Advanced Lifetimes - The Rust Programming Language

## Advanced Lifetimes

Back in Chapter 10, we learned how to annotate references with lifetime parameters to help Rust understand how the lifetimes of different references relate. We saw how most of the time, Rust will let you elide lifetimes, but every reference has a lifetime. There are three advanced features of lifetimes that we haven't covered though: lifetime subtyping, lifetime bounds, and trait object lifetimes.

### Lifetime Subtyping

Imagine that we want to write a parser. To do this, we'll have a structure that holds a reference to the string that we're parsing, and we'll call that struct `Context`. We'll write a parser that will parse this string and return success or failure. The parser will need to borrow the context to do the parsing. Implementing this would look like the code in Listing 19-12, which won't compile because we've left off the lifetime annotations for now:

    struct Context(&str);

    struct Parser {
        context: &Context,
    }

    impl Parser {
        fn parse(&self) -> Result<(), &str> {
            Err(&self.context.0[1..])
        }
    }

Listing 19-12: Defining a `Context` struct that holds a string slice, a `Parser` struct that holds a reference to a `Context` instance, and a `parse` method that always returns an error referencing the string slice

For simplicity's sake, our `parse` function returns a `Result<(), &str>`. That is, we don't do anything on success, and on failure we return the part of the string slice that didn't parse correctly. A real implementation would have more error information than that, and would actually return something created when parsing succeeds, but we're leaving those parts of the implementation off since they aren't relevant to the lifetimes part of this example. We're also defining `parse` to always produce an error after the first byte. Note that this may panic if the first byte is not on a valid character boundary; again, we're simplifying the example in order to concentrate on the lifetimes involved.

So how do we fill in the lifetime parameters for the string slice in `Context` and the reference to the `Context` in `Parser`? The most straightforward thing to do is to use the same lifetime everywhere, as shown in Listing 19-13:

    # #![allow(unused_variables)]
    #fn main() {
    struct Context<'a>(&'a str);

    struct Parser<'a> {
        context: &'a Context<'a>,
    }

    impl<'a> Parser<'a> {
        fn parse(&self) -> Result<(), &str> {
            Err(&self.context.0[1..])
        }
    }

    #}

Listing 19-13: Annotating all references in `Context` and `Parser` with the same lifetime parameter

This compiles fine. Next, in Listing 19-14, let's write a function that takes an instance of `Context`, uses a `Parser` to parse that context, and returns what `parse` returns. This won't quite work:

    fn parse_context(context: Context) -> Result<(), &str> {
        Parser { context: &context }.parse()
    }

Listing 19-14: An attempt to add a `parse_context` function that takes a `Context` and uses a `Parser`

We get two quite verbose errors when we try to compile the code with the addition of the `parse_context` function:

    error: borrowed value does not live long enough
      --> <anon>:16:5
       |
    16 |     Parser { context: &context }.parse()
       |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ does not live long enough
    17 | }
       | - temporary value only lives until here
       |
    note: borrowed value must be valid for the anonymous lifetime #1 defined on the
    body at 15:55...
      --> <anon>:15:56
       |
    15 |   fn parse_context(context: Context) -> Result<(), &str> {
       |  ________________________________________________________^
    16 | |     Parser { context: &context }.parse()
    17 | | }
       | |_^

    error: `context` does not live long enough
      --> <anon>:16:24
       |
    16 |     Parser { context: &context }.parse()
       |                        ^^^^^^^ does not live long enough
    17 | }
       | - borrowed value only lives until here
       |
    note: borrowed value must be valid for the anonymous lifetime #1 defined on the
    body at 15:55...
      --> <anon>:15:56
       |
    15 |   fn parse_context(context: Context) -> Result<(), &str> {
       |  ________________________________________________________^
    16 | |     Parser { context: &context }.parse()
    17 | | }
       | |_^

These errors are saying that both the `Parser` instance we're creating and the `context` parameter live from the line that the `Parser` is created until the end of the `parse_context` function, but they both need to live for the entire lifetime of the function.

In other words, `Parser` and `context` need to outlive the entire function and be valid before the function starts as well as after it ends in order for all the references in this code to always be valid. Both the `Parser` we're creating and the `context` parameter go out of scope at the end of the function, though (since `parse_context` takes ownership of `context`).

Let's look at the definitions in Listing 19-13 again, especially the signature of the `parse` method:

        fn parse(&self) -> Result<(), &str> {

Remember the elision rules? If we annotate the lifetimes of the references, the signature would be:

        fn parse<'a>(&'a self) -> Result<(), &'a str> {

That is, the error part of the return value of `parse` has a lifetime that is tied to the `Parser` instance's lifetime (that of `&self` in the `parse` method signature). That makes sense, as the returned string slice references the string slice in the `Context` instance that the `Parser` holds, and we've specified in the definition of the `Parser` struct that the lifetime of the reference to `Context` that `Parser` holds and the lifetime of the string slice that `Context` holds should be the same.

