[Pitch] Isolated conformances

[Douglas_Gregor]

@Douglas_Gregor

Y'all have convinced me to drop isolated P and go with @MainActor P. It's more general and clearer. I've updated the document accordingly. Thank you!

As an update-to-the-update, the implementation is now in sync with the proposal. I have toolchains for macOS and Linux (Windows coming soon), if anyone wants to kick the tires:

• macOS: https://ci.swift.org/view/all/job/swift-PR-toolchain-macos/1852/artifact/branch-main/swift-PR-79983-1852-osx.tar.gz
• Ubuntu 20.04: https://download.swift.org/tmp/pull-request/79983/1332/ubuntu2004/PR-ubuntu2004.tar.gz

There are three experimental features at play here for this proposal:

• IsolatedConformances: allows one to define and use isolated conformances, e.g., class MyClass: @MainActor P { }, the main part of the proposal.

• StrictSendableMetatypes: implements the restriction on sending metatypes in generic code. This enables just the source-breaking part of the proposal. Without it, isolated conformances are not data-race-safe, but you can try it out independently.

• UnspecifiedMeansMainActorIsolated: this experimental feature is the subject of the ongoing review of SE-0466. When enabled, it infers @MainActor for code that isn't explicit about its isolation. When isolated conformances are also enabled, we infer @MainActor for any conformances in the module of @MainActor types unless they are explicitly marked nonisolated.

  Doug