lowering-module.md

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 DefId – produces a set of Chalk program clauses. 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

Note: We've removed the Chalk unit tests in rust-lang/rust#69247. They will come back once we're ready to integrate next Chalk into rustc.

Here's a good example test. At the time of this writing, it looked like this:

#![feature(rustc_attrs)]

trait Foo { }

#[rustc_dump_program_clauses] //~ ERROR program clause dump
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 DefId (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. The stderr will be:

error: program clause dump
  --> $DIR/lower_impl.rs:5:1
   |
LL | #[rustc_dump_program_clauses]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: forall<T> { Implemented(T: Foo) :- ProjectionEq(<T as std::iter::Iterator>::Item == i32), TypeOutlives(T: 'static), Implemented(T: std::iter::Iterator), Implemented(T: std::marker::Sized). }