The problem is that the `parse_context` function returns the value returned from `parse`, so the lifetime of the return value of `parse_context` is tied to the lifetime of the `Parser` as well. But the `Parser` instance created in the `parse_context` function won't live past the end of the function (it's temporary), and the `context` will go out of scope at the end of the function (`parse_context` takes ownership of it).

We're not allowed to return a reference to a value that goes out of scope at the end of the function. Rust thinks that's what we're trying to do because we annotated all the lifetimes with the same lifetime parameter. That told Rust the lifetime of the string slice that `Context` holds is the same as that of the lifetime of the reference to `Context` that `Parser` holds.

The `parse_context` function can't see that within the `parse` function, the string slice returned will outlive both `Context` and `Parser`, and that the reference `parse_context` returns refers to the string slice, not to `Context` or `Parser`.

By knowing what the implementation of `parse` does, we know that the only reason that the return value of `parse` is tied to the `Parser` is because it's referencing the `Parser`'s `Context`, which is referencing the string slice, so it's really the lifetime of the string slice that `parse_context` needs to care about. We need a way to tell Rust that the string slice in `Context` and the reference to the `Context` in `Parser` have different lifetimes and that the return value of `parse_context` is tied to the lifetime of the string slice in `Context`.

We could try only giving `Parser` and `Context` different lifetime parameters as shown in Listing 19-15. We've chosen the lifetime parameter names `'s` and `'c` here to be clearer about which lifetime goes with the string slice in `Context` and which goes with the reference to `Context` in `Parser`. Note that this won't completely fix the problem, but it's a start and we'll look at why this isn't sufficient when we try to compile.

    struct Context<'s>(&'s str);

    struct Parser<'c, 's> {
        context: &'c Context<'s>,
    }

    impl<'c, 's> Parser<'c, 's> {
        fn parse(&self) -> Result<(), &'s str> {
            Err(&self.context.0[1..])
        }
    }

    fn parse_context(context: Context) -> Result<(), &str> {
        Parser { context: &context }.parse()
    }

Listing 19-15: Specifying different lifetime parameters for the references to the string slice and to `Context`

We've annotated the lifetimes of the references in all the same places that we annotated them in Listing 19-13, but used different parameters depending on whether the reference goes with the string slice or with `Context`. We've also added an annotation to the string slice part of the return value of `parse` to indicate that it goes with the lifetime of the string slice in `Context`.

Here's the error we get now:

    error[E0491]: in type `&'c Context<'s>`, reference has a longer lifetime than the data it references
     --> src/main.rs:4:5
      |
    4 |     context: &'c Context<'s>,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^
      |
    note: the pointer is valid for the lifetime 'c as defined on the struct at 3:0
     --> src/main.rs:3:1
      |
    3 | / struct Parser<'c, 's> {
    4 | |     context: &'c Context<'s>,
    5 | | }
      | |_^
    note: but the referenced data is only valid for the lifetime 's as defined on the struct at 3:0
     --> src/main.rs:3:1
      |
    3 | / struct Parser<'c, 's> {
    4 | |     context: &'c Context<'s>,
    5 | | }
      | |_^

Rust doesn't know of any relationship between `'c` and `'s`. In order to be valid, the referenced data in `Context` with lifetime `'s` needs to be constrained to guarantee that it lives longer than the reference to `Context` that has lifetime `'c`. If `'s` is not longer than `'c`, then the reference to `Context` might not be valid.

Which gets us to the point of this section: Rust has a feature called lifetime subtyping, which is a way to specify that one lifetime parameter lives at least as long as another one. In the angle brackets where we declare lifetime parameters, we can declare a lifetime `'a` as usual, and declare a lifetime `'b` that lives at least as long as `'a` by declaring `'b` with the syntax `'b: 'a`.

In our definition of `Parser`, in order to say that `'s` (the lifetime of the string slice) is guaranteed to live at least as long as `'c` (the lifetime of the reference to `Context`), we change the lifetime declarations to look like this:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Context<'a>(&'a str);
    #
    struct Parser<'c, 's: 'c> {
        context: &'c Context<'s>,
    }

    #}

Now, the reference to `Context` in the `Parser` and the reference to the string slice in the `Context` have different lifetimes, and we've ensured that the lifetime of the string slice is longer than the reference to the `Context`.

