2

I have read that Rust's compiler "inserts" memory management code during compile time, and this sounds kind of like "compile-time garbage collection".

What is the difference between these two ideas?

I've seen What does Rust have instead of a garbage collector? but that is about runtime garbage collection, not compile-time.

trent
  • 25,033
  • 7
  • 51
  • 90
Uclydde
  • 1,414
  • 3
  • 16
  • 33
  • 2
    Your question is likely to be closed as opinion-based, but look up [RAII](https://en.wikipedia.org/wiki/Resource_acquisition_is_initialization), it's a concept shared between C++ and Rust that's probably the closest to what a "compile-time garbage collection" could be referring to. – user4815162342 Mar 14 '21 at 20:23
  • 4
    Depends on what you mean behind that. "Garbage collection" means to remove objects from memory that don't have living references in a program. In Rust's case objects should be removed only when the owning variable goes out of scope. So Rust doesn't need garbage collection in either compile time or runtime. It deterministically knows where to delete an object, which is hardly can be called "collection", just plain removing from heap/stack – Alexey S. Larionov Mar 14 '21 at 20:26
  • 1
    I think the answers to the linked question are high quality and address your question -- if you have some lingering doubt or if you think I've closed this question in error, please [edit] the question to clarify how. This is definitely an interesting aspect of Rust and it's not quite similar to how any other language works (C++ comes close with RAII, but the defaults are different.) – trent Mar 14 '21 at 20:37
  • 1
    Also see [Why doesn't C++ have a garbage collector?](https://stackoverflow.com/questions/147130/why-doesnt-c-have-a-garbage-collector?rq=1) Although a bit dated, and obviously all the names are different between languages, a lot of the core arguments still apply. It's normal in C++ not to use a garbage collector, not to use `free` or `delete` manually, and yet not to have any leaks; the same is true in Rust. – trent Mar 14 '21 at 20:39
  • 3
    The duplicate answers do a good job of explaining what a "garbage collector" does and what Rust does instead. Whether the term "compile-time garbage collection" is an adequate description for what Rust does is probably off-topic. – kmdreko Mar 14 '21 at 20:39
  • @trentcl I have rephrased the question to be more about the differences between how rust handles memory, and how compile-time garbage collection works. The previous questions primarily talk about the more common runtime garbage collection. – Uclydde Mar 14 '21 at 20:43
  • "compile-time garbage collection" you serious ? this is not at all an adequate description of RAII, garbage is when you throw thing to trash and sometime you empty it that not at all what rust do. Next step is ["Malloc/free are a form of garbage collection"](https://www.reddit.com/r/programmingcirclejerk/comments/gibz0s/mallocfree_are_a_form_of_garbage_collection/) – Stargateur Mar 14 '21 at 20:47
  • 6
    The answer could be yes or no depending on what "compile-time garbage collection" *means*. Until then, this question is still unclear. – kmdreko Mar 14 '21 at 20:50
  • 1
    I don't know what exactly is compile time GC but Rust has kind of runtime GC enforced by the means of `Rc>` kind of type definitions. So something under the hood is checking the refrence count and freeing the allocated memory once the count is 0. So there is that if i am not mistaken. – Redu May 31 '23 at 19:21

1 Answers1

5

Compile-time garbage collection is commonly defined as follows:

A complementary form of automatic memory management is compile-time memory management (CTGC), where the decisions for memory management are taken at compile-time instead of at run-time. The compiler determines the life-time of the variables that are created during the execution of the program, and thus also the memory that will be associated with these variables. Whenever the compiler can guarantee that a variable, or more precisely, parts of the memory resources that this variable points to at run-time, will never ever be accessed beyond a certain program instruction, then the compiler can add instructions to deallocate these resources at that particular instruction without compromising the correctness of the resulting code.

(From Compile-Time Garbage Collection for the Declarative Language Mercury by Nancy Mazur)

Rust handles memory by using a concept of ownership and borrow checking. Ownership and move semantics describe which variable owns a value. Borrowing describes which references are allowed to access a value. These two concepts allow the compiler to "drop" the value when it is no longer accessible, causing the program to call the dtop method from the Drop trait).

However, the compiler itself doesn't handle dynamically allocated memory at all. It only handles drop checking (figuring out when to call drop) and inserting the .drop() calls. The drop implementation is responsible for determining what happens at this point, whether that is deallocating some dynamic memory (which is what Box's drop does, for example), or doing anything else. The compiler therefore never really enforces garbage collection, and it doesn't enforce deallocating unused memory. So we can't claim that Rust implements compile-time garbage collection, even if what Rust has is very reminiscent of it.

Enn Michael
  • 1,328
  • 14
  • 30