I enjoy coding in Rust, and I think it's a step in the right direction, but I'm still looking for a general purpose programming language which model IO/State effects the same way languages supporting functional programming do with pure functions. Haskell monads are not granular enough. The closer thing I've seen is Purescript, but still is not general purpose enough. Solidity's "pure" annotations are a step in the right direction, but I'm speaking about something much expressive. Also, it should have a Rust/C-like syntax in order to be able to be mainstream, but I'm happy with Haskell-like syntaxes.
Check out Nim. They recently added support for a function declaration in addition to a proc. Function automatically checks that it is a pure function with no side effects.
Considering the flexibility of the language as a whole, this is really cool!
