Add app layer to exceptions of no-layer-public-api rule (#57)

.changeset/honest-dryers-act.md

@@ -0,0 +1,5 @@
+---
+'@feature-sliced/steiger-plugin': patch
+---
+
+Add an exception for app layer to no-layer-public-api rule

packages/steiger-plugin-fsd/src/no-layer-public-api/README.md

@@ -11,6 +11,8 @@ According to the _public API rule on slices_:
 
 A corollary to this rule is that the layer itself should not have an index file.
 
+**Exception:** index files are allowed on `app` layer because some people prefer to have that as their app's entrypoint.
+
 Examples of project structures that pass this rule:
 
 ```

packages/steiger-plugin-fsd/src/no-layer-public-api/index.spec.ts

@@ -42,6 +42,10 @@ it('reports errors on a project with index files on layer level', () => {
         📂 ui
         📄 index.ts
       📄 index.ts
+    📂 app
+      📂 ui
+        📄 index.ts
+      📄 index.ts
   `)
 
   const diagnostics = noLayerPublicApi.check(root).diagnostics

packages/steiger-plugin-fsd/src/no-layer-public-api/index.ts

@@ -1,6 +1,9 @@
 import { getIndex, getLayers } from '@feature-sliced/filesystem'
 import type { Diagnostic, Rule } from '@steiger/types'
 
+/** Layers that are allowed to have an index file. */
+const exceptionLayers = ['app']
+
 /** Forbid index files on layer level. */
 const noLayerPublicApi = {
   name: 'no-layer-public-api',
@@ -9,7 +12,9 @@ const noLayerPublicApi = {
 
     for (const [layerName, layer] of Object.entries(getLayers(root))) {
       const index = getIndex(layer)
-      if (index !== undefined) {
+      const notAmongExceptions = !exceptionLayers.includes(layerName)
+
+      if (notAmongExceptions && index !== undefined) {
         diagnostics.push({
           message: `Layer "${layerName}" should not have an index file`,
           location: { path: index.path },