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Negative reasoning in Chalk

I’ve had the pleasure in recent weeks of working on Chalk, the project that Niko’s been blogging about:

The project has a few goals:

  • Recast Rust’s trait system explicitly in terms of logic programming, by “lowering” Rust code into a kind of logic program we can then execute queries against.

  • Provide a prototype for an implementation based on these principles in rustc.

  • Provide an executable, highly readable specification for the trait system.

We expect many benefits from this work. It will consolidate our existing, somewhat ad hoc implementation into something far more principled and expressive, which should behave better in corner cases, and be much easier to extend. For example, the current implementation already supports associated type constructors.

It also makes it much easier to gain confidence in what the trait system is doing, because we can understand it in relatively simple logical terms.

Open problems in paradise

All that said, Chalk isn’t finished, and it’s currently missing some of the core pieces of the real trait system.

I’ve been trying to puzzle out a tangle of related such open problems for Chalk. In particular, I want to work out how to:

  • Give a very precise and principled meaning for the Yes, No, and Maybe results you can receive.

  • Account for the various “mode switches” we employ in today’s trait system, which control the degree of negative reasoning permitted.

  • Account for rustc’s precedence rules that e.g. give more weight to where clauses than to blanket impls when it comes to type inference.

  • Support coherence checking, which requires (constrained) negative reasoning.

  • Leverage the orphan rules for reasoning.

  • Incorporate specialization soundly (ruling out lifetime dispatch).

The theme that ties all of these topics together is negative reasoning, i.e the ability to conclude definitively that something is not true. For the trait system, that usually means that a type definitively does not implement a trait. And what we’ve learned over time is, relying on this kind of reasoning can make your code brittle to changes in other crates: new impls are added all the time, and can invalidate these kinds of negative conclusions. We’ve carefully designed the existing trait system to strike the right balance between the ability to reason negatively and the ability of other crates to evolve, but the current implementation feels ad hoc and incomplete. The challenge is putting all of this on much firmer footing, by understanding it in terms of explicit logic programming, and keeping the underlying logic grounded in well-understood logical principles. (And that, by the way, would be a huge win, since we’ve often been quite fearful about negative reasoning in rustc, since it’s so easy to do it incorrectly.)

It turns out that Prolog has similar concerns about negation. In particular, the natural way of implementing negation in a Prolog engine is through failure: not P means you tried but failed to prove P given the facts currently present in the Prolog program. For this to be valid as logical negation, we have to view the program under a “closed world assumption”: the facts that follow from the program’s clauses, and only those facts, are true.

To understand the rest of this post, you’ll want to have read at least the first of Niko’s series.

Negative reasoning in Rust today

To get more clarity about the negative reasoning issues, let’s look at the various places they come into play in the current trait system.

The current system has two distinct “mode switches”:

  • Intercrate mode, which forces the trait system to account for the possibility that (1) downstream crates using this crate can introduce new types and trait impls that we can’t know about and (2) upstream crates could be changed to introduce new trait impls.

  • User-facing projection mode, which forces the trait system to account for the possibility that upstream crates could be changed to introduce new specializations (and thus alter the definition of an associated type).

We’ll see what this means concretely in a moment, but one observation right off the bat: these switches are not used orthogonally today. In particular, there is no code today that uses intercrate mode without also using user-facing projection mode.

Let’s go through the three major areas of the compiler that use the trait system and see how they employ these modes, and what the implications are.

Overlap checking and intercrate mode

Part of trait coherence is checking impls for overlap. Consider the following:

trait MyTrait { .. }
impl<T: Error> MyTrait for T { .. }
impl MyTrait for () { .. }

Do these two impls overlap? It depends on whether (): Error – or more precisely, whether we can definitively conclude not { (): Error }. If we are allowed to conclude that not { (): Error }, then we can conclude that the two impls don’t overlap.

