clean up the coherence rules

joint work with lcnr + errs

Author: nikomatsakis

Cargo.lock

@@ -30,6 +30,7 @@ formality-check = { version = "0.1.0", path = "crates/formality-check" }
 formality-prove = { version = "0.1.0", path = "crates/formality-prove" }
 formality-core = { version = "0.1.0", path = "crates/formality-core" }
 formality-smir = { version = "0.1.0", path = "crates/formality-smir" }
+expect-test = "1.4.0"
 
 [workspace]
 members = [

Cargo.toml

@@ -24,7 +24,9 @@ impl Check<'_> {
             self.orphan_check_neg(impl_a)?;
         }
 
-        // check for duplicate impls in the current crate
+        // check for duplicate impls in the current crate;
+        // the cartesian product below would otherwise consider every impl I
+        // as overlapping with itself.
         for (impl_a, i) in current_crate_impls.iter().zip(0..) {
             if current_crate_impls[i + 1..].contains(impl_a) {
                 bail!("duplicate impl in current crate: {:?}", impl_a)

crates/formality-check/src/coherence.rs

@@ -91,7 +91,7 @@ impl<T: Ord + Debug> ProvenSet<T> {
         }
     }
 
-    /// Convert to a non-empty set of proven results (if ok) or an error (otherwise).
+    /// Iterate through all solutions.
     pub fn iter<'a>(&'a self) -> Box<dyn Iterator<Item = &'a T> + 'a> {
         match &self.data {
             Data::Failure(_) => Box::new(std::iter::empty()),

crates/formality-core/src/judgment/proven_set.rs

@@ -1,5 +1,5 @@
 use crate::decls::Decls;
-use formality_core::{ProvenSet, Upcast};
+use formality_core::ProvenSet;
 use formality_types::rust::Term;
 
 use super::{Constraints, Env};
@@ -20,7 +20,7 @@ where
     assert_eq!(a.len(), b.len());
 
     if a.is_empty() && b.is_empty() {
-        return ProvenSet::singleton(Constraints::none(env.upcast()));
+        return ProvenSet::singleton(Constraints::none(env));
     }
 
     let a0 = a.remove(0);
@@ -45,7 +45,7 @@ where
     C: Term,
 {
     if a.is_empty() {
-        return ProvenSet::singleton(Constraints::none(env.upcast()));
+        return ProvenSet::singleton(Constraints::none(env));
     }
 
     let a0 = a[0].clone();

crates/formality-prove/src/prove/combinators.rs

@@ -25,7 +25,7 @@ where
 }
 
 impl Constraints {
-    pub fn none(env: Env) -> Self {
+    pub fn none(env: impl Upcast<Env>) -> Self {
         let v: Vec<(Variable, Parameter)> = vec![];
         Self::from(env, v)
     }
@@ -42,9 +42,10 @@ impl Constraints {
     }
 
     pub fn from(
-        env: Env,
+        env: impl Upcast<Env>,
         iter: impl IntoIterator<Item = (impl Upcast<Variable>, impl Upcast<Parameter>)>,
     ) -> Self {
+        let env = env.upcast();
         let substitution: Substitution = iter.into_iter().collect();
         assert!(env.encloses(substitution.range()));
         assert!(env.encloses(substitution.domain()));
@@ -72,7 +73,7 @@ impl Constraints {
         }
     }
 
-    /// Given constraings from solving the subparts of `(A /\ B)`, yield combined constraints.
+    /// Given constraints from solving the subparts of `(A /\ B)`, yield combined constraints.
     ///
     /// # Parameters
     ///

crates/formality-prove/src/prove/constraints.rs

@@ -1,6 +1,6 @@
-use formality_core::{judgment_fn, set, Set};
+use formality_core::{judgment_fn, ProvenSet, Upcast};
 use formality_types::grammar::{
-    Lt, Parameter, RigidName, RigidTy, TraitRef, TyData, Variable, Wcs,
+    AliasTy, BoundVar, Lt, Parameter, RigidName, RigidTy, TraitRef, TyData, Variable, Wcs,
 };
 
 use crate::{
@@ -37,24 +37,166 @@ use crate::{
 // `Vec<Foo>` is not. `LocalType<ForeignType>` is local. Type aliases and trait
 // aliases do not affect locality.
 
