You don't have to do it, but if you want to do it, being able to do so in a way that's verified and enforced by the toolchain beats doing so in a documentation comment.

Now every time I read a doc string saying that I need to "deepcopy" something in Python for some API usage pattern to work properly I cringe.

With a checker you're forced to think: do I really want to pass a copy/clone of this? Or do I want to let that function borrow it? Or borrow it mutably?

Assume you had some code that takes a file name and calls open on it. One day you decide you want to print that filename before you open it. Naive code will cause the name to “move” to print and unusable to the open in next line. Even though it is perfectly understood by all parties that there is no threading involved and print would finish before the next use of that string. Yes, I can create a borrow or clone, but having to think of it every single line of code even when there is only one thread of execution is really painful

Edit: I get print is a macro, but imagine a detailed logger for this case.

Here, if I have a `&T` and I try to call a function that has a `&mut T`, the compiler will tell me that's not gonna work - and then I can pick whether I want my function to take a `&mut T`, or if I want to make a clone and modify that, etc.

There's a learning curve, it's a set of habits to adopt, but once you embrace it it's really hard to go back to languages that don't have it! (See the rest of the comments for testimonials)

[0] https://crates.io/crates/rug

    let a = Integer::from(10);
    let b = a + a;

  let b = &a + &a;

GC protects you against double-free and use-after-free, but memory leaks? Nope.

  Foo* foo = new Foo();
  ...code that uses foo...

In a practical sense... is it your personal experience that memory leaks are equally prevalent in GC and non-GC languages? I've spent decades working in each (primarily C++ and Java, but also Pascal, C, C#, Smalltalk...) and my experience is that memory leaks were a _much_ bigger issue, in practice, in the non-GC languages.

It is my experience that when people work with GC languages, they treat the GC as a blackbox (which it is) and simply won't bother investigating: do they have memory leaks? Of course not, they are using a GC after all, all memory-related problems solved, right? Right... With code that relies on free(), I can use a debugger and check which free() calls are hit for which pointers. Even better, I may use an arena allocator where appropriate and don't bother with free() at all. With a GC I'm just looking at some vague heap graph. Am I leaking memory? Who knows... "Do those numbers look right to you?"

Memory management issues are usually symptoms of architectural issues. A GC won't fix your architecture, but it will make your memory management issues less visible.

It is my experience that most memory problems in C come from out-of-bounds writes (which includes a lot more than just array access), not from anything related to free(). A GC doesn't help here.

> In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that memory which is no longer needed is not released.

In most of your scenarios, e.g. "pushed to some queue", the object in question is still needed. Hence, this is not a leak. Presumably, the entry will eventually be removed from the queue, and GC will then reclaim the object.

At any rate, I think we're moving past the point of productive discussion. My experience in practice is that memory leaks are more common / harder to avoid in non-GC languages. Of course a true memory leak (per the definition above) is possible in a GC language, but I just don't see it much in practice. Perhaps your experience is different.

If I have a queue with elements and I won’t be accessing some of it in the next part of the program, is it a leak? Also, remember that certain GCd languages intern strings, is it a memory leak since it will not necessarily use it anymore?

To provide an example for a different case: programs written in Gallina have the weak normalization property, implying that they always terminate.

With a "proper" GC engine, anything that is no longer able to be referenced can be safely collected. Barring bugs in the GC, none of those can leak. But, you can unintentionally keep references to things for a lot longer (possibly unlimited longer) than you need to. Which looks like a memory leak, but is actually unintentional liveness.

And to prove the difference between "cannot be referenced" (a property that can in principle be checked by consulting a snapshot of RAM at an instant in time) and "will not be referenced" (a larger set, we will never reference things that cannot be referenced, but we may not reference things that are reachable, depending on code) feels like it is requiring solving the halting problem.

And for free()-related problems, I've definitely seen code crash with use-after-free (and double-free).

Both situations prevent you from reusing memory previously used by other objects, which isn't being utilized for anything useful at that point. The distinction is valid formally, but from a practical point of view sounds rather academic.

> And to prove the difference between "cannot be referenced" (a property that can in principle be checked by consulting a snapshot of RAM at an instant in time) and "will not be referenced" (a larger set, we will never reference things that cannot be referenced, but we may not reference things that are reachable, depending on code) feels like it is requiring solving the halting problem.

As usual, looking for a general solution to such a problem is probably a fool's errand. It's much easier to write code simple enough that it's obvious where things are referenced. Rust's lifetime semantics help with that (if your code isn't all that simple, it will be apparent in the overload of punctuation). If not Rust, then at least it would be good if you could check liveness of an object in a debugger. In C you can check whether a particular allocation was undone by a free() or equivalent. I'm not aware of any debugger for e.g. Java which would let me point at a variable and ask it to notify me when it's garbage collected, but it sounds like something that shouldn't be too hard to do, if the debugger is integrated with the compiler.

Often you don't need to care about the extra allocations and can just deserialize to owned types.

The code for owned deserialization certainly ends up looking more elegant.

I want to say that seems like a valid concern but in practice I've seen it come up only rarely.

Eg cloning a string leads to an extra allocation and a memcopy.

If you want to get a similar performance profile to GC languages, you have to stick your types behind a `Rc<T>>/Arc<T>` or `Rc<RefCell<T>> / Arc<Mutex<T>>` if you need mutability.

But modern allocators hold up pretty well to a GC, which amortizes the allocations. The extra memcopying can be less detrimental than one might think.

  const obj1 = { a: 32, b: 42 };
  function foo(ref) { ref.a = 0; }

  foo(obj1);
  console.log(obj1);

Basically all I was saying is one can not approach writing Rust with a Java/JavaScript mindset. (That a variable is just a bucket holding a value). Care needs to be taken when referencing a variable as it may need to be moved, copied/cloned or referenced. In the case of copying, another memory allocation is done. So if someone approaches Rust from the standpoint of "this word represents a value, and I'm going to use it all over the place", they can find themselves blindly allocating memory.

However when you take a high level feature like overriding operators which can be done elegantly in Python, for a complied language, Rust's way is quite concise, readable and to my eyes quite pretty.

Edit: Typos