That was a very long-winded example, but as we mentioned at the start of this chapter, these features are pretty niche. You won't often need this syntax, but it can come up in situations like this one, where you need to refer to something you have a reference to.

### Lifetime Bounds

In Chapter 10, we discussed how to use trait bounds on generic types. We can also add lifetime parameters as constraints on generic types, which are called lifetime bounds. For example, consider a type that is a wrapper over references. Recall the `RefCell<T>` type from Chapter 15: its `borrow` and `borrow_mut` methods return the types `Ref` and `RefMut`, respectively. These types are wrappers over references that keep track of the borrowing rules at runtime. The definition of the `Ref` struct is shown in Listing 19-16, without lifetime bounds for now:

    struct Ref<'a, T>(&'a T);

Listing 19-16: Defining a struct to wrap a reference to a generic type; without lifetime bounds to start

Without constraining the lifetime `'a` in relation to the generic parameter `T`, we get an error because Rust doesn't know how long the generic type `T` will live:

    error[E0309]: the parameter type `T` may not live long enough
     --> <anon>:1:19
      |
    1 | struct Ref<'a, T>(&'a T);
      |                   ^^^^^^
      |
      = help: consider adding an explicit lifetime bound `T: 'a`...
    note: ...so that the reference type `&'a T` does not outlive the data it points at
     --> <anon>:1:19
      |
    1 | struct Ref<'a, T>(&'a T);
      |                   ^^^^^^

Since `T` can be any type, `T` could itself be a reference or a type that holds one or more references, each of which could have their own lifetimes. Rust can't be sure `T` will live as long as `'a`.

Fortunately, Rust gave us helpful advice on how to specify the lifetime bound in this case:

    consider adding an explicit lifetime bound `T: 'a` so that the reference type
    `&'a T` does not outlive the data it points at.

Listing 19-17 shows how to apply this advice by specifying the lifetime bound when we declare the generic type `T`. This code now compiles because the `T: 'a` syntax specifies that `T` can be any type, but if it contains any references, the references must live at least as long as `'a`:

    # #![allow(unused_variables)]
    #fn main() {
    struct Ref<'a, T: 'a>(&'a T);

    #}

Listing 19-17: Adding lifetime bounds on `T` to specify that any references in `T` live at least as long as `'a`

We could choose to solve this in a different way, shown in the definition of a `StaticRef` struct in Listing 19-18, by adding the `'static` lifetime bound on `T`. This means if `T` contains any references, they must have the `'static` lifetime:

    # #![allow(unused_variables)]
    #fn main() {
    struct StaticRef<T: 'static>(&'static T);

    #}

Listing 19-18: Adding a `'static` lifetime bound to `T` to constrain `T` to types that have only `'static` references or no references

Types without any references count as `T: 'static`. Because `'static` means the reference must live as long as the entire program, a type that contains no references meets the criteria of all references living as long as the entire program (since there are no references). Think of it this way: if the borrow checker is concerned about references living long enough, then there's no real distinction between a type that has no references and a type that has references that live forever; both of them are the same for the purpose of determining whether or not a reference has a shorter lifetime than what it refers to.

### Trait Object Lifetimes

In Chapter 17, we learned about trait objects that consist of putting a trait behind a reference in order to use dynamic dispatch. However, we didn't discuss what happens if the type implementing the trait used in the trait object has a lifetime. Consider Listing 19-19, where we have a trait `Foo` and a struct `Bar` that holds a reference (and thus has a lifetime parameter) that implements trait `Foo`, and we want to use an instance of `Bar` as the trait object `Box<Foo>`:

    # #![allow(unused_variables)]
    #fn main() {
    trait Foo { }

    struct Bar<'a> {
        x: &'a i32,
    }

    impl<'a> Foo for Bar<'a> { }

    let num = 5;

    let obj = Box::new(Bar { x: &num }) as Box<Foo>;

    #}

Listing 19-19: Using a type that has a lifetime parameter with a trait object

This code compiles without any errors, even though we haven't said anything about the lifetimes involved in `obj`. This works because there are rules having to do with lifetimes and trait objects:

- The default lifetime of a trait object is `'static`.
- If we have `&'a X` or `&'a mut X`, then the default is `'a`.
- If we have a single `T: 'a` clause, then the default is `'a`.
- If we have multiple `T: 'a`-like clauses, then there is no default; we must be explicit.

When we must be explicit, we can add a lifetime bound on a trait object like `Box<Foo>` with the syntax `Box<Foo + 'a>` or `Box<Foo + 'static>`, depending on what's needed. Just as with the other bounds, this means that any implementer of the `Foo` trait that has any references inside must have the lifetime specified in the trait object bounds as those references.

Next, let's take a look at some other advanced features dealing with traits!