-/// True if `goal` may be remote. This is
-pub fn may_be_remote(decls: Decls, env: Env, assumptions: Wcs, goal: TraitRef) -> Set<Constraints> {
+judgment_fn! {
+    /// True if `goal` may be implemented in a crate that is not the current one.
+    /// This could be a downstream crate that we cannot see, or it could be a future
+    /// (semver-compatible) version of an upstream crate.
+    ///
+    /// Note that per RFC #2451, upstream crates are not permitted to add blanket impls
+    /// and so new upstream impls for local types cannot be added.
+    pub fn may_be_remote(
+        decls: Decls,
+        env: Env,
+        assumptions: Wcs,
+        goal: TraitRef,
+    ) => () {
+        debug(assumptions, goal, decls, env)
+
+        (
+            (may_be_downstream_trait_ref(decls, env, assumptions, goal) => ())
+            --- ("may be defined downstream")
+            (may_be_remote(decls, env, assumptions, goal) => ())
+        )
+
+        (
+            // In principle this rule could be removed and preserve soundness,
+            // but then we would accept code that is very prone to semver failures.
+            (may_not_be_provable(is_local_trait_ref(decls, env, assumptions, goal)) => ())
+            --- ("may be added by upstream in a minor release")
+            (may_be_remote(decls, env, assumptions, goal) => ())
+        )
+    }
+}
+
+judgment_fn! {
+    /// True if an impl defining this trait-reference could appear in a downstream crate.
+    fn may_be_downstream_trait_ref(
+        decls: Decls,
+        env: Env,
+        assumptions: Wcs,
+        goal: TraitRef,
+    ) => () {
+        debug(goal, assumptions, env, decls)
+
+        (
+            // There may be a downstream parameter at position i...
+            (&goal.parameters => p)
+            (may_be_downstream_parameter(&decls, &env, &assumptions, p) => ())
+            --- ("may_be_downstream_trait_ref")
+            (may_be_downstream_trait_ref(decls, env, assumptions, goal) => ())
+        )
+    }
+}
+
+judgment_fn! {
+    fn may_be_downstream_parameter(
+        decls: Decls,
+        env: Env,
+        assumptions: Wcs,
+        parameter: Parameter,
+    ) => () {
+        debug(parameter, assumptions, env, decls)
+
+        (
+            // existential variables *could* be inferred to downstream types; depends on the substitution
+            // we ultimately have.
+            --- ("type variable")
+            (may_be_downstream_parameter(_decls, _env, _assumptions, TyData::Variable(Variable::ExistentialVar(_))) => ())
+        )
+
+        // If we can normalize `goal` to something else, and that normalized
+        // form may be downstream.
+        (
+            // (a) there is some parameter in the alias that may be downstream
+            (parameters.iter() => p)
+            (if may_contain_downstream_type(&decls, &env, &assumptions, p))
+
+            // (b) the alias cannot be normalized to something that may not be downstream
+            (may_not_be_provable(normalizes_to_not_downstream(&decls, &env, &assumptions, AliasTy { name: name.clone(), parameters: parameters.clone() })) => ())
+            --- ("via normalize")
+            (may_be_downstream_parameter(decls, env, assumptions, AliasTy { name, parameters }) => ())
+        )
+    }
+}
+
+fn may_contain_downstream_type(
+    decls: &Decls,
+    env: &Env,
+    assumptions: &Wcs,
+    parameter: impl Upcast<Parameter>,
+) -> bool {
     assert!(env.is_in_coherence_mode());
+    let parameter = parameter.upcast();
 
-    let c = is_local_trait_ref(decls, &env, assumptions, goal);
+    let Parameter::Ty(ty) = parameter else {
+        return false;
+    };
 
-    if !c.is_proven() {
-        // Cannot possibly be local, so always remote.
-        return set![Constraints::none(env)];
+    match ty.data() {
+        TyData::RigidTy(RigidTy {
+            name: _,
+            parameters,
+        }) => parameters
+            .iter()
+            .any(|p| may_contain_downstream_type(decls, env, assumptions, p)),
+        TyData::AliasTy(_) => prove_normalize(decls, env, assumptions, ty)
+            .iter()
+            .any(|(c, p)| {
+                let assumptions = c.substitution().apply(assumptions);
+                may_contain_downstream_type(decls, env, &assumptions, p)
+            }),
+        TyData::PredicateTy(p) => match p {
+            formality_types::grammar::PredicateTy::ForAll(binder) => {
+                let (_, ty) = binder.open();
+                may_contain_downstream_type(decls, env, assumptions, ty)
+            }
+        },
+        TyData::Variable(v) => match v {
+            Variable::ExistentialVar(_) => true,
+            Variable::UniversalVar(_) => panic!("universals are unexpected"),
+            Variable::BoundVar(BoundVar {
+                debruijn,
+                var_index: _,
+                kind: _,
+            }) => {
+                assert!(debruijn.is_none(), "must have been opened on the way down");
+                true
+            }
+        },
     }
+}
 
-    if c.iter().any(Constraints::unconditionally_true) {
-        // If this is unconditionally known to be local, then it is never remote.
-        return set![];
+fn may_not_be_provable(op: ProvenSet<Constraints>) -> ProvenSet<()> {
+    if let Some(constraints) = op
+        .iter()
+        .find(|constraints| constraints.unconditionally_true())
+    {
+        ProvenSet::failed(
+            "may_not_be_provable",
+            format!("found a solution {constraints:?}"),
+        )
+    } else {
+        ProvenSet::singleton(())
     }
+}
 
