the Pages layer.

packages/steiger-plugin-fsd/src/no-ui-in-app/index.spec.ts

@@ -0,0 +1,46 @@
+import { expect, it } from 'vitest'
+
+import noUiInApp from './index.js'
+import { joinFromRoot, parseIntoFsdRoot } from '../_lib/prepare-test.js'
+
+it('reports no errors on a project without the "ui" segment on the "app" layer', () => {
+  const root = parseIntoFsdRoot(`
+    📂 shared
+      📂 ui
+        📄 index.ts
+    📂 pages
+      📂 home
+        📂 ui
+        📄 index.ts
+    📂 app
+      📂 providers
+        📄 index.ts
+  `)
+
+  expect(noUiInApp.check(root)).toEqual({ diagnostics: [] })
+})
+
+it('reports errors on a project with the "ui" segment on the "app" layer', () => {
+  const root = parseIntoFsdRoot(`
+    📂 shared
+      📂 ui
+        📄 index.ts
+    📂 pages
+      📂 home
+        📂 ui
+        📄 index.ts
+    📂 app
+      📂 providers
+        📄 index.ts
+      📂 ui
+        📄 index.ts
+  `)
+
+  const diagnostics = noUiInApp.check(root).diagnostics
+  expect(diagnostics).toEqual([
+    {
+      message: 'Layer "app" should not have "ui" segment.',
+      location: { path: joinFromRoot('app', 'ui') },
+    },
+  ])
+})

packages/steiger-plugin-fsd/src/no-ui-in-app/index.ts

@@ -0,0 +1,27 @@
+import type { Diagnostic, Rule } from '@steiger/types'
+import { NAMESPACE } from '../constants.js'
+import { getLayers, getSegments } from '@feature-sliced/filesystem'
+
+const noUiInApp = {
+  name: `${NAMESPACE}/no-ui-in-app`,
+  check(root) {
+    const diagnostics: Array<Diagnostic> = []
+
+    const layers = getLayers(root)
+
+    if (layers.app !== undefined) {
+      const segments = getSegments(layers.app)
+
+      if (segments.ui !== undefined) {
+        diagnostics.push({
+          message: 'Layer "app" should not have "ui" segment.',
+          location: { path: segments.ui.path },
+        })
+      }
+    }
+
+    return { diagnostics }
+  },
+} satisfies Rule
+
+export default noUiInApp

packages/steiger/README.md

@@ -69,6 +69,7 @@ Currently, Steiger is not extendable with more rules, though that will change in
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-reserved-folder-names/README.md"><code>no-reserved-folder-names</code></a></td> <td>Forbid subfolders in segments that have the same name as other conventional segments.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-segmentless-slices/README.md"><code>no-segmentless-slices</code></a></td> <td>Forbid slices that don't have any segments.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-segments-on-sliced-layers/README.md"><code>no-segments-on-sliced-layers</code></a></td> <td>Forbid segments (like ui, lib, api ...) that appear directly in sliced layer folders (entities, features, ...)</td> </tr>
+  <tr> <td><a href="./packages/steiger-plugin-fsd/src/no-ui-in-app/README.md"><code>no-ui-in-app</code></a></td> <td>Forbid having the <code>ui</code> segment on the App layer.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/public-api/README.md"><code>public-api</code></a></td> <td>Require slices (and segments on sliceless layers like Shared) to have a public API definition.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/repetitive-naming/README.md"><code>repetitive-naming</code></a></td> <td>Ensure that all entities are named consistently in terms of pluralization.</td> </tr>
   <tr> <td><a href="./packages/steiger-plugin-fsd/src/segments-by-purpose/README.md"><code>segments-by-purpose</code></a></td> <td>Discourage the use of segment names that group code by its essence, and instead encourage grouping by purpose</td> </tr>

packages/steiger/migrations/convert-config-to-flat.js

@@ -63,6 +63,7 @@ const ruleNames = [
   'no-reserved-folder-names',
   'no-segmentless-slices',
   'no-segments-on-sliced-layers',
+  'no-ui-in-app',
   'public-api',
   'repetitive-naming',
   'segments-by-purpose',