fortify

A simple and convenient way to bundle owned data with a borrowing type.

Example

use fortify::*;

// Define a borrowing type. The `Lower` trait specifies that it is covariant in its first
// lifetime parameter.
#[derive(Lower)]
struct Example<'a> {
   a: &'a i32,
   b: &'a mut i32,
}

// Construct a fortified value that makes an "external" reference to `a` and an "internal"
// reference to `b`, which is owned by the Fortify.
let a = 1;
let mut example: Fortify<Example> = fortify! {
    let mut b = 1;
    b += 1;
    yield Example {
        a: &a,
        b: &mut b
    };
};

// Use `with_mut` for general mutable access to the wrapped value. Note that the reference
// to `b` is still valid even though `b` is not in scope in this block.
example.with_mut(|example| {
    assert_eq!(*example.a, 1);
    assert_eq!(*example.b, 2);
    *example.b += 1;
    assert_eq!(*example.b, 3);
});

The Problem

One of the main selling points of Rust is its borrow checker, which allows you to define types that make references to outside data while ensuring that the data will remain valid for as long the type is using it. In theory, this is the ideal memory management scheme: it costs nothing to "drop" a reference, we're not putting off any work for a deferred garbage collection and we're completely protected from use after free errors.

However, many Rust programmers feel uneasy putting this technique into practice, often resorting to simpler methods such as reference counting. A key reason for this is the infectious nature of lifetimes: storing a reference in a type requires you to specify a lifetime, and with the rare exception of 'static data, this requires you to have a lifetime parameter on the type. Then, to use the type in another type, you need to specify a lifetime parameter for that type, and so on. The chain only ends when you finally introduce the value as a local variable, allowing new unique lifetimes to be created for it.

But this isn't always possible. Sometimes you need a value to be allocated at a higher stack level than you have access to. It's difficult to identify these situations ahead of time, and when you do, your whole design collapses. No wonder Rust programmers are hesitant about using references.

There's a few existing solutions to mitigate this problem, but all of them have their issues:

• Using an arena allocator such as bumpalo or typed-arena, you can allocate data with a particular lifetime from a lower level in the stack. However, the memory can't be freed until the allocator is dropped, so there is a practical limit to how long the allocator can be kept alive.
• The owning_ref crate allows you to avoid specifying the lifetime for a reference by bundling it with the data it is referencing. However, it has numerous soundness issues and is no longer being maintained.
• There have been proposals for allowing self-referential structs. In lieu of language support, the rental and ouroboros crates enable a limited form of this. However, the implementation of self-referential structs is not as simple or intuitive as one would expect. There are limitations to what may be stored in the struct and access to struct fields must be restricted in order to adhere to Rust's aliasing rules.

This crate introduces yet another solution to problem, with the goal of being flexible and convenient enough to enable fearless use of references and borrowing types.

The Solution

The Fortify<T> wrapper allows a wrapped value to make references to hidden, supplementary data owned by the wrapper itself. For example, a Fortify<&'static str> can be a regular &'static str, or it can be a reference to a string stored inside the Fortify. T isn't limited to being a reference, it can be any type that has a lifetime parameter, with similar effect. The implication here is that you can turn any borrowing type (i.e. a type with a lifetime parameter) into an owning type by setting its lifetime to 'static and putting it in a Fortify wrapper.

How is this okay? Doesn't a &'static str always have to reference something in the 'static lifetime?

The key insight is that you can never get a &'static str from a Fortify<&'static str>. Instead, you can get a &'a str where 'a is tied to lifetime of the Fortify. The wrapper has a complex relationship with its wrapped type that can't normally be expressed in Rust (hence the need for this crate). It's implementation requires being able to manipulate the lifetime parameter of the enclosed type.

So if I use a type with multiple lifetime parameters, how does the wrapper know which lifetime it "works" on?

All wrapped types must implement the Lower<'a> trait, which specifies how to substitute the covariant lifetime parameters in the type. This trait can be automatically derived, in which case it will only operate on the first lifetime parameter in the parameter list.

How do I create a Fortify<T>?

There are many ways to create an instance of the wrapper, with the simplest being a direct conversion from T. However, the preferred and most general method is the fortify! macro:

let example: Fortify<&'static str> = fortify! {
     let mut str = String::new();
     str.push_str("Foo");
     str.push_str("Bar");
     yield str.as_str();
};

This captures all of the local variables in a block of code and stores them inside the Fortify wrapper. The final yield statement provides the wrapped value that is exposed to the outside. Note that it may make references to the local variables.

How do I use it?

You can use borrow to get an immutable reference to the wrapped value with appropriately shortened lifetime. Mutable access is a bit more complicated, and requires the use of with_mut.

assert_eq!(example.borrow(), &"FooBar");
// or
example.with_mut(|s| assert_eq!(s, &"FooBar"));

Can I use Fortify<T> with a non-'static lifetime?

Of course! The Fortify wrapper merely introduces an additional option for references (pointing to owned data inside the wrapper). It is always possible to forgo this option and construct a Fortify<T> directly from a T. You can even have a mixed value which makes some references to external data and some references to owned data, as in the first example.