-    // Otherwise it is ambiguous
-    set![Constraints::none(env).ambiguous()]
+judgment_fn! {
+    fn normalizes_to_not_downstream(
+        decls: Decls,
+        env: Env,
+        assumptions: Wcs,
+        parameter: Parameter,
+    ) => Constraints {
+        debug(parameter, assumptions, env, decls)
+
+        (
+            (prove_normalize(&decls, &env, &assumptions, parameter) => (c1, parameter))
+            (let assumptions = c1.substitution().apply(&assumptions))
+            (is_not_downstream(&decls, &env, assumptions, parameter) => c2)
+            --- ("ambiguous")
+            (normalizes_to_not_downstream(decls, env, assumptions, parameter) => c1.seq(c2))
+        )
+    }
 }
 
 judgment_fn! {
@@ -73,26 +215,29 @@ judgment_fn! {
         )
 
         (
+            // There is a local parameter at position i...
             (0 .. goal.parameters.len() => i)
             (is_local_parameter(&decls, &env, &assumptions, &goal.parameters[i]) => c1)
+
+            // ...and in positions 0..i, there are no downstream parameters.
             (let assumptions = c1.substitution().apply(&assumptions))
             (let goal = c1.substitution().apply(&goal))
-            (for_all(&decls, &env, &assumptions, &goal.parameters[..i], &not_downstream) => c2)
+            (for_all(&decls, &env, &assumptions, &goal.parameters[..i], &is_not_downstream) => c2)
             --- ("local parameter")
             (is_local_trait_ref(decls, env, assumptions, goal) => c1.seq(c2))
         )
     }
 }
 
 judgment_fn! {
-    /// "not_downstream(..., P)" means that `P` cannot be instantiated with a type from
+    /// `is_not_downstream(..., P)` means that `P` cannot be instantiated with a type from
     /// a downstream crate (i.e., a crate that has us as a dependency).
     ///
     /// NB. Since RFC 2451, the judgment applies to the outermost type only. In other words,
     /// the judgment holds for (e.g.) `Vec<T>`, which could be instantiated
     /// with something like `Vec<DownstreamType>`, but that is not considered downstream
     /// as the outermost type (`Vec`) is upstream.
-    fn not_downstream(
+    fn is_not_downstream(
         decls: Decls,
         env: Env,
         assumptions: Wcs,
@@ -104,20 +249,20 @@ judgment_fn! {
             // Since https://rust-lang.github.io/rfcs/2451-re-rebalancing-coherence.html,
             // any rigid type is adequate.
             --- ("rigid")
-            (not_downstream(_decls, env, _assumptions, RigidTy { .. }) => Constraints::none(env))
+            (is_not_downstream(_decls, env, _assumptions, RigidTy { .. }) => Constraints::none(env))
         )
 
         (
             // Lifetimes are not relevant.
             --- ("lifetime")
-            (not_downstream(_decls, env, _assumptions, _l: Lt) => Constraints::none(env))
+            (is_not_downstream(_decls, env, _assumptions, _l: Lt) => Constraints::none(env))
         )
 
         (
             // existential variables *could* be inferred to downstream types; depends on the substitution
             // we ultimately have.
             --- ("type variable")
-            (not_downstream(_decls, env, _assumptions, TyData::Variable(Variable::ExistentialVar(_))) => Constraints::none(env).ambiguous())
+            (is_not_downstream(_decls, env, _assumptions, TyData::Variable(Variable::ExistentialVar(_))) => Constraints::none(env).ambiguous())
         )
     }
 }

crates/formality-prove/src/prove/is_local.rs

@@ -165,7 +165,7 @@ fn equate_variable(
     // For each universal variable that we replaced with an existential variable
     // above, we now have to prove that goal. e.g., if we had `X = Vec<!Y>`, we would replace `!Y` with `?Z`
     // (where `?Z` is in a lower universe than `X`), but now we must prove that `!Y = ?Z`
-    // (this may be posible due to assumptions).
+    // (this may be possible due to assumptions).
     let goals: Wcs = universe_subst
         .iter()
         .filter(|(v, _)| v.is_a::<UniversalVar>())

crates/formality-prove/src/prove/prove_eq.rs

@@ -61,9 +61,9 @@ judgment_fn! {
 
         (
             (if env.is_in_coherence_mode())!
-            (may_be_remote(decls, env, assumptions, trait_ref) => c)
+            (may_be_remote(decls, &env, assumptions, trait_ref) => ())
             ----------------------------- ("coherence / remote impl")
-            (prove_wc(decls, env, assumptions, Predicate::IsImplemented(trait_ref)) => c.ambiguous())
+            (prove_wc(decls, env, assumptions, Predicate::IsImplemented(trait_ref)) => Constraints::none(&env).ambiguous())
         )
 
         (

crates/formality-prove/src/prove/prove_wc.rs

@@ -215,10 +215,10 @@ impl TraitId {
 /// We need a better name for this lol.
 #[term]
 pub enum PR {
-    #[cast]
-    Predicate(Predicate),
     #[cast]
     Relation(Relation),
+    #[cast]
+    Predicate(Predicate),
 }
 
 impl PR {