Module drone_core::token

The Token trait and its common patterns.

A token is a zero-sized type, at most one instance of which ever exists. This concept is ubiquitous in Drone. It is used for representing memory-mapped registers, threads, one-time initializers, mutable statics ownership. While affinity (also called move-semantics in Rust) could be represented by Rust type-system, the other properties couldn’t. Therefore the concept relies on the two unsafe contracts below.

1. Implementing the trait is unsafe, and it is the implementer responsibility to ensure the following:

   • The type must not implement Clone.
   • The type must be instantiated only inside Token::take method.
   • The type must be zero-sized.

2. Calling Token::take is unsafe, and it is the caller responsibility to ensure that at most one instance of the type ever exists.

Tokens are often nested to minimize the usage of unsafe Token::take constructor. It is supposed to instantiate all needed tokens at the very beginning of the program and pass the instances further to the code.

Since tokens are zero-sized, Token::take is no-op from the assembly perspective. Likewise passing the instance around doesn’t consume the stack, and storing the instance inside other types doesn’t consume the memory.

Simple Tokens

Here is a usage example of tokens of their simplest form - simple_token! macro. In this example we implement one-timer initializers.

use drone_core::token::{simple_token, unsafe_simple_tokens, Token};

simple_token! {
    /// The token for Foo initializer.
    pub struct FooInitToken;
}

simple_token! {
    /// The token for Bar initializer.
    pub struct BarInitToken;
}

// Here is `unsafe`, we need to ensure that `FooInitToken` and `BarInitToken`
// are not used anywhere else.
unsafe_simple_tokens! {
    /// The group token for all initializers.
    pub struct Inits {
        FooInitToken,
        BarInitToken,
    }
}

// Define one-time initializers. They should accept tokens by-value and
// shouldn't return them.

fn init_foo(token: FooInitToken) {
    // Initialize Foo.
}

fn init_bar(token: BarInitToken) {
    // Initialize Bar.
}

// Your entry point.
fn main() {
    // Various unsafe initializations goes here.
    unsafe {
        // Calling unsafe `take()`, we need to ensure that it is the only place
        // we call it and we are not in a cycle or recursion.
        let ini = Inits::take();
        // Pass the token instance to your safe entry point.
        trunk(ini);
    }
}

fn trunk(ini: Inits) {
    init_foo(ini.foo_init);
    init_bar(ini.bar_init);
    // Calling them again won't compile, because the tokens were consumed.
    // init_foo(ini.foo_init);
    // init_bar(ini.bar_init);
}

Static Tokens

Mutable statics are unsafe in Rust. One way to make them safe is to use interior-mutability. For example Mutex ensures that concurrent access to the data is safe. If you don’t need simultaneous access to the static but still need other static characteristics like known and stable address, you can use static tokens:

use drone_core::token::{unsafe_static_tokens, StaticToken, Token};

// Define some statics.
static mut FOO: usize = 0;
static mut BAR: &str = "data";

// Here is `unsafe`, we need to ensure that `FOO` and `BAR` are not used
// anywhere else.
unsafe_static_tokens! {
    /// The group token for all statics.
    pub struct Statics {
        FOO: usize,
        BAR: &'static str,
    }
}

// Your entry point.
fn main() {
    // Various unsafe initializations goes here.
    unsafe {
        // Calling unsafe `take()`, we need to ensure that it is the only place
        // we call it and we are not in a cycle or recursion.
        let stc = Statics::take();
        // Pass the token instance to your safe entry point.
        trunk(stc);
    }
}

fn trunk(mut stc: Statics) {
    // Borrow a mutable reference.
    add_one(stc.foo.get());
    add_one(stc.foo.get());
    assert_eq!(*stc.foo.get(), 2);
    assert_eq!(core::mem::size_of_val(&stc), 0);
    // Permanently convert to `&'static usize`. Note that `foo` is no longer a ZST.
    let foo = stc.foo.into_static();
    // Calling it again won't compile, because the token was consumed.
    // let foo = stc.foo.into_static();
}

fn add_one(foo: &mut usize) {
    *foo += 1;
}

Macros

simple_token	Defines a new simple Token.
unsafe_simple_tokens	Defines a new token for the set of simple Tokens.
unsafe_static_tokens	Defines a new token for the set of StaticTokens.

Traits

StaticToken	A token for a mutable static variable.
Token	A zero-sized affine type, at most one instance of which ever exists.