The lowering module in rustc

The program clauses described in the lowering rules section are actually created in the rustc_traits::lowering module.

The program_clauses_for query

The main entry point is the program_clauses_for query, which – given a def-id – produces a set of Chalk program clauses. These queries are tested using a dedicated unit-testing mechanism, described below. The query is invoked on a DefId that identifies something like a trait, an impl, or an associated item definition. It then produces and returns a vector of program clauses.

Unit tests

Unit tests are located in src/test/ui/chalkify. A good example test is the lower_impl test. At the time of this writing, it looked like this:

#![feature(rustc_attrs)]

trait Foo { }

#[rustc_dump_program_clauses] //~ ERROR Implemented(T: Foo) :-
impl<T: 'static> Foo for T where T: Iterator<Item = i32> { }

fn main() {
    println!("hello");
}

The #[rustc_dump_program_clauses] annotation can be attached to anything with a def-id. (It requires the rustc_attrs feature.) The compiler will then invoke the program_clauses_for query on that item, and emit compiler errors that dump the clauses produced. These errors just exist for unit-testing, as we can then leverage the standard ui test mechanisms to check them. In this case, there is a //~ ERROR Implemented annotation which is intentionally minimal (it need only be a prefix of the error), but the stderr file contains the full details:

error: Implemented(T: Foo) :- ProjectionEq(<T as std::iter::Iterator>::Item == i32), TypeOutlives(T \
: 'static), Implemented(T: std::iter::Iterator), Implemented(T: std::marker::Sized).
  --> $DIR/lower_impl.rs:15:1
   |
LL | #[rustc_dump_program_clauses] //~ ERROR Implemented(T: Foo) :-
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error