A useful way to think about this:
- All data in Rust has exactly one owner.
- If you need some kind of multiple ownership, you have to make the owner be a reference-counted cell, such as Rc or Arc.
- All data can be accessed by one reader/writer, or N readers, but not both at the same time.
- There is both compile time and run time machinery to strictly enforce this.
Once you get that, you can see what the borrow checker is trying to do for you.
> Function Overloads
Strictly speaking, Rust doesn't support overloaded functions. Function overloading is when you define the same function but with different arguments, and the language selects the correct one based on the argument type. In this case, it's two different implementations of a trait for two different types. That said, it's close enough that this isn't really an issue, more of just a technical note, since this is a series trying to get into details.
> I can't find an explanation in the Rust documentation but I expect the reason is that someone could implement another trait that provides .into_iter() on whatever the x is in for y in x, thus resulting in a compiler error because there would be two candidate implementations.
Yep, I'm not sure that there is an official explanation anywhere else, but this is exactly what I would assume as well. This ensures that the correct implementation is called. This is also one of the reasons why adding a trait implementation isn't considered a breaking change, even if it could create a compiler error, because you can always expand it yourself to explicitly select the correct choice. Of course, these situations are usually treated more carefully then they have to be, because breakage isn't fun, even if it's technically allowed.
> But wait, you say, I'm doing exactly this in the first program, and indeed you are.
It's not the same, as the next paragraphs explain.
> We are able to examine the function and realize it's safe, but because the compiler wants to use local reasoning, it's not able to do so.
This is a super important point!
Because nothing in Rust is identifier-based. Unlike python, all syntax magic (even the ? operator) relies on traits defined by `core`.
for x in y {
let mut iter = IntoIterator::into_iter(y);
while let Some(x) = y.next() {
x?
match x {
Ok(x) => x,
Err(e) => {
return Err(From::from(e));
}
}
x + y
core::ops::Add::add(x, y)
All of those traits are expected to live explicitly in the core crate at well known paths. Otherwise you'd be writing methods with absolutely no idea how the language would interact with it. And if you had a Set type implement `add`, it'd have to accept exactly 2 arguments to be compatible with the language's `add` or something equally as unergonomic.
It's traits all the way down! There'd be no explanation needed because it'd be antithetical and contradictory to traits to begin with. Once one understands how traits are intended to be used, the explanation for why there aren't identifier based resolution semantics becomes obvious.
Essentially, you wouldn't and shouldn't make that tradeoff for anything other than system programming.
This is what you should be doing when working with C/C++, except there is no compiler to call you names there if you don’t.
If you’re saying ‘use a GC language unless requirements are strict about it’, yeah hard to disagree.
Another point, is that the rust ecosystem is absolutely insanely good ( i've recently worked with uniffi and wasmbindgen, and those are 5 years ahead of anything else i've seen..)
guess it's "instead of a &mut self"
sidcool•5h ago
bnjms•4h ago