Forbid index files on the layer level.
According to the public API rule on slices:
Every slice (and segment on layers that don't have slices) must contain a public API definition.
Modules outside of this slice/segment can only reference the public API, not the internal file structure of the slice/segment. https://feature-sliced.design/docs/reference/slices-segments#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:
📂 shared
📂 ui
📄 index.ts
📂 lib
📄 index.ts
📂 entities
📂 user
📂 ui
📂 model
📄 index.ts
📂 pages
📂 home
📂 ui
📄 index.ts
Examples of project structures that fail this rule:
📂 shared
📂 ui
📄 index.ts
📄 index.ts // ❌
📂 entities
📂 user
📂 ui
📄 index.ts
📂 pages
📂 home
📂 ui
📄 index.ts
📂 editor
📂 ui
📄 index.ts
📄 index.ts // ❌
Layers contain slices, and slices are meant to be independent partitions of code by their business domain. It doesn't make much sense to import things from two different business domains from the same place. In addition to that, index files cause issues with tree-shaking for some bundlers, so having as little of them as possible helps to keep the bundle size down.