📜 keyword generics Charter

One of Rust's defining features is the ability to write functions which are generic over their input types. That allows us to write a function once, leaving it up to the compiler to generate the right implementations for us.

When we introduce a new keyword for something which used to be a trait, we not only gain new functionality - we also lose the ability to be generic over that keyword. This proposal seeks to change that by introducing keyword generics: the ability to be generic over specific keywords.

This proposal is scoped to the const and async keywords only, but is designed to be leveraged by other keywords as well in the future. Keywords are valuable, generics are valuable, users of Rust shouldn't have to choose between the two.

Proposal

We're in the process of adding new features to Rust. The Const WG is creating an extension to Rust which enables arbitrary computation at compile time. While the Async WG is in the process of adding capabilities for asynchronous computation. We've noticed that both these efforts have a lot in common, and may in fact require similar solutions. This document describes a framework for thinking about these language features, describes their individual needs, and makes the case that we should be considering a generalized language design for "keywords" (aka "definitely not effects"), so that we can ensure that the Rust language and standard library remain consistent in the face of extensions.

A broad perspective on language extensions

const fn and async fn are similar language extensions, but the way they extend the language is different:

• const fn creates a subset of "base Rust", enabling functions to be executed during compilation. const functions can be executed in "base" contexts, while the other way around isn't possible.
• async fn creates a superset of "base Rust", enabling functions to be executed asynchronously. async types cannot be executed in "base" contexts ¹, but "base" in async contexts is possible.

                      +---------------------------+                               
                      | +-----------------------+ |     Compute values:
                      | | +-------------------+ | |     - types
                      | | |                   | | |     - numbers
                      | | |    const Rust     |-------{ - functions               
                      | | |                   | | |     - control flow            
 Access to the host:  | | +-------------------+ | |     - traits (planned)                 
 - networking         | |                       | |     - containers (planned)
 - filesystem  }--------|      "base" Rust      | |                               
 - threads            | |                       | |                               
 - system time        | +-----------------------+ |     
                      |                           |     Control over execution:      
                      |         async Rust        |---{ - ad-hoc concurrency      
                      |                           |     - ad-hoc cancellation     
                      +---------------------------+     - ad-hoc pausing/resumption

In terms of standard library these relationships also mirror each other. "Base" Rust will want to do everything during runtime what const rust can do, but in addition to that also things like network and filesystem IO. Async Rust will in turn want to do everything "base" Rust can do, but in addition to that will also want to introduce methods for ad-hoc concurrency, cancellation, and execution control. It will also want to do things which are blocking in "base" Rust as non-blocking in async Rust.

And it doesn't stop with const and async Rust; it's not hard to imagine that other annotations for "can this panic", "can this return an error", "can this yield values" may want to exist as well. All of which would present extensions to the "base" Rust language, which would need to be introduced in a way which keeps it feeling like a single language - instead of several disjoint languages in a trenchcoat.

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