​​Traits: Roadmap sketch
​​
​​Idea stage
  • ​​associated traits, quantification over traits
  • ​​gadts (where​ clauses on enum variants)
  • ​​Trait object improvements
  • ​​AsRef​ inference limitations — some way to avoid the “every time I add an impl, code breaks” problem (probably opt-in, tied to the edition)
​​
​​Pre-RFC stage
  • ​​HRTB for types
  • ​​Centril has been working on this
  • ​​Related to generic closures, but the latter is an important part of the motivation here
  • ​​Fields in traits
​​
​​RFC stage
  • ​​Delegation
  • ​​Hasn’t seen much movement; mostly language team’s fault!
  • ​​trickiest bits: interactions with Drop, interactions with provided methods
  • ​​Some weirdness with specialization (unsound in one model, to fix unsoundness the interaction with specialization becomes weird)
​​
​​Implementation stage
  • ​​GATs
  • ​​Implied bounds
  • ​​Trait aliases (needs more design)
  • ​​Existential types (centril working on fcp)
  • ​​Associated type defaults
  • ​​Specialization
  • ​​Needs spec-level rework, possibly additional RFC
  • ​​Associated type bound syntax improvements
  • ​​Needs tests
  • ​​Inherent associated types
  • ​​Re-re-balancing coherence

​​

​​Named existential types

  • ​​There are probably implementation issue aside from syntax changes (which will need implementation changes).
​​

​​let foo: impl Bar;​ / Impl Trait in Bindings

​​#![feature(impl_trait_in_bindings)]
​​fn foo() {
​​    let x: impl core::fmt::Debug = 12;
​​    static x: impl core::fmt::Debug = 12;
​​    const x: impl core::fmt::Debug = 12;