Crate escher[−][src]

Self-referencial structs using async stacks

Escher is an extremely simple library providing a safe and sound API to build self-referencial structs. It works by (ab)using the async await trasformation of rustc. If you’d like to know more about the inner workings please take a look at the How it works section and the source code.

Compared to the state of the art escher:

Is only around 100 lines of well-commented code
Contains only two unsafe calls that are well argued for
Uses rustc for all the analysis. If it compiles, the self references are correct

Usage

This library provides the Escher wrapper type that can hold self-referencial data and expose them safely through the as_ref() and as_mut() functions.

You construct a self reference by calling Escher’s constructor and providing an async closure that will initialize your self-references on its stack. Your closure will be provided with a capturer r that has a single capture() method that consumes r.

Note: It is important to .await the result .capture() in order for escher to correctly initialize your struct.

Once all the data and references are created you can capture the desired ones. Simple references to owned data can be captured directly (see first example).

To capture more than one variable or capture references to non-owned data you will have to define your own reference struct that derives Rebindable (see second example).

Examples

Simple `&str` view into an owned `Vec<u8>`

The simplest way to use Escher is to create a reference of some data and then capture it:

use escher::Escher;

let escher_heart = Escher::new(|r| async move {
    let data: Vec<u8> = vec![240, 159, 146, 150];
    let sparkle_heart = std::str::from_utf8(&data).unwrap();

    r.capture(sparkle_heart).await;
});

assert_eq!("💖", *escher_heart.as_ref());

Capturing both a `Vec<u8>` and a `&str` view into it

In order to capture more than one things you can define a struct that will be used to capture the variables:

use escher::{Escher, Rebindable};

#[derive(Rebindable)]
struct VecStr<'this> {
    data: &'this Vec<u8>,
    s: &'this str,
}

let escher_heart = Escher::new(|r| async move {
    let data: Vec<u8> = vec![240, 159, 146, 150];

    r.capture(VecStr{
        data: &data,
        s: std::str::from_utf8(&data).unwrap(),
    }).await;
});

assert_eq!(240, escher_heart.as_ref().data[0]);
assert_eq!("💖", escher_heart.as_ref().s);

Capturing a mutable `&mut str` view into a `Vec<u8>`

If you capture a mutable reference to some piece of data then you cannot capture the data itself like the previous example. This is mandatory as doing otherwise would create two mutable references into the same piece of data which is not allowed.

use escher::Escher;

let mut name = Escher::new(|r| async move {
    let mut data: Vec<u8> = vec![101, 115, 99, 104, 101, 114];
    let name = std::str::from_utf8_mut(&mut data).unwrap();

    r.capture(name).await;
});

assert_eq!("escher", *name.as_ref());
name.as_mut().make_ascii_uppercase();
assert_eq!("ESCHER", *name.as_ref());

Capturing multiple mixed references

use escher::{Escher, Rebindable};

#[derive(Rebindable)]
struct MyStruct<'this> {
    int_data: &'this Box<i32>,
    int_ref: &'this i32,
    float_ref: &'this mut f32,
}

let mut my_value = Escher::new(|r| async move {
    let int_data = Box::new(42);
    let mut float_data = Box::new(3.14);

    r.capture(MyStruct{
        int_data: &int_data,
        int_ref: &int_data,
        float_ref: &mut float_data,
    }).await;
});

assert_eq!(Box::new(42), *my_value.as_ref().int_data);
assert_eq!(3.14, *my_value.as_ref().float_ref);

*my_value.as_mut().float_ref = (*my_value.as_ref().int_ref as f32) * 2.0;

assert_eq!(84.0, *my_value.as_ref().float_ref);

Structs

Capturer	An instance of `Capturer` is given to the closure passed to Escher::new and is used to capture a reference from the async stack.
Escher	A containter of a self referencial struct. The self-referencial struct is constructed with the aid of the async/await machinery of rustc, see Escher::new.

Traits

RebindTo	The `RebindTo` trait defines a type level function that allows you convert a type that holds references of lifetime `'a` to a type that holds references of lifetime `'b`.
Rebindable	Marker trait for any type that implements RebindTo for any lifetime. All types can derive this trait using the Rebindable derive macro.

Type Definitions

Rebind

Type-level function that takes a lifetime 'a and a type T computes a new type U that is identical to T except for its lifetimes that are now bound to 'a.

Derive Macros

Rebindable

This trait can be derived for any struct, enum, or union to make its lifetimes rebindable and thus compatible with the Rebind type level function.