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Merge pull request #223 from tmandry/improve-chalk-overview
Update chalk overview
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‎src/traits/chalk-overview.md‎

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@@ -16,23 +16,45 @@ existing, somewhat ad-hoc implementation into something far more principled and
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expressive, which should behave better in corner cases, and be much easier to
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extend.
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## Resources
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## Chalk Structure
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* [Chalk Source Code](https://github.com/rust-lang-nursery/chalk)
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* [Chalk Glossary](https://github.com/rust-lang-nursery/chalk/blob/master/GLOSSARY.md)
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* The traits section of the rustc guide (you are here)
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Chalk has two main "products". The first of these is the
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[`chalk_engine`][chalk_engine] crate, which defines the core [SLG
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solver][slg]. This is the part rustc uses.
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### Blog Posts
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The rest of chalk can be considered an elaborate testing harness. Chalk is
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capable of parsing Rust-like "programs", lowering them to logic, and
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performing queries on them.
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* [Lowering Rust traits to logic](http://smallcultfollowing.com/babysteps/blog/2017/01/26/lowering-rust-traits-to-logic/)
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* [Unification in Chalk, part 1](http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/)
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* [Unification in Chalk, part 2](http://smallcultfollowing.com/babysteps/blog/2017/04/23/unification-in-chalk-part-2/)
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* [Negative reasoning in Chalk](http://aturon.github.io/blog/2017/04/24/negative-chalk/)
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* [Query structure in chalk](http://smallcultfollowing.com/babysteps/blog/2017/05/25/query-structure-in-chalk/)
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* [Cyclic queries in chalk](http://smallcultfollowing.com/babysteps/blog/2017/09/12/tabling-handling-cyclic-queries-in-chalk/)
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* [An on-demand SLG solver for chalk](http://smallcultfollowing.com/babysteps/blog/2018/01/31/an-on-demand-slg-solver-for-chalk/)
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Here's a sample session in the chalk repl, chalki. After feeding it our
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program, we perform some queries on it.
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```rust,ignore
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?- program
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Enter a program; press Ctrl-D when finished
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| struct Foo { }
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| struct Bar { }
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| struct Vec<T> { }
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| trait Clone { }
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| impl<T> Clone for Vec<T> where T: Clone { }
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| impl Clone for Foo { }
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?- Vec<Foo>: Clone
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Unique; substitution [], lifetime constraints []
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?- Vec<Bar>: Clone
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No possible solution.
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?- exists<T> { Vec<T>: Clone }
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Ambiguous; no inference guidance
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```
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You can see more examples of programs and queries in the [unit
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tests][chalk-test-example].
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## Parsing
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Next we'll go through each stage required to produce the output above.
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### Parsing ([chalk_parse])
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Chalk is designed to be incorporated with the Rust compiler, so the syntax and
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concepts it deals with heavily borrow from Rust. It is convenient for the sake
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transformation that a program goes through in order to become a format that
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chalk can understand.
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## Lowering
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### Rust Intermediate Representation ([rust_ir])
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After getting the AST we convert it to a more convenient intermediate
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representation called [`rust_ir`][rust_ir]. This is sort of analogous to the
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[HIR] in Rust. The process of converting to IR is called *lowering*.
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After parsing, there is a "lowering" phase. This aims to convert traits/impls
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into "program clauses". A [`ProgramClause` (source code)][programclause] is
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essentially one of the following:
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The [`rust_ir::Program`][rust_ir-program] struct contains some "rust things"
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but indexed and accessible in a different way. For example, if you have a
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type like `Foo<Bar>`, we would represent `Foo` as a string in the AST but in
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`rust_ir::Program`, we use numeric indices (`ItemId`).
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The [IR source code][ir-code] contains the complete definition.
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### Chalk Intermediate Representation ([chalk_ir])
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Once we have Rust IR it is time to convert it to "program clauses". A
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[`ProgramClause`] is essentially one of the following:
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* A [clause] of the form `consequence :- conditions` where `:-` is read as
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"if" and `conditions = cond1 && cond2 && ...`
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*See also: [Goals and Clauses][goals-and-clauses]*
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Lowering is the phase where we encode the rules of the trait system into logic.
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For example, if we have the following Rust:
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This is where we encode the rules of the trait system into logic. For
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example, if we have the following Rust:
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```rust,ignore
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impl<T: Clone> Clone for Vec<T> {}
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This rule dictates that `Vec<T>: Clone` is only satisfied if `T: Clone` is also
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satisfied (i.e. "provable").
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### Well-formedness checks
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Similar to [`rust_ir::Program`][rust_ir-program] which has "rust-like
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things", chalk_ir defines [`ProgramEnvironment`] which which is "pure logic".
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The main field in that struct is `program_clauses`, which contains the
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[`ProgramClause`]s generated by the rules module.
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#### Rules
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The `rules` module ([source code][rules-src]) defines the logic rules we use
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for each item in the Rust IR. It works by iterating over every trait, impl,
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etc. and emitting the rules that come from each one.
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*See also: [Lowering Rules][lowering-rules]*
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#### Well-formedness checks
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As part of lowering to logic, we also do some "well formedness" checks. See
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the [`rules::wf` source code][rules-wf-src] for where those are done.
