Zig's Incremental Compilation Internals

    const std = @import("std");
    const File = std.Io.File;
    
    pub fn main(init: std.process.Init) !void {
        _ = try File.stdout().writeStreamingAll(init.io, "Hello, World!\n");
    }

Yes, like I keep saying: two clusters each within an order of magnitude, separated by three orders of magnitude feel to me like two distinct things. That does not at all feel arbitrary. I think that claim is pretty self-explanatory. You appear to think it reduces to "because I feel it so" and in some sense it does. I am applying my own judgement and values in constructing those clusters. Someone who felt that any amount of memory safety was unacceptable would structure them differently. Someone who cared naught about memory safety would similarly group them differently too.

Yes, but they come with speed gains, so you can't say that you pay "performance overheads" when Java removes some of the performance overheads that programs in low-level languages and replaces them with others. You could similarly say that you pay performance overheads when going in the other direction.

> and the experience of the large number of former C/C++ devs I've worked with after they learned Rust

And it's not my experience or a large number of C/C++ devs I work with.

> the only people I've talked to with that experience didn't really try to learn Rust and went in hoping that it wouldn't work for them

Then your exposure isn't wide enough.

> I don't think I understand what you're saying here.

What I'm saying is that we can't say that the value is merely in the existence of a clear syntactic distinction between safe and unsafe code, because that distinction exists in C, only in C, the clearly delineated line between safe and unsafe code is that between `int main(void) {}` and anything that isn't that; i.e. any program other than that explicitly opts into unsafety. So any meaningful discussion about memory safe languages must include what you can do in the safe subset. In C's "safe subset" (the empty program), you can do nothing, and that's what makes it not valuable. But for my needs, what you can do in Rust's safe subset (compared to both Java and Zig) is also far too little (to justify the cost).

> To me, "I can't write code that's memory unsafe without explicitly opting into it" seems like an objective statement

It is, but what I'm trying to say is that it alone doesn't have much value. In C you also "can't write code that's memory unsafe without explicitly opting into it" by writing anything other than the empty program, but obviously you wouldn't consider C's memory-safe subset suitable because you can't use it to do what you want to do in C. Rust's value is not, therefore, in that it has a memory-safe subset, but that it has a useful memory-safe subset. It's just that the utility of that subset depends on the kinds of programs you'd want to use a low-level language in the first place.

> it seems like you're refusing to pay a pretty small price for mostly ideological purity rather than pragmatism.

Quite the opposite. The price of Rust's complexity, implicitness, and compilation time is too high for what little safety I get in return, that I don't want to pay it for pragmatic reasons.

> I'm struggling to imagine what the circumstances are where these are genuine concerns rather than theoretical or premature optimizations. What are some examples of programs where you'd get better characteristics running them if they were written in Java rather than Rust due to the lack of enough "memory safety" in Rust?

It's nothing to do with memory safety. Low-level programs sacrifice optimisation opportunities available to Java because above all else they need to offer low-level control. That low-level control can translate to good performance sometimes (especially in smaller programs), and sometimes it translates to worse performance (especially in large programs). The huge C++ programs I worked on migrated to Java not (just) for safety but also for better performance than C++ (again, it's easy to get excellent performance in low-level languages when the programs are small or specialised; it gets harder and harder as they grow). So we got better performance than C++ while also getting better safety than Rust, a much simpler language than Rust (or C++), and faster build cycles than Rust (or C++). But the topic of how Java reduces the overheads that C/C++/Rust/Zig programs often have when they grow large (although Zig makes it easier than the other them to reduce them) is a whole complicated topic. I might give a talk about it at the upcoming Devoxx.