Matr

Matr (Metaprogramming with Associated Types in Rust, pronounced like "matter") is a metaprogramming library for Rust.

This is meant to be a sort of standard library for metaprogramming in Rust, providing basic types and algorithms, but with the expectation that users of this crate will want to define their own types and meta-functions; the meta-"type system" is designed to be extensible.

Apart from bool and type, all other meta-types defined here (i.e., the directories under src/) don't access anything beyond what a user of this library could, so they also serve as examples for users of this library on how to write their own meta-types.

It's also possible to define custom sub-traits of Kind with additional restrictions/capabilities, see EqualityComparableKind and Equals for an example.

This library uses traits (notably Expr<K>) for meta-values, so that the Rust typechecker can detect type errors in meta-functions (e.g. attempting to return an Expr<Bool> in a meta-function that should return an Expr<USize>). Since traits can't have trait parameters (only type parameters, that could be constrained via traits) each meta-type has an associated Kind struct (e.g. Bool for booleans).

At the time of writing (March 2023) this is still a work in progress, and requires a nightly version of rustc due to the use of #![feature(specialization)] and #![feature(const_trait_impl)] (that are not yet stabilized).

This is not an officially supported Google product.

Why not use const fns for metaprogramming?

If your use case is covered by const fns, it's simpler to use those instead of this. However, const fns can't compute types; if you're looking for a library to manipulate types you're in the right place.

This library also provides a ConstFn metatype that allows to also have a const fn as the result of the meta-computation.

That is meant to be used in cases where you have some shared logic between the code used to compute a type and to compute a ConstFn. If you only need the ConstFn, then this library is overkill for your use case, you should just use plain const fns instead.

Why not use macros for metaprogramming?

If your use case is covered by macros, it's simpler to use those instead of this. However, macros only have access to the AST and not to the full type information (which is possible using this library).