CODEWITHSUNDEEP
rustgarbage-collection
Uclydde
Uclydde

Reputation: 1414

How does Rust's memory management differ from compile-time garbage collection?

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.

Upvotes: 2

Views: 1956

Answers (1)

David Lichmann
David Lichmann

Reputation: 1506

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.

Upvotes: 5

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