`async fn` inside inherent impl produces generic parse error on 2015 edition

[comex]

When compiling for the 2018 edition, rustc allows async fn both outside of an impl and within an inherent impl.

When compiling for the 2015 edition, async fn works outside of an impl, but putting one inside an inherent impl produces this error:

error: missing `fn`, `type`, or `const` for impl-item declaration
 --> src/lib.rs:4:9
  |
4 | impl A { async fn inside_impl() {} }
  |         ^ missing `fn`, `type`, or `const`

Either this should be allowed, or it should result in a better error message that recommends switching to the 2018 edition.

[withoutboats]

Since async blocks and some versions of the final await syntax cannot be permitted (for backward compatibility reasons), it seems preferable to disallow all async/await features on 2015 to simplifier the user story: async/await works on 2018.