Should we be able to draw such a conclusion? On the one hand, currently () does not implement the Error trait (both are defined in std), and hence the two impls here do not overlap. However, if std was ever changed so that () implemented Error, these impls would overlap and could not be allowed. In other words, std adding such an impl would irrevocably break this code! And we’d like for std to be able to add trait implementations without requiring a new major version of Rust.

Part of the rebalancing coherence RFC was a decision that these kinds of negative conclusions can only be drawn about type/trait combinations that are fully under the current crate’s control. In other words, it connects negative reasoning to the orphan rule, which says which impls a crate is allowed to provide. (There is also a mechanism, called fundamental, to promise that certain impls won’t be provided in the future, but we’ll ignore that for now.) By limiting negative reasoning in this way, we can “future proof” crates against changes their dependencies will likely make, such as introducing impls. While such changes can still cause type inference ambiguities, they can never cause irrevocable breakage.

To illustrate where we do allow negative reasoning for overlap checking, consider the following variant:

trait MyTrait { .. }
impl<T: Error> MyTrait for T { .. }

struct MyStruct { .. } // does not implement Error
impl MyTrait for MyStruct { .. }

Here, we allow the trait system to conclude that not { MyStruct: Error }, because whether or not MyStruct: Error is entirely under this crate’s control, so there is no risk of an innocent upstream change breaking this crate.

Here’s a more subtle case:

trait MyTrait { .. }
trait Aux { .. } // no impls in this crate

impl<T: Error> MyTrait for T { .. }

struct MyStruct<U> { .. } // no impl for Error in this crate
impl<T> MyTrait for MyStruct<T> { .. }
impl<T: Aux> Error for MyStruct<T> { .. }

This example has a lot going on. The key point is that the current crate defines an Aux trait, but does not implement it for any types. Hence, there is no T you could mention in this crate such that T: Aux, and hence no type T such that MyStruct<T>: Error. Can we thus conclude that for all T, not { MyStruct<T>: Error }? No! Because a downstream crate using this one could define a new type Foo and implement Aux for it, and then suddenly MyStruct<Foo> would have two applicable impls of MyTrait.

For that reason, we consider not only the way that existing, upstream crates could provide new impls over time, but also consider that downstream crates can introduce new types, and new trait impls for them, that we will never be able to know about here.

All of the above restrictions on negative reasoning are part of intercrate mode, which is only used by overlap checking.

Type checking and user-facing projection mode

Another case of negative reasoning arises through specialization. Consider:

// Crate A

trait Foo { type T; }
impl<T> Foo for T {
    default type T = bool;
}

// Crate B

use crate_a::Foo;

fn main() {
    let x: <bool as Foo>::T = true;
}

Should this compile? More specifically, is it valid for crate B to conclude that the associated type T for bool’s implementation of Foo is bool?

It would be sound to make that assumption, since we know that crate A is the only crate that can implement Foo for bool (due to the orphan rules), and chose not to specialize the impl. However, in the future, crate A could be modified with an additional impl:

impl Foo for bool {
    type T = ();
}

And that change would break crate B. So this is again a question of what changes an upstream crate should be able to make in a minor revision.

Currently, we tilt things in favor of crate A being able to add such an impl, and thus do not allow the original example to compile. This is again a form of constraining negative reasoning: we do not allow crate B to conclude that there is not a more specialized impl that applies, because there could be one in the future.

Interestingly, in the current implementation you could not write fn main even in crate A, where all of the relevant impls are under the crate’s direct control. I consider this a bug.

In any case, this negative reasoning restriction is called “user-facing projection mode” (as opposed to “trans-facing”, which we’ll see below). It’s turned on during both type checking and overlap checking.

Why type checking does not turn on intercrate mode

Today, type checking and overlap checking differ in one (big) way with respect to negative reasoning: type checking does not turn on intercrate mode. Why?

