Rust Const Generics

rust_const_generics.md

Presentation on Const Generics as of 1.50.0 stable and 1.52.0-nightly

Challenges

• Beware no, or misleading, documentation
  • Nothing in Rust Book.
  • RFC https://rust-lang.github.io/rfcs/2000-const-generics.html and various issues rust-lang/rust#68436 have tips or examples, but many are invalid!
• Syntax may seem arbitrary:
  • Need ::< turbofish at some places, ordinary < elsewhere.
  • Need {...} curly brackets around some const generic parameter values, not around others.
  • Some error messages feel like going in circles.

Learning

• If your constant generic parameters are not integer/char/boolean, don't downgrade your domain.
  • Use #![feature(const_generics)] to allow structs and enums as const generic parameters.
  • Such struct/enums need #[derive(PartialEq, Eq)] (or their implementation).
  • Trust nightly.
• Even though the purpose of (const) generics is to reuse code, the actual const generics code is repetitious. You'll have to repeat where clause in any impl that involves a type with const generic parameter, if the const generic parameter affects size of any field(s).