The concept of rust lifetime

Question

Question

I usually use c++ lang, and recently I'm learning rust lang but now confusing the concept of lifetime.
My understanding for lifetime is as follows. Is this correct?

• Lifetime is an attribute of instance.
• Lifetime represents the valid scope of instance.

Background of above question

The following code is a sample code at here.

    {
        let r;                // ---------+-- 'a
                              //          |
        {                     //          |
            let x = 5;        // -+-- 'b  |
            r = &x;           //  |       |
        }                     // -+       |
                              //          |
        println!("r: {}", r); //          |
    }                         // ---------+

The document said that 'a is a lifetime, and 'b is also a lifetime.
but if my understanding is correct, 'a is not lifetime, just scope of symbol r... Is `a really lifetime?

P.S.

There are two things named "a lifetime": value's lifetime, and the lifetime attached to a reference.

Thank you! Maybe I understood a little bit more than before...

Answer 1

A lifetime is not so much an attribute of an instance, it's more the opposite way around; when an instance is borrowed (a reference is taken), it is borrowed for a specific lifetime. The borrow checker will try to minimise the lifetime of the borrow so that it can be as permissive as possible whilst ensuring code is still safe.

Lifetimes are your way of communicating how long you need a reference to last to the compiler, and given that information, the compiler will check that nothing violates that rule (by holding the reference for longer), and that the reference is available for at least as long as you require it. This can be thought of in much the same way that a compiler can check type to ensure you don't assign a float to an integer.

Lifetimes are also independent of scope. Rust now has non-lexical lifetimes (https://github.com/rust-lang/rfcs/pull/2094 — see What are non-lexical lifetimes? for more detailed explanation), meaning that within a given scope, the borrow checker is capable of determining that borrows have shorter lifetimes than the containing scope.