Consider the following, quite contrived example:

trait Foo<T> {
    fn make() -> T;
}
trait Bar<T> {}

#[derive(Debug)]
struct Local;

impl<T> Foo<T> for () where (): Bar<T> {
    fn make() -> T { panic!() }
}

impl Foo<Local> for () {
    fn make() -> Local { Local }
}

fn main() {
    println!("{:?}", <()>::make());
}

This code compiles and prints “Local”. That’s because, from what this crate can see, no type implements Bar<T>, so only the second impl of Foo is viable. That conclusion is then fed into type inference, which decides to interpret <()>::make() as <() as Foo<Local>>::make().

This is all kosher because, for soundness, the only thing that matters about type inference is that, in the end, we get something that typechecks. And we’ve made a deliberate decision to not make type inference future-proofed against changes in other crates, since that creates serious ergonomic problems (see the linked post, particularly the section on conditional impls).

Trans

On the other hand, when it comes time to actually generate code, we are no longer interested in future-proofing (which has already been taken care of in the static checking described above), and instead expect to get a clear-cut answer to all questions we ask of the trait system–in part because all of the questions will involve fully monomorphized types. In particular, we need to allow default associated types to be revealed, so that we can generate code.

Thus, when using the trait system within trans, we allow full-blown negative reasoning.

So, putting together all of the above: the trait system engages in various forms of negative reasoning, but at different times this reasoning is restricted in different ways. Only the trans point of view correlates directly with Prolog’s “negation as failure”/closed world view. The question now is, can we understand the restricted forms of negative reasoning in logical terms as well?

It turns out that there’s a very satisfying answer: use a modal logic.

Modal logic makes truth relative to a possible world, rather than being an absolute thing. In one world, the sky is blue; in another world, it’s red. That’s the story for “facts”. But the basic rules of logic apply no matter what world you’re talking about: 1+1 = 2 in every possible world.

An important aspect of modal logics is modalities, which are basically ways of qualifying a statement by what world(s) you are talking about. The statement I just made above is an example of a modal statement: 1+1 = 2 in every possible world. There are lots of possible modalities, like “in every future world” or “in some possible world” and so on.

There’s a lot more to say about modal logic, but this post is going to tell the story in a Rust-centric way. You can find more background here.

Possible worlds in Rust

What does all this mean for Rust? We can give a rational reconstruction of what the compiler currently does via modal logic, and use it to guide the development of Chalk, resolving a number of open questions along the way.

First off, a “possible world” for us will be a full crate graph, with a particular crate being considered “the current crate” (the one actively being compiled). In the simplest case, there’s just one crate, like in the following two examples:

// Program A
struct MyType;
trait MyTrait {}
impl MyTrait for MyType {}

and

// Program B
struct MyType;
trait MyTrait {}

Both crates define MyType and MyTrait, but in the first one MyType: MyTrait, while in the second one, not { MyType: MyTrait }. The facts on the ground depend on the crate you’re compiling. And when you’re asking Chalk a question, that question is normally grounded in the particular crate graph you’ve lowered to Chalk’s logic. In other words, statements made directly about the current world are interpreted in, well, a “closed world” way: we know precisely what the world is, and can give firm answers on that basis. That’s the appropriate interpretation for trans, as we saw above.

Second–and this is really the key idea–we add a modality to Chalk to make statements about all compatible worlds to the one we’re currently in. This is how we capture the idea of “future proofed” reasoning of the kind we want in type and coherence checking. A world is compatible with the current world if:

  • The current crate is unchanged.
  • All dependencies of the current crate still exist, but may be extended in semver-compatible ways (i.e., ways that would only require a minor version bump).
  • Downstream crates (that use the current crate) can come, go, and otherwise change in arbitrary ways.

