[wrs]

An important “unblocker” for me when learning Rust after decades of other languages was internalizing that assignment is destructive move by default. Like many Rust intros, this sort of glides past that in the “ownership” section, but I felt like it should be a big red headline.

If you’re coming from C++ especially, where the move/copy situation is ridiculously confusing (IMO), but also from a simpler “reference by default” language like Java, this has profound impact on what’s intuitive in the language.

For the C++ comparison, this is a pretty good article: https://radekvit.medium.com/move-semantics-in-c-and-rust-the...

[scotty79]

Also very important realization is that things that are moved around (assigned to variable, moved into or returned from a function, kept as a part of the tuple or a field of a struct) must have fixed and known size. And the variable itself is not a handle. It's a fixed sized area in the memory that you named and moved something into it.

This makes completely logical why some things must be Box<>'ed and borrowed. Why you cannot treat Trait or even impl Trait like any other type. Why sometimes it's ok to have impl Trait as your return type while in other cases it's impossible and you must Box<> it.

Third important realization is that things borrowed out of containers might be moved 'behind the scenes' by the container while you hold the borrow, so you are not allowed to mutate container while you are holding borrows to any of its contents. So it's ok, to instead hold indexes or copies or clones of keys if you need.

Another observation is that any struct that contains a borrow is a borrow itself. Despite using completely different syntax for how it's declared, created, it behaves exactly like a borrow and is just as restrictive.

Last thing are lifetimes, which don't have consistent (let alone intuitive) syntax of what should outlive what so are kinda hard to wrap your head around so you should probably start with deep understanding what should outlive what in your code and then look up how to express it in Rust syntax.

Rust is syntactically very similar to other languages, but semantically is fundamentally different beast. And while familar syntax is very good for adoption I'd also prefer tutorials that while showing the syntax explain why it means something completely different than what you think.

[alltheworlds]

As someone who hasn't looked at Rust, reading all this makes me never want to. Am I missing something? Is it actually easier to reason about the language than it feels like from reading this text?

[zozbot234]

This is just stuff that's flagged automatically by the compiler. You don't need to "reason" about it, the compiler will tell you how to fix it if it does come up.

[coderenegade]

I found the copy/move situation in Rust to be far less intuitive than in C++. In C++, move semantics are obvious because they rely on std::move and the && operator, whereas in Rust, similar behavior seemed to depend on the object type. Even more confusingly, Rust has its own move operator as well, despite destructive move being the default behavior for assignment.

I found it frustrating enough that I put the language down and just went back to using C++.

[scotty79]

> In C++, move semantics are obvious because ...

In Rust it's also obvious because every = is a move. Confusion comes from tutorials pretending for too long that it's not.

> whereas in Rust, similar behavior seemed to depend on the object type.

It's best to think of that as an exception to the rule created specifically for numbers and other similar small, cheaply copied things.

If you need to move `a` but `a` has a trait Copy then you copy instead.

> Rust has its own move operator as well, despite destructive move being the default behavior for assignment.

I don't think that's true? Rust has a `move` keyword but it's a part of closure definition that makes it take all the variables from its environment by move, even if it doesn't need it. Unless you are talking about something else...

[legerdemain]

  > In Rust it's also obvious because every = is a move.

False, depending on the presence of an explicit type annotation on the left-hand side, an = triggers either a move or a reborrow.

[scotty79]

How does it look like? Do you mean this? https://haibane-tenshi.github.io/rust-reborrowing/

If you assume = always moves you can just never use automatic reborrowing (do &mut * instead) and you'll loose nothing.

I don't think that's a very common pattern, fully optional and it can be just treated as another exception to the rule, just like Copy types. You don't have to manually write .copy() in some cases, and you don't have to manually write &mut* in some others. But that's what happens. Move is still done.

So that's total of two exceptions. Way easier to be tackled individually when the time comes than assuming = means something else, or something complex from the start.

[puffoflogic]

There's a way of thinking of `Copy` which makes it not an exception: `Copy` variables/places are simply not rendered invalid/uninitialized when they are the source of a move operation, unlike non-`Copy` sources. They're still moved from bitwise like all other rust values!

[scotty79]

I like it!

[legerdemain]

I've posted similar minimal examples before. The first one triggers a move (and then fails to compile), while the second one triggers a reborrow. Sure, you can explicitly write out every instance of reborrowing with `&mut *`. That would require you to understand every instance that triggers it, and would also be unbelievably noisy, since automatic dereferencing and automatic reborrowing are actually incredibly common.

  > pub fn main() {
  >     let mut x = String::from("moo");
  >     let y = &mut x;
  >     let z = y;
  >     println!("{:?}", y)
  > }
  > 
  > pub fn main() {
  >     let mut x = String::from("moo");
  >     let y = &mut x;
  >     let z: &mut _ = y;
  >     println!("{:?}", y)
  > }

[scotty79]

Ok. So that's just triggering auto-insertion of &mut *

It's completely optional fearure. You don't have to rely on it. Like auto-insertion of semicolons in JS maybe it's better if you don't. Although likelihood of this biting you is way lower than in case of relying on semicolons autoinsertion.

[saghm]

I'd be pretty surprised if the C++ move semantics were considered more intuitive by someone not familiar with either and then learning both for the same time. Rust's semantics do depend on the type in that anything that implements Copy will be implicitly copied rather than moved, but I'm sure I understand why that's unintuitive. I'm also not sure what you mean by Rust having it's own move operator; the only thing I can think of is the `move` keyword used for indicating that closures should capture by move rather than by reference, but it's not used outside of closures as far as I'm aware, so I suspect that the confusion here is more due to expecting things to behave like C++ rather than the Rust semantics being "unintuitive" in a vacuum.

At a high level, I think the most unintuitive part of moving in C++ compared to Rust is that it can silently degrade into a copy without anything indicating it. In Rust, trying to use a value after it's been moved will give you a compiler error, at which point you can reconsider whether you do in fact want to explicitly copy or if you made a mistake or need to refactor. In C++, the only way I'm aware of to verify whether a value is actually moved or not is to use a debugger. The benefit for requiring explicit copies is similar to having bindings be immutable by default and requiring an explicit `mut` annotation; if you start out enforcing the constraint that things should be moved or immutable, fixing it later if you find out it won't work that way only requires adding `.clone()` or `mut` in one place. On the other hand, if you start out with implicit copies or mutability by default and then want to change it later, it can be a lot more work to refactor the places where the variable is used to not violate this, and it may not even be possible in some cases.

[bear24rw]

> In C++, the only way I'm aware of to verify whether a value is actually moved or not is to use a debugger.

Not as simple as if there was a proper borrow checker but its sort of possible:

https://awesomekling.github.io/Catching-use-after-move-bugs-...