I try to compile a Rust library I found on GitHub.

Although I put this line inside the root Cargo.toml file:
#![feature(sync_unsafe_cell)]

When I compile the code I get the following error in several places:
UnsafeCell<Vec<bool>> cannot be shared between threads safely

However, after changing the package edition to "2018", the error is vanished.
In addition, I looked here, and it seems the feature is pretty new.

Does anyone can explain me why?

Thanks.