It does pollute the whole module, but this is much less of a big deal than it might seem because Rust modules are so lightweight - you don't even need a new file to create a new module, you can just do:
mod name_of_module {
// code goes here
}
And it's often possible to wrap the tricky unsafe bits in a safe interface (that e.g. uses a mutex to enforce safety). So that anyone who is contributing to higher-level code doesn't need to worry about it. This is much better than C or C++ code where it's trivially easy to introduce memory unsafety or even Undefined Behaviour in even the boring "glue" parts of the codebase.
This leads to a really nice gentle onboarding flow where inexperienced users can start out contributing to the safe parts of the project, and (optionally) move on to gnarly unsafe bits later when they are already familiar with the project's codebase. It also dramatically reduces review workload for maintainers as they can rely on the compiler enforcing invariants outside of unsafe modules.
This works less well for really low-level code like embedded code or kernel code. But it's still a lot better than nothing.
Though my usual experience with writing performance-sensitive code is: if you just write naive inefficient code on the first try, there's a high probably that you need to rearchitect the whole system to get a more performant design, it's not something you can do incrementally. Maybe in these kinds of projects it's not wise to let random contributors handle your code though... (I'm geared more into graphics and numerical computing so the experience might be different from others.)
Maybe there's a reason why game developers have primarily used scripting languages - give out a safe managed GC-backed runtime for the majority of developers, and let only a select few who understand the system to develop the core C++ engine. Maybe Safe Rust can be used this way (as a "fast" scripting language), to separate between these two worlds... but the problems is even Safe Rust is just really difficult to grok for newcomers, and the hoops they go through to circumvent the borrow checker either falls into using Copy/Clone all over the place (slow) or smart pointers (slow) or array indices with bound checking (maybe less slow but more cumbersome, and also prone to logical invalidation errors if you're not careful)
> Maybe Safe Rust can be used this way (as a "fast" scripting language), to separate between these two worlds... but the problems is even Safe Rust is just really difficult to grok for newcomers, and the hoops they go through to circumvent the borrow checker either falls into using Copy/Clone all over the place (slow) or smart pointers (slow) or array indices with bound checking (maybe less slow but more cumbersome, and also prone to logical invalidation errors if you're not careful)
Are smart pointers like Box, Rc and even Arc any slower than any scripting system you'd "hand out" to most developers from your tightly written C++ core engine?
One thing that I see is that 90% of code is simple enough that you don't need to have any ceremony around ownership beyond writing a & in front of a value or type, 5% is harder than that but doable, and 5% requires extensive expertise to avoid allocations, or using Arc. I'd wager that the distribution of code that a GC can optimize during runtime is comparable, if not worse, at higher memory consumption.
I've compared some simple networked applications written in Java and Rust for that purpose, and performance ended up being comparable, but with 100x memory consumption, even when using GraalVM.
This leads to a really nice gentle onboarding flow where inexperienced users can start out contributing to the safe parts of the project, and (optionally) move on to gnarly unsafe bits later when they are already familiar with the project's codebase. It also dramatically reduces review workload for maintainers as they can rely on the compiler enforcing invariants outside of unsafe modules.
This works less well for really low-level code like embedded code or kernel code. But it's still a lot better than nothing.