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As part of lowering from the AST to the internal IR, we also do some "well
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formedness" checks. See the [source code][well-formedness-checks] for where
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those are done. The call to `record_specialization_priorities` checks
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"coherence" which means that it ensures that two impls of the same trait for the
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same type cannot exist.
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*See also: [Well-formedness checking][wf-checking]*
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## Intermediate Representation (IR)
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#### Coherence
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The second intermediate representation in chalk is called, well, the "ir". :)
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The [IR source code][ir-code] contains the complete definition. The
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`ir::Program` struct contains some "rust things" but indexed and accessible in
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a different way. This is sort of analogous to the [HIR] in Rust.
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The function `record_specialization_priorities` in the `coherence` module
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([source code][coherence-src]) checks "coherence", which means that it
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ensures that two impls of the same trait for the same type cannot exist.
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For example, if you have a type like `Foo<Bar>`, we would represent `Foo` as a
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string in the AST but in `ir::Program`, we use numeric indices (`ItemId`).
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### Solver ([chalk_solve])
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In addition to `ir::Program` which has "rust-like things", there is also
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`ir::ProgramEnvironment` which is "pure logic". The main field in that struct is
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`program_clauses` which contains the `ProgramClause`s that we generated
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previously.
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Finally, when we've collected all the program clauses we care about, we want
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to perform queries on it. The component that finds the answer to these
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queries is called the *solver*.
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## Rules
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*See also: [The SLG Solver][slg]*
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The `rules` module works by iterating over every trait, impl, etc. and emitting
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the rules that come from each one. See [Lowering Rules][lowering-rules] for the
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most up-to-date reference on that.
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## Crates
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The `ir::ProgramEnvironment` is created [in this module][rules-environment].
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Chalk's functionality is broken up into the following crates:
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- [**chalk_engine**][chalk_engine]: Defines the core [SLG solver][slg].
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- [**chalk_ir**][chalk_ir]: Defines chalk's internal representation of
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types, lifetimes, and goals.
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- [**chalk_solve**][chalk_solve]: Combines `chalk_ir` and `chalk_engine`,
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effectively.
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- [`chalk_engine::context`][engine-context] provides the necessary hooks.
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- [**chalk_parse**][chalk_parse]: Defines the raw AST and a parser.
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- [**chalk**][doc-chalk]: Brings everything together. Defines the following
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modules:
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- [`rust_ir`][rust_ir], containing the "HIR-like" form of the AST
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- `rust_ir::lowering`, which converts AST to `rust_ir`
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- `rules`, which implements logic rules converting `rust_ir` to `chalk_ir`
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- `coherence`, which implements coherence rules
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- Also includes [chalki][chalki], chalk's REPL.
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[Browse source code on GitHub](https://github.com/rust-lang-nursery/chalk)
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## Testing
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TODO: Basically, [there is a macro](https://github.com/rust-lang-nursery/chalk/blob/94a1941a021842a5fcb35cd043145c8faae59f08/src/solve/test.rs#L112-L148)
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that will take chalk's Rust-like syntax and run it through the full pipeline
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described above.
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[This](https://github.com/rust-lang-nursery/chalk/blob/94a1941a021842a5fcb35cd043145c8faae59f08/src/solve/test.rs#L83-L110)
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is the function that is ultimately called.
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chalk has a test framework for lowering programs to logic, checking the
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lowered logic, and performing queries on it. This is how we test the
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implementation of chalk itself, and the viability of the [lowering
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rules][lowering-rules].
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## Solver
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The main kind of tests in chalk are **goal tests**. They contain a program,
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which is expected to lower to logic successfully, and a set of queries
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(goals) along with the expected output. Here's an
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[example][chalk-test-example]. Since chalk's output can be quite long, goal
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tests support specifying only a prefix of the output.
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See [The SLG Solver][slg].
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**Lowering tests** check the stages that occur before we can issue queries
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to the solver: the [lowering to rust_ir][chalk-test-lowering], and the
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[well-formedness checks][chalk-test-wf] that occur after that.
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[rustc-issues]: https://github.com/rust-lang-nursery/rustc-guide/issues
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[chalk]: https://github.com/rust-lang-nursery/chalk
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[lowering-to-logic]: ./lowering-to-logic.html
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### Testing internals
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Goal tests use a [`test!` macro][test-macro] that takes chalk's Rust-like
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syntax and runs it through the full pipeline described above. The macro
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ultimately calls the [`solve_goal` function][solve_goal].
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Likewise, lowering tests use the [`lowering_success!` and
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`lowering_error!` macros][test-lowering-macros].
