If I understand the article correctly, even if the programmer is reasoning and coding functionally in an imperative language, the compiler can not assert that the code is safe for concurrency and parallelism. (You may find you get other benefits from programming functionally.)
Rust seems to have solved some of this with its borrow checking semantics that automatically prove which parts of code are automatically paralellizable without running into issues like simple data races - or am I wrong?
The Rust compiler doesn't automatically parallelize, but all single-threaded Rust code upholds the invariants necessary to be thread-safe, so it's relatively easy (and sometimes entirely trivial) to turn a single-threaded program into a parallel program.
