I'm... not sure what you mean by this. Something like #include technically exists, but is absolutely not used everywhere, and is certainly not the 'default build system'.[1]
That is one, very, very tiny aspect of the C preprocessor.
And https://github.com/SirCmpwn/rust-poc/blob/master/includes.rs... is significant: the code included from another file is under a namespace, and isn't going to clash like randomly throwing #includes around will, and you don't need to worry about things like double inclusion...
In other words, I think this is a very superficial comparison, and I'm not even sure why it upsets you so. Rust won't be a perfect language for everyone, and that's okay.
Well, you could move the mod { } to the included file to accomplish the same result (like the `namespace` keyword in C#), but that'd break the example I'm trying to give. I don't really want to rehash this argument on Hacker News, I'm just providing context. There are numerous flaws in the module system that derive from this basic point.
#include in C is just textual substitution of the contents of the specified file in place of the #include statement, there is not a concept of namespace in C.
I can't really effectively explain how all of this works without providing a lot of background on systems languages in general, especially background on how linking works. Sorry.
Where was this discussion held?
1: http://doc.rust-lang.org/std/macro.include_str!.html is what I'm thinking of here.