​​Rust 2021 Edition Planning

​​Rust 2020 Roadmap Goal:

  • ​​Follow-through on “unfinished business”
  • ​​Prepare for a Rust 2021 Edition
  • ​​Breaking changes in by October 2020
​​

​​My hope for this meeting

  • ​​We can try to sharpen a bit the things we think are top priorities and why, keeping “mid 2021” as a kind of time-frame
​​

​​Ergonomics initiative and other “unfinished business” from Rust 2018

​​​+Topic: Ergonomics Initiative Roundup​ 
​​

​​impl Trait

​​Still a number of things left to be resolved here:
  • ​​impl Trait in traits
  • ​​linked to associated types and generic associated types
  • ​​requires a bit of design work
  • ​​impl Trait in trait impls
  • ​​some differences between RFC and the actual inference that we do
​​
​​This can be a major enabler. For example, in at least some cases, it can enable async fn in traits (not the hardest ones), by permitting e.g.
​​
​​trait Service {
​​  type Future: Future<...>;
​​
​​  fn process_request(&mut self) -> Self::Future
​​}
​​
​​impl Service for MyType {
​​  type Future = impl Future;
​​  fn process_request(&mut self) -> Self::Future { async move { .. } }
​​}
​​
  • ​​Smaller details:
  • ​​turbofish is not allowed in method calls self.foo::<A, B>​, needs to be decided
  • ​​boats: my inclination would be to make impl Trait not specifiable
​​trait Foo {
​​  fn foo<T>(x: impl FnOnce() -> T);
​​}
​​
​​self.foo::<u32> // error today, but it would be nice to support it
  • ​​then you can use impl Trait for things you wouldn’t want to specify
  • ​​downside: you can’t switch from foo<T: Debug>(t: T)​ to fn foo(t: impl Debug)​
  • ​​true today, wouldn’t always have worked before either
  • ​​e.g., fn foo<A, B>(b: B, a: A)​ probably wouldn’t work?
  • ​​door is also not forever closed, it we did permit it, they would have to be optional
  • ​​scottmcm: implications of having to figure out the order doesn’t feel great to me, I’d rather say make it a real generic

​​Implicit mod submodule