In many user-cases, provideSomeLayer will be utilized:

• to provide the non-ZEnv part of the environment
• to provide the non-(ZEnv + test) part of the environment (ZIO Test)

We could have helpers, provideCustomLayer, affixed to the data types, which simply call provideSomeLayer, and which fix the remainder to be ZEnv (in ZIO Core) and ZEnv + TestEnvironment (in ZIO Test).

Then a lot of user-land tests could be simplified to spec.provideCustomLayer(myCustomLayer), and a lot of App run functions could switch to using provideSomeLayer.