Chalk integration

 - add proper canonicalization logic
 - add conversions from/to Chalk IR

12 files changed, 669 insertions, 146 deletions

diff --git a/crates/ra_hir/src/code_model_api.rs b/crates/ra_hir/src/code_model_api.rs
index 8f1ed1086..9dcae50a5 100644
--- a/crates/ra_hir/src/code_model_api.rs
+++ b/crates/ra_hir/src/code_model_api.rs
@@ -9,7 +9,7 @@ use crate::{
     type_ref::TypeRef,
     nameres::{ModuleScope, Namespace, ImportId, CrateModuleId},
     expr::{Body, BodySourceMap},
-    ty::InferenceResult,
+    ty::{ TraitRef, InferenceResult},
     adt::{EnumVariantId, StructFieldId, VariantDef},
     generics::HasGenericParams,
     docs::{Documentation, Docs, docs_from_ast},
@@ -696,6 +696,10 @@ impl Trait {
         db.trait_data(self)
     }
 
+    pub fn trait_ref(self, db: &impl HirDatabase) -> TraitRef {
+        TraitRef::for_trait(db, self)
+    }
+
     pub(crate) fn resolver(&self, db: &impl DefDatabase) -> Resolver {
         let r = self.module(db).resolver(db);
         // add generic params, if present
diff --git a/crates/ra_hir/src/db.rs b/crates/ra_hir/src/db.rs
index 398e00c42..b2c4fccf2 100644
--- a/crates/ra_hir/src/db.rs
+++ b/crates/ra_hir/src/db.rs
@@ -1,4 +1,4 @@
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 
 use ra_syntax::{SyntaxNode, TreeArc, SourceFile, SmolStr, ast};
 use ra_db::{SourceDatabase, salsa};
@@ -8,16 +8,16 @@ use crate::{
     Function, FnSignature, ExprScopes, TypeAlias,
     Struct, Enum, StructField,
     Const, ConstSignature, Static,
-    DefWithBody,
+    DefWithBody, Trait,
+    ids,
     nameres::{Namespace, ImportSourceMap, RawItems, CrateDefMap},
-    ty::{InferenceResult, Ty, method_resolution::CrateImplBlocks, TypableDef, CallableDef, FnSig},
+    ty::{InferenceResult, Ty, method_resolution::CrateImplBlocks, TypableDef, CallableDef, FnSig, TypeCtor},
     adt::{StructData, EnumData},
-    impl_block::{ModuleImplBlocks, ImplSourceMap},
+    impl_block::{ModuleImplBlocks, ImplSourceMap, ImplBlock},
     generics::{GenericParams, GenericDef},
     type_ref::TypeRef,
-    traits::TraitData, Trait, ty::TraitRef,
+    traits::TraitData,
     lang_item::{LangItems, LangItemTarget},
-    ids
 };
 
 #[salsa::query_group(DefDatabaseStorage)]
@@ -39,6 +39,12 @@ pub trait DefDatabase: SourceDatabase {
     #[salsa::interned]
     fn intern_type_alias(&self, loc: ids::ItemLoc<ast::TypeAliasDef>) -> ids::TypeAliasId;
 
+    // Interned IDs for Chalk integration
+    #[salsa::interned]
+    fn intern_type_ctor(&self, type_ctor: TypeCtor) -> ids::TypeCtorId;
+    #[salsa::interned]
+    fn intern_impl_block(&self, impl_block: ImplBlock) -> ids::GlobalImplId;
+
     #[salsa::invoke(crate::ids::macro_def_query)]
     fn macro_def(&self, macro_id: MacroDefId) -> Option<Arc<mbe::MacroRules>>;
 
@@ -144,8 +150,12 @@ pub trait HirDatabase: DefDatabase {
     #[salsa::invoke(crate::ty::method_resolution::CrateImplBlocks::impls_in_crate_query)]
     fn impls_in_crate(&self, krate: Crate) -> Arc<CrateImplBlocks>;
 
-    #[salsa::invoke(crate::ty::traits::implements)]
-    fn implements(&self, trait_ref: TraitRef) -> Option<crate::ty::traits::Solution>;
+    #[salsa::invoke(crate::ty::traits::impls_for_trait)]
+    fn impls_for_trait(&self, krate: Crate, trait_: Trait) -> Arc<[ImplBlock]>;
+
+    #[salsa::invoke(crate::ty::traits::solver)]
+    #[salsa::volatile]
+    fn chalk_solver(&self, krate: Crate) -> Arc<Mutex<chalk_solve::Solver>>;
 }
 
