Rust has strict rules for memory management, which includes the concept of memory lifetimes. A memory lifetime is the duration for which a particular piece of memory is valid and can be accessed. Lifetimes can be defined in function signatures, struct definitions, and other code constructs. The borrow checker enforces rules around memory lifetimes, to ensure that memory is accessed safely and without any undefined behavior.
Memory lifetimes are determined by the ownership and borrowing system. Every value has an owner, which is responsible for allocating and freeing the memory associated with the value. When a value is borrowed, the borrower is given a reference to the memory owned by the owner. The borrower must return the reference before the owner goes out of scope, or else the program will not compile.
For example, consider the following code:
fn main() {
let x = 5;
let y = &x;
println!("{}", y);
}Here, x is an integer with a value of 5. The & operator creates a reference to x and assign it to y. The println!() macro prints the value of y. The lifetime of x begins when it is created and ends when it goes out of scope at the end of the main() function. The lifetime of y is the same as the lifetime of x, because it is a reference to the memory owned by x. The borrow checker ensures that y is returned before x goes out of scope.
Memory lifetimes are strict, and can be complex to learn, because they help ensure that programs are safe and free from undefined behavior, and enable high-performance memory-safe code without the need for garbage collection or other runtime memory management.