% Concurrency and Paralellism

Rust as a language doesn't *really* have an opinion on how to do concurrency or
parallelism. The standard library exposes OS threads and blocking sys-calls
because *everyone* has those, and they're uniform enough that you can provide
an abstraction over them in a relatively uncontroversial way. Message passing,
green threads, and async APIs are all diverse enough that any abstraction over
them tends to involve trade-offs that we weren't willing to commit to for 1.0.

However the way Rust models concurrency makes it relatively easy design your own
concurrency paradigm as a library and have *everyone else's* code Just Work
with yours. Just require the right lifetimes and Send and Sync where appropriate
and you're off to the races. Or rather, not having races. Races are bad.