 #[test]
diff --git a/crates/ra_hir/src/generics.rs b/crates/ra_hir/src/generics.rs
index 1c71e21ea..2e52c5871 100644
--- a/crates/ra_hir/src/generics.rs
+++ b/crates/ra_hir/src/generics.rs
@@ -9,7 +9,7 @@ use ra_syntax::ast::{self, NameOwner, TypeParamsOwner, TypeBoundsOwner};
 
 use crate::{
     db::DefDatabase,
-    Name, AsName, Function, Struct, Enum, Trait, TypeAlias, ImplBlock, Container, path::Path, type_ref::TypeRef
+    Name, AsName, Function, Struct, Enum, Trait, TypeAlias, ImplBlock, Container, path::Path, type_ref::TypeRef, AdtDef
 };
 
 /// Data about a generic parameter (to a function, struct, impl, ...).
@@ -157,6 +157,15 @@ impl From<Container> for GenericDef {
     }
 }
 
+impl From<crate::adt::AdtDef> for GenericDef {
+    fn from(adt: crate::adt::AdtDef) -> Self {
+        match adt {
+            AdtDef::Struct(s) => s.into(),
+            AdtDef::Enum(e) => e.into(),
+        }
+    }
+}
+
 pub trait HasGenericParams {
     fn generic_params(self, db: &impl DefDatabase) -> Arc<GenericParams>;
 }
diff --git a/crates/ra_hir/src/ids.rs b/crates/ra_hir/src/ids.rs
index 4102951c9..6875b006d 100644
--- a/crates/ra_hir/src/ids.rs
+++ b/crates/ra_hir/src/ids.rs
@@ -323,6 +323,25 @@ impl AstItemDef<ast::TraitDef> for TraitId {
     }
 }
 
+fn from_chalk<T: salsa::InternKey>(chalk_id: chalk_ir::RawId) -> T {
+    T::from_intern_id(salsa::InternId::from(chalk_id.index))
+}
+fn to_chalk<T: salsa::InternKey>(salsa_id: T) -> chalk_ir::RawId {
+    chalk_ir::RawId { index: salsa_id.as_intern_id().as_u32() }
+}
+
+impl From<chalk_ir::TraitId> for TraitId {
+    fn from(trait_id: chalk_ir::TraitId) -> Self {
+        from_chalk(trait_id.0)
+    }
+}
+
+impl From<TraitId> for chalk_ir::TraitId {
+    fn from(trait_id: TraitId) -> Self {
+        chalk_ir::TraitId(to_chalk(trait_id))
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct TypeAliasId(salsa::InternId);
 impl_intern_key!(TypeAliasId);
@@ -354,3 +373,38 @@ impl MacroCallId {
         )
     }
 }
+
+/// This exists just for chalk, because chalk doesn't differentiate between
+/// enums and structs.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct TypeCtorId(salsa::InternId);
+impl_intern_key!(TypeCtorId);
+
+impl From<chalk_ir::StructId> for TypeCtorId {
+    fn from(struct_id: chalk_ir::StructId) -> Self {
+        from_chalk(struct_id.0)
+    }
+}
+
+impl From<TypeCtorId> for chalk_ir::StructId {
+    fn from(adt_id: TypeCtorId) -> Self {
+        chalk_ir::StructId(to_chalk(adt_id))
+    }
+}
+
+/// This exists just for chalk, because our ImplIds are only unique per module.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct GlobalImplId(salsa::InternId);
+impl_intern_key!(GlobalImplId);
+
+impl From<chalk_ir::ImplId> for GlobalImplId {
+    fn from(impl_id: chalk_ir::ImplId) -> Self {
+        from_chalk(impl_id.0)
+    }
+}
+
+impl From<GlobalImplId> for chalk_ir::ImplId {
+    fn from(impl_id: GlobalImplId) -> Self {
+        chalk_ir::ImplId(to_chalk(impl_id))
+    }
+}
diff --git a/crates/ra_hir/src/resolve.rs b/crates/ra_hir/src/resolve.rs
index bc9170cdc..707556ef8 100644
--- a/crates/ra_hir/src/resolve.rs
+++ b/crates/ra_hir/src/resolve.rs
@@ -6,7 +6,7 @@ use ra_syntax::ast;
 use rustc_hash::FxHashMap;
 
 use crate::{
-    ModuleDef,
+    ModuleDef, Trait,
     code_model_api::Crate,
     MacroCallId,
     MacroCallLoc,
@@ -18,7 +18,6 @@ use crate::{
     expr::{scope::{ExprScopes, ScopeId}, PatId},
     impl_block::ImplBlock,
     path::Path,
-    Trait,
 };
 