Let’s see an example. Suppose we have a two crate dependency graph:

// WORLD 1

////////////////////////////////////////////////////////////
// crate A
////////////////////////////////////////////////////////////

trait Foo {}
struct CrateAType;

////////////////////////////////////////////////////////////
// crate B -- the current crate
////////////////////////////////////////////////////////////

extern crate crate_a;
use crate_a::*;

struct CrateBType;

Here’s a compatible world, one that extends crate A in a minor-bump kind of way:

// WORLD 2

////////////////////////////////////////////////////////////
// crate A
////////////////////////////////////////////////////////////

trait Foo {}
struct CrateAType;
impl Foo for CrateAType {}   // <- changed

////////////////////////////////////////////////////////////
// crate B -- the current crate
////////////////////////////////////////////////////////////

extern crate crate_a;
use crate_a::*;

struct CrateBType;

Now, here’s an interesting thing: in the world 1, not { CrateAType: Foo }. But in this new, compatible world 2, CrateAType: Foo! The facts on the ground have changed in a meaningful way.

Could we go the other way around? That is, if we’re currently talking about the world 2, is world 1 considered compatible? No. Because removing a trait impl is not a semver-compatible change. What that means in practice is that, when jumping to a compatible world, you can go from not { Foo: Bar } to Foo: Bar, but not the other way around.

Here’s a world that is incompatible with the world 1:

// World 3

////////////////////////////////////////////////////////////
// crate A
////////////////////////////////////////////////////////////

trait Foo {}
struct CrateAType;

////////////////////////////////////////////////////////////
// crate B -- the current crate
////////////////////////////////////////////////////////////

extern crate crate_a;
use crate_a::*;

struct CrateBType;
impl Foo for CrateBType {}   // <- changed

The change here is very similar to the change in world 2, but the key difference is which crate was changed: crate B, the “current crate”, is different in this world, and that makes it incompatible with world 1. This is how we get a distinction for the “local” crate, which we control and therefore don’t need to future-proof against. So if not { CrateBType: Foo } and crate B is the current crate, we know that in any compatible world, not { CrateBType: Foo } will still be true.

Now let’s talk about the other kind of change allowed to the world: arbitrary changes to downstream crates. In world 1, we didn’t have any crates downstream from crate B. Here’s a world that does:

// WORLD 4

////////////////////////////////////////////////////////////
// crate A
////////////////////////////////////////////////////////////

trait Foo {}
struct CrateAType;

////////////////////////////////////////////////////////////
// crate B -- the current crate
////////////////////////////////////////////////////////////

extern crate crate_a;
use crate_a::*;

struct CrateBType;

////////////////////////////////////////////////////////////
// crate C -- a downstream crate
////////////////////////////////////////////////////////////

extern crate crate_a;
extern crate crate_b;
use crate_a::*;

struct CrateCType;
impl Foo for CrateCType {}

So, in world 1 we could conclude not { exists<T> { T: Foo } }. But in world 4 here, we have CrateCType: Foo. That’s the kind of thing that we assume can happen during coherence checking, but today we don’t in typechecking. As we’ll see later, though, this single notion of compatible worlds will end up sufficing for both.

Finally, let’s look at one more world, this time involving downstream crates:

// WORLD 5

////////////////////////////////////////////////////////////
// crate A
////////////////////////////////////////////////////////////

trait Foo {}
struct CrateAType;

////////////////////////////////////////////////////////////
// crate B -- the current crate
////////////////////////////////////////////////////////////

extern crate crate_a;
use crate_a::*;

struct CrateBType;

////////////////////////////////////////////////////////////
// crate C -- a downstream crate
////////////////////////////////////////////////////////////

extern crate crate_a;
extern crate crate_b;
use crate_a::*;
use crate_b::*;

impl Foo for CrateBType {}    // <- Note the type here

This world is not just incompatible with world 1–it’s not even a possible world! That’s because crate C violates the orphan rule by providing an impl for a type and trait it does not define (Foo for CrateBType). In other words, when we consider “all compatible worlds”, we take into account the orphan rules when doing so. And that’s why, starting from world 1, we know that in all compatible worlds, not { CrateBType: Foo }.

