Sometimes you introduce certain restrictions in the language just for the sake of simplifying the compiler. This is allowed, valid and will be understood by the developers.
For example, for decades most static languages required a symbol to be defined before its first usage. Removing this restiction seemed like a nice little cute convenience but the outcome of it is this horrific slowdown of the build process. Personally I would agree to the restriction if I knew the compiler could be 5x faster which is pretty realistic to achieve, it seems.
Those kinds of restrictions can have their own productivity pitfalls. For instance, it might be the case that requiring declaration prior to usage explicitly will speed up the compiler, but it might also mean that you can't move blocks of code around without paying careful attention to the order of declaration. I think having a permissive compiler, but also being able to profile the compilation process is a reasonable compromise.
Possibly, but there are other kinds of compromises such as allowing arbitrary order only within a class, the C++ way. Or another one: the first (quick) pass treats all function and reference type definitions as forward declarations and skips the details. This way the compiler might be able to do more permissive compilation though not 100% accurate like Swift currently does.
This is a pitfall of Rust as well. I suppose there will always be a tradeoff between stronger static analysis and compilation time, and I'm firmly in the camp that I want my compiler to catch more issues so I don't have unexpected runtime failures, but iteration time matters too, and I'm hopeful improvements can be made in this area.