 #[derive(Debug, Clone, Default)]
diff --git a/crates/ra_hir/src/ty.rs b/crates/ra_hir/src/ty.rs
index 094de62a3..538148956 100644
--- a/crates/ra_hir/src/ty.rs
+++ b/crates/ra_hir/src/ty.rs
@@ -15,7 +15,7 @@ pub(crate) mod display;
 use std::sync::Arc;
 use std::{fmt, mem};
 
-use crate::{Name, AdtDef, type_ref::Mutability, db::HirDatabase, Trait};
+use crate::{Name, AdtDef, type_ref::Mutability, db::HirDatabase, Trait, GenericParams};
 use display::{HirDisplay, HirFormatter};
 
 pub(crate) use lower::{TypableDef, type_for_def, type_for_field, callable_item_sig};
@@ -118,6 +118,7 @@ pub enum Ty {
         /// surrounding impl, then the current function).
         idx: u32,
         /// The name of the parameter, for displaying.
+        // FIXME get rid of this
         name: Name,
     },
 
@@ -177,6 +178,30 @@ impl Substs {
         }
         &self.0[0]
     }
+
+    /// Return Substs that replace each parameter by itself (i.e. `Ty::Param`).
+    pub fn identity(generic_params: &GenericParams) -> Substs {
+        Substs(
+            generic_params
+                .params_including_parent()
+                .into_iter()
+                .map(|p| Ty::Param { idx: p.idx, name: p.name.clone() })
+                .collect::<Vec<_>>()
+                .into(),
+        )
+    }
+
+    /// Return Substs that replace each parameter by a bound variable.
+    pub fn bound_vars(generic_params: &GenericParams) -> Substs {
+        Substs(
+            generic_params
+                .params_including_parent()
+                .into_iter()
+                .map(|p| Ty::Bound(p.idx))
+                .collect::<Vec<_>>()
+                .into(),
+        )
+    }
 }
 
 impl From<Vec<Ty>> for Substs {
@@ -198,6 +223,14 @@ impl TraitRef {
     pub fn self_ty(&self) -> &Ty {
         &self.substs.0[0]
     }
+
+    pub fn subst(mut self, substs: &Substs) -> TraitRef {
+        self.substs.walk_mut(&mut |ty_mut| {
+            let ty = mem::replace(ty_mut, Ty::Unknown);
+            *ty_mut = ty.subst(substs);
+        });
+        self
+    }
 }
 
 /// A function signature as seen by type inference: Several parameter types and
@@ -376,6 +409,20 @@ impl Ty {
         })
     }
 
+    /// Substitutes `Ty::Bound` vars (as opposed to type parameters).
+    pub fn subst_bound_vars(self, substs: &Substs) -> Ty {
+        self.fold(&mut |ty| match ty {
+            Ty::Bound(idx) => {
+                if (idx as usize) < substs.0.len() {
+                    substs.0[idx as usize].clone()
+                } else {
+                    Ty::Bound(idx)
+                }
+            }
+            ty => ty,
+        })
+    }
+
     /// Returns the type parameters of this type if it has some (i.e. is an ADT
     /// or function); so if `self` is `Option<u32>`, this returns the `u32`.
     fn substs(&self) -> Option<Substs> {
diff --git a/crates/ra_hir/src/ty/infer.rs b/crates/ra_hir/src/ty/infer.rs
index c7772a7f6..a6d08dbcb 100644
--- a/crates/ra_hir/src/ty/infer.rs
+++ b/crates/ra_hir/src/ty/infer.rs
@@ -44,9 +44,13 @@ use crate::{
 };
 use super::{
     Ty, TypableDef, Substs, primitive, op, ApplicationTy, TypeCtor, CallableDef, TraitRef,
-    traits::{ Solution, Obligation, Guidance},
+    traits::{Solution, Obligation, Guidance},
 };
 