The compat modality

The discussion for Rust so far has focused on the underlying meaning of worlds. But we want to “surface” that meaning through a modality that we can use when making statements or asking questions in Chalk. We’ll do this via the compat modality. (For modal logic aficionados, this is basically the “box” modality, where the reachable worlds are the “compatible” ones, described below.)

The basic idea is that if we pose a query Q, that’s understood in terms of the current world, but if we ask compat { Q }, we’re asking if Q is true in all compatible worlds.

Revisiting our example above, if world 1 is the current world, here are some fact’s we’ll be able to deduce

  • not { CrateAType: Foo }
  • not { CrateBType: Foo }
  • not { exists<T> { T: Foo } }
  • compat { not { CrateBType: Foo } } – in every compatible world, we still know that CrateBType does not implement Foo

And here are some statements that will not hold:

  • compat { not { CrateAType: Foo } }, because of examples like world 2.
  • compat { CrateAType: Foo }, because of world 1 itself.
  • compat { not { exists<T> { T: Foo } } }, for similar reasons

Note that for a given query Q, we might not be able to show compat { Q } or compat { not { Q } }, because some compatible worlds satisfy Q, and some don’t. So, within the compat modality, you don’t get the law of the excluded middle.

There’s more to say about this modality and how it’s implemented, but we can already put some cards on the table: when we’re type or coherence checking, the queries we pose to Chalk will be placed within a compat modality, which essentially “future proofs” their conclusions. For trans, we’ll pose queries directly about the current world.

In other words, having the compat modality means we can decide whether to make a “closed world assumption” or not, depending on what we’re trying to do.

Not taking Yes or No for an answer

To finish telling the story around modalities in Chalk, as well as to fully capture the current trait system’s behavior, we need to talk a little bit about what kinds of answers you can get when you pose a query to Chalk.

Traditional logic programming gives you two kinds of answers: Yes (with some information about how the query was resolved) and No. So for example, take the following Rust program:

trait Foo: Display {
    fn new() -> Self;
}
impl Foo for u8 { ... }
impl Foo for bool { ... }

If you ask exists<T> { T: Foo } in a traditional Prolog engine, you’ll get something like “Yes, T = u8”; if you ask again, you’ll get “Yes, T = bool”, and if you ask a final time, you’ll get “No”.

That’s not quite what we want for Rust. There, “existential” questions come up primarily when we’re in the middle of type inference and we don’t know what a particular type is yet. Think about a program using the Foo trait like so:

fn main() {
    println!("{}", Foo::new());
}

When we go to type check this function, we don’t immediately know what the type returned by Foo::new() is going to be, or which impl to use. While it’s true that there do exist types we could use, we don’t want to pick one at random for type inference. Instead, we want an error asking the programmer to clarify which type they wanted to use.

On the other hand, when there’s only one choice of type given other constraints, we allow type inference to assume the programmer must have meant that type:

struct Foo;

trait Convert<T> {
    fn convert(self) -> T;
}

impl Convert<bool> for Foo {
    fn convert(self) -> bool { true }
}

fn main() {
    println!("{}", Foo.convert()) // prints `true`
}

Similarly, when it comes time to generate code, we expect there to be a unique choice of impl to draw each method call from!

These considerations have led both rustc and Chalk to adopt a kind of three-way answer system: Yes, No, and Maybe. The precise meaning of these outcomes has been a bit muddy, but part of what I want to advocate for is the following setup:

  • Yes: in the current world, there is a unique way of choosing the existentials (inference variables) to make the query true; here’s what it is.

  • No: the query does not hold in the current world.

  • Maybe: the query may or may not hold in the current world; optionally, here’s a suggestion of what to choose for the existentials if you get stuck.

