Again, I haven't actually dug into this; maybe someone more knowledgeable about this can point me in the right direction here?
Lack of move and copy ctors in rust greatly simplifies things like this, and makes it very explicit when code is running, but the trade-off is that intrusive data structures are hard to do on the stack in rust.
The self-reference would point to the old object. See this for an example: http://ideone.com/sEFtbN
(Intrusive datastructures on the heap are doable with some tricks.)
They're not very essential so it works out.
I imagine Rust does something similar, copying bytes if the underlying type has the `Copy` trait and calling some actual code if not, but I'm not familiar with the details.
[1]: http://en.cppreference.com/w/cpp/concept/TriviallyCopyable
Rust's answer to copy ctors is Clone, which is always explicitly called. Variable use in rust is a move. Trivially copyable types (Copy) will be copied without compile-time invalidating the old type.
> copying bytes if the underlying type has the `Copy` trait and calling some actual code if not,
It does not. Moves and copies are both "memcopy these bytes", the only difference is if you can use the previous copy or not. (This is also, of course, subject to the optimizer, which may elide the copy.)
> either way some constructor of the object must be called if it exists (though it might be inlined and optimized away).
Yeah, this is what I was getting at; this has to happen in C++, but not in Rust. You are right to point out that this only matters for things that aren't trivially copyable.
It would be cool to have those things, but it also means that there's less "magic" stuff going on, which is nice. And it makes the semantics of stuff like this a lot simpler.
> Does `drop()` get called on objects that have been copied from?
Nope. In fact, Copy types can't have a Drop at all, but types that move don't have their Drop impl called when they move.
struct Foo { s1: String, s2: &str, }
where s2 is always intended to point at s1's backing storage. What's unfortunate here is that Rust will disallow this, as it doesn't understand that s2 is pointing to some data on the heap, with a stable address, not the parts of the String struct in s1 that are part of the struct itself. So what this means is, in plain Rust, this type isn't movable, Rust is concerned about the invalidation.
However, you can get around this restriction with some unsafe code to teach Rust about it; this is the premise of the "owning-ref" crate.
This isn't the case. They only need to borrow it. A borrowed value isn't allowed to move (the borrow itself can be copied around and shared within the scope of the borrow), so that works out.
Most Rust iterators only borrow the container or iterator they operate on. It's only explicitly moving iterators like .into_iter() (which extracts elements by-move) that don't.
Neither does Rust.
> The subscript checking variants of C and C++ have to use "fat pointers" which carry along size information.
So do Rust's Slices.
> The overhead for this is large and nobody uses that.
People use std::vector all the time for this purpose in C++. It has about the performance you'd expect, with very little overhead except where you want it in bounds-checking.
I don't think there's actually a performance difference here. Rust's default is safer because it requires dropping to unsafe code to do something dangerous, but the same optimizations are available in both.