+mod unify;
+
+pub(super) use unify::Canonical;
+
 /// The entry point of type inference.
 pub fn infer(db: &impl HirDatabase, def: DefWithBody) -> Arc<InferenceResult> {
     db.check_canceled();
@@ -321,30 +325,27 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     fn resolve_obligations_as_possible(&mut self) {
         let obligations = mem::replace(&mut self.obligations, Vec::new());
         for obligation in obligations {
-            // FIXME resolve types in the obligation first
-            let (solution, var_mapping) = match &obligation {
+            let mut canonicalizer = self.canonicalizer();
+            let solution = match &obligation {
                 Obligation::Trait(tr) => {
-                    let (tr, var_mapping) = super::traits::canonicalize(tr.clone());
-                    (self.db.implements(tr), var_mapping)
+                    let canonical = canonicalizer.canonicalize_trait_ref(tr.clone());
+                    super::traits::implements(
+                        canonicalizer.ctx.db,
+                        canonicalizer.ctx.resolver.krate().unwrap(),
+                        canonical,
+                    )
                 }
             };
             match solution {
                 Some(Solution::Unique(substs)) => {
-                    for (i, subst) in substs.0.iter().enumerate() {
-                        let uncanonical = var_mapping[i];
-                        // FIXME the subst may contain type variables, which would need to be mapped back as well
-                        self.unify(&Ty::Infer(InferTy::TypeVar(uncanonical)), subst);
-                    }
+                    canonicalizer.apply_solution(substs.0);
                 }
                 Some(Solution::Ambig(Guidance::Definite(substs))) => {
-                    for (i, subst) in substs.0.iter().enumerate() {
-                        let uncanonical = var_mapping[i];
-                        // FIXME the subst may contain type variables, which would need to be mapped back as well
-                        self.unify(&Ty::Infer(InferTy::TypeVar(uncanonical)), subst);
-                    }
+                    canonicalizer.apply_solution(substs.0);
                     self.obligations.push(obligation);
                 }
                 Some(_) => {
+                    // FIXME use this when trying to resolve everything at the end
                     self.obligations.push(obligation);
                 }
                 None => {
diff --git a/crates/ra_hir/src/ty/infer/unify.rs b/crates/ra_hir/src/ty/infer/unify.rs
new file mode 100644
index 000000000..7918b007b
--- /dev/null
+++ b/crates/ra_hir/src/ty/infer/unify.rs
@@ -0,0 +1,81 @@
+//! Unification and canonicalization logic.
+
+use super::{InferenceContext, Ty, TraitRef, InferTy, HirDatabase};
+
+impl<'a, D: HirDatabase> InferenceContext<'a, D> {
+    pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b, D>
+    where
+        'a: 'b,
+    {
+        Canonicalizer { ctx: self, free_vars: Vec::new() }
+    }
+}
+
+// TODO improve the interface of this
+
+// TODO move further up?
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub(crate) struct Canonical<T> {
+    pub value: T,
+    pub num_vars: usize,
+}
+
+pub(super) struct Canonicalizer<'a, 'b, D: HirDatabase>
+where
+    'a: 'b,
+{
+    pub ctx: &'b mut InferenceContext<'a, D>,
+    pub free_vars: Vec<InferTy>,
+}
+
+impl<'a, 'b, D: HirDatabase> Canonicalizer<'a, 'b, D>
+where
+    'a: 'b,
+{
+    fn add(&mut self, free_var: InferTy) -> usize {
+        self.free_vars.iter().position(|&v| v == free_var).unwrap_or_else(|| {
+            let next_index = self.free_vars.len();
+            self.free_vars.push(free_var);
+            next_index
+        })
+    }
+
+    pub fn canonicalize_ty(&mut self, ty: Ty) -> Canonical<Ty> {
+        let value = ty.fold(&mut |ty| match ty {
+            Ty::Infer(tv) => {
+                let inner = tv.to_inner();
+                // TODO prevent infinite loops? => keep var stack
+                if let Some(known_ty) = self.ctx.var_unification_table.probe_value(inner).known() {
+                    self.canonicalize_ty(known_ty.clone()).value
+                } else {
+                    let free_var = InferTy::TypeVar(self.ctx.var_unification_table.find(inner));
+                    let position = self.add(free_var);
+                    Ty::Bound(position as u32)
+                }
+            }
+            _ => ty,
+        });
+        Canonical { value, num_vars: self.free_vars.len() }
+    }
+
+    pub fn canonicalize_trait_ref(&mut self, trait_ref: TraitRef) -> Canonical<TraitRef> {
+        let substs = trait_ref
+            .substs
+            .0
+            .iter()
+            .map(|ty| self.canonicalize_ty(ty.clone()).value)
+            .collect::<Vec<_>>();
+        let value = TraitRef { trait_: trait_ref.trait_, substs: substs.into() };
+        Canonical { value, num_vars: self.free_vars.len() }
+    }
+
+    pub fn apply_solution(&mut self, solution: Canonical<Vec<Ty>>) {
+        // the solution may contain new variables, which we need to convert to new inference vars
+        let new_vars =
+            (0..solution.num_vars).map(|_| self.ctx.new_type_var()).collect::<Vec<_>>().into();
+        for (i, ty) in solution.value.into_iter().enumerate() {
+            let var = self.free_vars[i].clone();
+            self.ctx.unify(&Ty::Infer(var), &ty.subst_bound_vars(&new_vars));
+        }
+    }
+}
diff --git a/crates/ra_hir/src/ty/lower.rs b/crates/ra_hir/src/ty/lower.rs
index 7fac084ce..2bbd17068 100644
--- a/crates/ra_hir/src/ty/lower.rs
+++ b/crates/ra_hir/src/ty/lower.rs
@@ -238,6 +238,11 @@ impl TraitRef {
         let segment = path.segments.last().expect("path should have at least one segment");
         substs_from_path_segment(db, resolver, segment, &resolved.generic_params(db), true)
     }
+
+    pub(crate) fn for_trait(db: &impl HirDatabase, trait_: Trait) -> TraitRef {
+        let substs = Substs::identity(&trait_.generic_params(db));
+        TraitRef { trait_, substs }
+    }
 }
 