What is this business about getting stuck? In general, when we’re type checking a function body, we don’t always know the type of everything at the time we encounter it. Take, for example, the following:

fn main() {
    let mut opt = None;
    opt = Some(true);
}

When we are typechecking the first line, we know that opt will have type Option<?T>, but we don’t know what ?T is; it’s an inference variable. Later on in checking, as we encounter further constraints, we’ll learn that ?T must be bool. By the time we finish typechecking a function body, all inference variables must be so resolved; otherwise, we wouldn’t know how to generate the code!

This inference process is interleaved with querying the trait system, as in the Convert example above. So in general we need the trait system to feed back information about unknown types. But for the case of Maybe, the trait system is saying that there might be multiple ways of implementing the trait, and the suggested types being returned should only be used as a “fallback” if type checking otherwise can’t make any progress.

Let’s take a look at a couple of ways that this version of Maybe helps.

Leveraging Maybe for where clause precedence

In the current trait system, where clauses are given precedence over other impls when it comes to type inference:

pub trait Foo<T> { fn foo(&self) -> T; }
impl<T> Foo<()> for T { fn foo(&self) { } }
impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }

pub fn foo<T>(t: T) where T: Foo<bool> {
   println!("{:?}", <T as Foo<_>>::foo(&t));
}
fn main() { foo(false); }

The program prints false. What’s happening here is that the call to foo within println! does not provide enough information by itself to know whether we want the Foo<bool> impl or the Foo<()> impl, both of which apply. In other words, there’s not a unique way to resolve the type. However, the current trait system assumes that if you have an explicit where clause, it should take precedence over impls, and hence influence type inference.

It wasn’t initially clear how this would carry over to Chalk, where we’re trying to take a “pure logic” stance on things, and hence would prefer not to bake in various notions of precedence and so on.

However, with the reading of Maybe given above, we can yield a Maybe answer here and recommend to type inference that it choose bool if it gets stuck, but we’ve made clear that this is a sort of “extra-logical” step.

Leveraging Maybe for type checking under compat

Similarly, recall that in the current trait system, there are two different mode switches, but so far we’ve only talked about a single compat modality to add to Chalk.

The key, again, is to leverage Maybe. In particular, we can have both type checking and coherence checking make queries to Chalk within the compat modality. But if they get a Maybe answer back, they will interpret it differently:

  • Coherence, which is trying to be conservative, will consider a Maybe to mean “Yes, these could potentially overlap”, and hence produce an error.

  • Type checking, as explained above, will take the fallbacks suggested by Maybe under advisement, and if it gets stuck, will apply them and see whether it can make further progress.

Here again was the example that distinguished the modes that type checking and coherence used:

trait Foo<T> {
    fn make() -> T;
}
trait Bar<T> {}

#[derive(Debug)]
struct Local;

impl<T> Foo<T> for () where (): Bar<T> {
    fn make() -> T { panic!() }
}

impl Foo<Local> for () {
    fn make() -> Local { Local }
}

fn main() {
    println!("{:?}", <()>::make());
}

The key point was: can you deduce that not { exists<T> { (): Bar<T> } }, and hence that only the second impl of Foo could possibly apply?

In the system proposed by this post, we’d follow a chain of events like the following:

  • The type checker asks: exists<T> { compat { (): Make<T> } }
    • We check the first impl, and end up asking: compat { (): Bar<?T> }
      • We return Maybe, since we’re within compat and there are indeed some compatible worlds for which (): Bar<?T> for some ?T; but we have no idea what that ?T should be.
    • We check the second impl, and get Yes with ?T = Local.
    • Since there were multiple possibilities, we don’t have a unique answer; but only one of the possibilities gave us a suggestion for the inference variables. So we return Maybe with ?T = Local
  • The type checker takes under advisement that ?T should be unified with Local if nothing else constrains it.

In other words, unlike with the current trait implementation, we don’t have to pretend that we actually get a unique answer here; we can work within the future-proofed compat modality, and get back a Maybe answer with the suggestion we wanted.

It’s quite nice that all of the static checking takes place under the “future proofing” of the compat modality, whereas trans talks only about the world as it is, under a closed world assumption.

