rust/lifetimes.md

% Lifetimes

Rust enforces these rules through lifetimes. Lifetimes are effectively just names for scopes somewhere in the program. Each reference, and anything that contains a reference, is tagged with a lifetime specifying the scope it's valid for.

Within a function body, Rust generally doesn't let you explicitly name the lifetimes involved. This is because it's generally not really necessary to talk about lifetimes in a local context; rust has all the information and can work out everything. It's also a good thing because the scope of a borrow is often significantly smaller than the scope its referent is actually valid for. Rust will introduce many anonymous scopes and temporaries to make your code just work.

However once you cross the function boundary, you need to start talking about lifetimes. Lifetimes are denoted with an apostrophe: 'a, 'static. To dip our toes with lifetimes, we're going to pretend that we're actually allowed to label scopes with lifetimes, and desugar the examples from the start of this chapter.

Our examples made use of aggressive sugar -- high fructose corn syrup even -- around scopes and lifetimes, because writing everything out explicitly is extremely noisy. All Rust code relies on aggressive inference and elision of "obvious" things.

One particularly interesting piece of sugar is that each let statement implicitly introduces a scope. For the most part, this doesn't really matter. However it does matter for variables that refer to each other. As a simple example, let's completely desugar this simple piece of Rust code:

let x = 0;
let y = &x;
let z = &y;

The borrow checker always tries to minimize the extent of a lifetime, so it will likely desugar to the following:

// NOTE: `'a: {` and `&'b x` is not valid syntax!
'a: {
    let x: i32 = 0;
    'b: {
        // lifetime used is 'b because that's *good enough*.
        let y: &'b i32 = &'b x;
        'c: {
            // ditto on 'c
            let z: &'c &'b i32 = &'c y;
        }
    }
}

Wow. That's... awful. Let's all take a moment to thank Rust for being a diabetes-inducing torrent of syrupy-goodness.

Actually passing references to outer scopes will cause Rust to infer a larger lifetime:

let x = 0;
let z;
let y = &x;
z = y;

The borrow checker always tries to minimize the extent of a lifetime, so it will likely desugar to something like the following:

// NOTE: `'a: {` and `&'b x` is not valid syntax!
'a: {
    let x: i32 = 0;
    'b: {
        let z: &'b i32;
        'c: {
            // Must use 'b here because this reference is
            // being passed to that scope.
            let y: &'b i32 = &'b x;
            z = y;
        }
    }
}

Example: references that outlive referents

Alright, let's look at some of those examples from before:

fn as_str(data: &u32) -> &str {
    let s = format!("{}", data);
    &s
}

desugars to:

fn as_str<'a>(data: &'a u32) -> &'a str {
    'b: {
        let s = format!("{}", data);
        return &'a s;
    }
}

This signature of as_str takes a reference to a u32 with some lifetime, and promises that it can produce a reference to a str that can live just as long. Already we can see why this signature might be trouble. That basically implies that we're going to find a str somewhere in the scope the scope the reference to the u32 originated in, or somewhere even earlier. That's a bit of a big ask.

We then proceed to compute the string s, and return a reference to it. Since the contract of our function says the reference must outlive 'a, that's the lifetime we infer for the reference. Unfortunately, s was defined in the scope 'b, so the only way this is sound is if 'b contains 'a -- which is clearly false since 'a must contain the function call itself. We have therefore created a reference whose lifetime outlives its referent, which is literally the first thing we said that references can't do. The compiler rightfully blows up in our face.

To make this more clear, we can expand the example:

fn as_str<'a>(data: &'a u32) -> &'a str {
    'b: {
        let s = format!("{}", data);
        return &'a s
    }
}

fn main() {
    'c: {
        let x: u32 = 0;
        'd: {
            // An anonymous scope is introduced because the borrow does not
            // need to last for the whole scope x is valid for. The return
            // of as_str must find a str somewhere *before* this function
            // call. Obviously not happening.
            println!("{}", as_str::<'d>(&'d temp));
        }
    }
}

Shoot!

Of course, the right way to write this function is as follows:

fn to_string(data: &u32) -> String {
    format!("{}", data)
}

We must produce an owned value inside the function to return it! The only way we could have returned an &'a str would have been if it was in a field of the &'a u32, which is obviously not the case.

(Actually we could have also just returned a string literal, which as a global can be considered to reside at the bottom of the stack; though this limits our implementation just a bit.)

Example 2: aliasing a mutable reference

How about the other example:

let mut data = vec![1, 2, 3];
let x = &data[0];
data.push(4);
println!("{}", x);

'a: {
    let mut data: Vec<i32> = vec![1, 2, 3];
    'b: {
        // 'b is as big as we need this borrow to be
        // (just need to get to `println!`)
        let x: &'b i32 = Index::index::<'b>(&'b data, 0);
        'c: {
            // Temporary scope because we don't need the
            // &mut to last any longer.

            // NOTE: Vec::push is not valid syntax
            Vec::push(&'c mut data, 4);
        }
        println!("{}", x);
    }
}

The problem here is is bit more subtle and interesting. We want Rust to reject this program for the following reason: We have a live shared reference x to a descendent of data when try to take a mutable reference to data when we call push. This would create an aliased mutable reference, which would violate the second rule of references.

However this is not at all how Rust reasons that this program is bad. Rust doesn't understand that x is a reference to a subpath of data. It doesn't understand Vec at all. What it does see is that x has to live for 'b to be printed. The signature of Index::index subsequently demands that the reference we take to data has to survive for 'b. When we try to call push, it then sees us try to make an &'c mut data. Rust knows that 'c is contained within 'b, and rejects our program because the &'b data must still be live!

Here we see that the lifetime system is much more coarse than the reference semantics we're actually interested in preserving. For the most part, that's totally ok, because it keeps us from spending all day explaining our program to the compiler. However it does mean that several programs that are totally correct with respect to Rust's true semantics are rejected because lifetimes are too dumb.