How does memory management work for the Rust language?
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In low-level languages like C, you have to manually manage memory: you request memory when you want it, and you’re responsible for freeing it when done. If you forget to free it, you can leak memory. If you use memory after freeing it, you could have a security problem or a crash. This is error-prone, so many languages have tried to make this easier for programmers.
Some languages use garbage collection. Periodically, the runtime engine scans to see if any memory blocks no longer have variables referencing them. This makes it pretty easy to avoid leaks, but the garbage collector can run at odd times and can make extremely high-performance code difficult. An example of this is Java.
Some languages use reference counting – they count how many variables reference memory blocks and free the memory when the count reaches zero. This is more predictable than a garbage collector but it adds a bit of extra computation every time a variable references memory, so it gets in the way of extremely high-performance code. An example of this is Python.
Rust takes a novel approach. Only one variable is allowed to reference a block of memory at a time. Another variable can “borrow” that reference, but it must return it when done. Verifying that the borrowing is done correctly is done at compile time, not run time. Memory is freed when the owning variable goes out of scope, similar to reference counting.
As a very rough approximation, you could say Rust is a bit more similar to reference counting, but where the reference counting is done at compile time, so it doesn’t slow down the code at all.
Overall this means that Rust code has no overhead for memory management, and programmers don’t have to ever worry about forgetting to free, nor do they have to worry about using memory that’s already been freed. Rust handles both cases automatically with no runtime cost.
The catch is that when multiple variables do need to reference the same memory block, Rust requires syntax to keep track of that borrowing, and it can be tricky to get right.
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