Implementing not and compat in Chalk

Before we close out this post, it’s worth being a bit more concrete about how not and compat would be implemented in Chalk (neither exists today).

Negation

For not { Q }, we basically follow Prolog-style negation-as-failure:

  • Attempt to solve Q, then dispatch on the answer we got:
    • If we got Yes, return No
    • If we got No,
      • If there are no existential variables within Q, return Yes
      • Otherwise, return Maybe, with no type suggestions
    • If we got Maybe, return Maybe, with no type suggestions

As you can see, with negation we don’t get information about existential variables, which is the same in traditional Prolog.

The compat modality

The implementation of compat { Q } is a bit more complex. First of all, it’s important to realize that Chalk already operates on an explicit “world”, namely the program you’ve lowered to it. When you ask questions, it will use this world as one source of facts (together with where clauses, basically). So the question is: how do we tweak this setup to capture the idea of “Evaluate this in any world compatible with the current one”?

We certainly can’t literally construct every such world, as there are an infinite number of them. But fortunately, we don’t have to. The role of the world in Chalk, as I said above, is to provide a core source of facts. To model “some arbitrary compatible world”, we just need to capture the various facts that might be true in such a world. This can be done in a “lazy” kind of way: in the compat modality, whenever we are seeing whether a particular fact is true by virtue of the current world, we see whether it’s a fact that could be made true in some compatible world, and if so yield Maybe (with no suggested types).

Let’s look at this concretely, revisiting world 1:

// WORLD 1

////////////////////////////////////////////////////////////
// crate A
////////////////////////////////////////////////////////////

trait Foo {}
struct CrateAType;

////////////////////////////////////////////////////////////
// crate B -- the current crate
////////////////////////////////////////////////////////////

extern crate crate_a;
use crate_a::*;

struct CrateBType;

If we ask CrateAType: Foo, we’ll get No. But if we ask compat { CrateAType: Foo }, then Chalk should switch into “compatible world” mode, so that when it’s consulting the world whether CrateAType: Foo, it will determine that such an impl could be added by crate A, and hence return Maybe. But compat { CrateBType: Foo } will return No, because we know the current crate controls the existence of such an impl. And hence, compat { not { CrateBType: Foo } } returns Yes. (Other than turning on “compatible world” mode, the compat modality just re-invokes the solver and returns up whatever was found.)

This strategy has much in common with the current rustc implementation, but now we have explicit negation (which does the right thing both inside and outside the modality), and we can get away with just this single modality, versus rustc’s two different “global switches”. Moreover, we’ve rationally reconstructed the behavior by connecting it to modal logic, which puts us on better footing for exploring extensions (like negative trait impls and so on).

A word about associated types: as Niko’s latest post discusses, when we lower impls we separately lower the fact that the impl exists from the various projections it provides. For compat, we only need to handle Type: Trait kinds of facts; the “applicative fallback” for associated type projection takes care of the rest.

Altogether, we avoid actually constructing some particular compatible world, and avoid having to guess meaningful facts about it; we just say “Maybe” to any question that could have a different answer in some compatible world.

What’s next

Putting together everything proposed in this post, we’ve achieved quite a bit:

  • A clear meaning for Yes/No/Maybe.
  • A story for the various “modes” in today’s trait system, which means we can support type checking, coherence checking, and trans.
  • A story for integrating the orphan rules into Chalk.
  • A story for integrating the where clause precedence rules into Chalk.
  • A clear treatment of negative reasoning in general, which will allow us to much more confidently employ it in the future.

What’s missing to achieve parity with rustc is specialization. It turns out that the modal foundation laid here provides most of what we need for specialization. However, there are some additional concerns around “lifetime dispatch”, which render rustc’s implementation of specialization unsound. The Chalk implementation should provide a testbed for finally solving those issues. I plan to have a follow-up post about that in the near future.

The design presented here is also just an “on paper” design. I’ll be working to implement it over the coming weeks.