[Pitch #4] Incremental migration to concurrency checking

Hey all,

We've revised and now implemented the proposal for incremental migration to concurrency checking. Only a few changes since the last pitch:

  • Collapsed the three concurrency checking modes down to two (minimal and strict).
  • Renamed the attribute to @preconcurrency.
  • Clarified some of the behavior about when warnings vs. errors are produced.

Here is the full proposal.