[samus]

If it doesn't ship with the default distribution of the language, then there's the risk that multiple mutually incompatible libraries emerge or that the adoption by the ecosystem is spotty. Best example: the async runtime mess in the Rust ecosystem.

[neonsunset]

There is no async runtime mess. Tokio is the preferred one, and given Rust's goals it is difficult to do it better (pluggable async executors and the degree of flexibility such abstraction offers - running both on big servers and bare cooperative multi-tasking on microcontrollers).

In addition, multiple versions of transitive dependencies can coexist in Rust without conflicting with each other, there is no such risk.

[samus]

Quite so. But I am under the impression that not every library supports Tokio as a runtime. And while it might be possible to run multiple runtimes in the same process, or use compatibility wrappers, it sounds like trouble.

[pjmlp]

It is definitely a mess, given incompatible semantics making it an herculean effort to write runtime agnostic async libraries.