A language-level reasync has been discussed before, but no proposals have advanced. The pitch post quotes Doug Gregor's observation, where he notes that the motivation is easy, but the compiler implementation is a bunch of work, and that a peer macro can plausibly cover the common case without language-level changes.
This pitch also proposes addition of the macros as a swiftlang-org package, rather than as part of the standard library.
The full proposal discusses this further in the Alternatives Considered section.