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## More Resources
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* [Chalk Source Code](https://github.com/rust-lang-nursery/chalk)
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* [Chalk Glossary](https://github.com/rust-lang-nursery/chalk/blob/master/GLOSSARY.md)
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### Blog Posts
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* [Lowering Rust traits to logic](http://smallcultfollowing.com/babysteps/blog/2017/01/26/lowering-rust-traits-to-logic/)
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* [Unification in Chalk, part 1](http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/)
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* [Unification in Chalk, part 2](http://smallcultfollowing.com/babysteps/blog/2017/04/23/unification-in-chalk-part-2/)
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* [Negative reasoning in Chalk](http://aturon.github.io/blog/2017/04/24/negative-chalk/)
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* [Query structure in chalk](http://smallcultfollowing.com/babysteps/blog/2017/05/25/query-structure-in-chalk/)
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* [Cyclic queries in chalk](http://smallcultfollowing.com/babysteps/blog/2017/09/12/tabling-handling-cyclic-queries-in-chalk/)
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* [An on-demand SLG solver for chalk](http://smallcultfollowing.com/babysteps/blog/2018/01/31/an-on-demand-slg-solver-for-chalk/)
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[goals-and-clauses]: ./goals-and-clauses.html
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[HIR]: ../hir.html
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[lowering-forall]: ./lowering-to-logic.html#type-checking-generic-functions-beyond-horn-clauses
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[lowering-rules]: ./lowering-rules.html
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[lowering-to-logic]: ./lowering-to-logic.html
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[slg]: ./slg.html
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[wf-checking]: ./wf.html
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[ast]: https://en.wikipedia.org/wiki/Abstract_syntax_tree
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[chalk-ast]: https://github.com/rust-lang-nursery/chalk/blob/master/chalk-parse/src/ast.rs
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[chalk]: https://github.com/rust-lang-nursery/chalk
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[rustc-issues]: https://github.com/rust-lang-nursery/rustc-guide/issues
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[universal quantification]: https://en.wikipedia.org/wiki/Universal_quantification
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[lowering-forall]: ./lowering-to-logic.html#type-checking-generic-functions-beyond-horn-clauses
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[programclause]: https://github.com/rust-lang-nursery/chalk/blob/94a1941a021842a5fcb35cd043145c8faae59f08/src/ir.rs#L721
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[clause]: https://github.com/rust-lang-nursery/chalk/blob/master/GLOSSARY.md#clause
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[goals-and-clauses]: ./goals-and-clauses.html
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[well-formedness-checks]: https://github.com/rust-lang-nursery/chalk/blob/94a1941a021842a5fcb35cd043145c8faae59f08/src/ir/lowering.rs#L230-L232
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[ir-code]: https://github.com/rust-lang-nursery/chalk/tree/master/chalk-ir
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[HIR]: ../hir.html
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[`ProgramClause`]: https://rust-lang-nursery.github.io/chalk/doc/chalk_ir/enum.ProgramClause.html
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[`ProgramEnvironment`]: https://rust-lang-nursery.github.io/chalk/doc/chalk_ir/struct.ProgramEnvironment.html
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[chalk_engine]: https://rust-lang-nursery.github.io/chalk/doc/chalk_engine/index.html
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[chalk_ir]: https://rust-lang-nursery.github.io/chalk/doc/chalk_ir/index.html
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[chalk_parse]: https://rust-lang-nursery.github.io/chalk/doc/chalk_parse/index.html
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[chalk_solve]: https://rust-lang-nursery.github.io/chalk/doc/chalk_solve/index.html
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[doc-chalk]: https://rust-lang-nursery.github.io/chalk/doc/chalk/index.html
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[engine-context]: https://rust-lang-nursery.github.io/chalk/doc/chalk_engine/context/index.html
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[rust_ir-program]: https://rust-lang-nursery.github.io/chalk/doc/chalk/rust_ir/struct.Program.html
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[rust_ir]: https://rust-lang-nursery.github.io/chalk/doc/chalk/rust_ir/index.html
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[binders-struct]: https://github.com/rust-lang-nursery/chalk/blob/94a1941a021842a5fcb35cd043145c8faae59f08/src/ir.rs#L661
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[chalk-ast]: https://github.com/rust-lang-nursery/chalk/blob/master/chalk-parse/src/ast.rs
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[chalk-test-example]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/test.rs#L115
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[chalk-test-lowering-example]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/rust_ir/lowering/test.rs#L8-L31
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[chalk-test-lowering]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/rust_ir/lowering/test.rs
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[chalk-test-wf]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/rules/wf/test.rs#L1
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[chalki]: https://rust-lang-nursery.github.io/chalk/doc/chalki/index.html
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[clause]: https://github.com/rust-lang-nursery/chalk/blob/master/GLOSSARY.md#clause
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[coherence-src]: https://github.com/rust-lang-nursery/chalk/blob/master/src/coherence.rs
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[ir-code]: https://github.com/rust-lang-nursery/chalk/blob/master/src/rust_ir.rs
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[rules-environment]: https://github.com/rust-lang-nursery/chalk/blob/94a1941a021842a5fcb35cd043145c8faae59f08/src/rules.rs#L9
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[slg]: ./slg.html
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[rules-src]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/rules.rs
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[rules-wf-src]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/rules/wf.rs
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[solve_goal]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/test.rs#L85
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[test-lowering-macros]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/test_util.rs#L21-L54
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[test-macro]: https://github.com/rust-lang-nursery/chalk/blob/4bce000801de31bf45c02f742a5fce335c9f035f/src/test.rs#L33

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