Chalk is a library that implements the Rust trait system, based on Prolog-ish logic rules.

See the Chalk book for more information.

How does chalk relate to rustc? The plan is to have rustc use the chalk-solve crate (in this repo) to answer questions about Rust programs, for example, "Does Vec<u32> implement Debug?". Internally, chalk converts Rust-specific information into logic and uses a logic engine to find the answer to the original query. For more details, see this explanation in the chalk book.

Where does the name come from? chalk is named after Chalkidiki, the area where Aristotle was born. Since Prolog is a logic programming language, this seemed a suitable reference.

Here are some blog posts talking about chalk:

• Lowering Rust Traits to Logic
  • Explains the basic concepts at play
• Unification in Chalk, Part 1
  • An introduction to unification
• Unification in Chalk, Part 2
  • Extending the system for associated types
• Negative reasoning in Chalk
  • How to prove that something is not true
• Query structure in chalk
  • The basic chalk query structure, with pointers into the chalk implementation
• Cyclic queries in chalk
  • Handling cyclic relations and enabling the implementation of implied bounds and other long-desired features in an elegant way

There is a repl mainly for debugging purposes which can be run by cargo run. Some basic examples are in libstd.chalk:

$ cargo run
?- load libstd.chalk
?- Vec<Box<i32>>: Clone
Unique; substitution [], lifetime constraints []

If you'd like to contribute, consider joining the Traits Working Group. We hang out on the rust-lang zulip in the #wg-traits stream.

See the contributing chapter in the chalk book for more info.