 /// Build the declared type of an item. This depends on the namespace; e.g. for
@@ -299,7 +304,7 @@ fn fn_sig_for_fn(db: &impl HirDatabase, def: Function) -> FnSig {
 /// function body.
 fn type_for_fn(db: &impl HirDatabase, def: Function) -> Ty {
     let generics = def.generic_params(db);
-    let substs = make_substs(&generics);
+    let substs = Substs::identity(&generics);
     Ty::apply(TypeCtor::FnDef(def.into()), substs)
 }
 
@@ -341,7 +346,7 @@ fn type_for_struct_constructor(db: &impl HirDatabase, def: Struct) -> Ty {
         return type_for_struct(db, def); // Unit struct
     }
     let generics = def.generic_params(db);
-    let substs = make_substs(&generics);
+    let substs = Substs::identity(&generics);
     Ty::apply(TypeCtor::FnDef(def.into()), substs)
 }
 
@@ -357,7 +362,7 @@ fn fn_sig_for_enum_variant_constructor(db: &impl HirDatabase, def: EnumVariant)
         .map(|(_, field)| Ty::from_hir(db, &resolver, &field.type_ref))
         .collect::<Vec<_>>();
     let generics = def.parent_enum(db).generic_params(db);
-    let substs = make_substs(&generics);
+    let substs = Substs::identity(&generics);
     let ret = type_for_enum(db, def.parent_enum(db)).subst(&substs);
     FnSig::from_params_and_return(params, ret)
 }
@@ -369,36 +374,25 @@ fn type_for_enum_variant_constructor(db: &impl HirDatabase, def: EnumVariant) ->
         return type_for_enum(db, def.parent_enum(db)); // Unit variant
     }
     let generics = def.parent_enum(db).generic_params(db);
-    let substs = make_substs(&generics);
+    let substs = Substs::identity(&generics);
     Ty::apply(TypeCtor::FnDef(def.into()), substs)
 }
 
-fn make_substs(generics: &GenericParams) -> Substs {
-    Substs(
-        generics
-            .params_including_parent()
-            .into_iter()
-            .map(|p| Ty::Param { idx: p.idx, name: p.name.clone() })
-            .collect::<Vec<_>>()
-            .into(),
-    )
-}
-
 fn type_for_struct(db: &impl HirDatabase, s: Struct) -> Ty {
     let generics = s.generic_params(db);
-    Ty::apply(TypeCtor::Adt(s.into()), make_substs(&generics))
+    Ty::apply(TypeCtor::Adt(s.into()), Substs::identity(&generics))
 }
 
 fn type_for_enum(db: &impl HirDatabase, s: Enum) -> Ty {
     let generics = s.generic_params(db);
-    Ty::apply(TypeCtor::Adt(s.into()), make_substs(&generics))
+    Ty::apply(TypeCtor::Adt(s.into()), Substs::identity(&generics))
 }
 
 fn type_for_type_alias(db: &impl HirDatabase, t: TypeAlias) -> Ty {
     let generics = t.generic_params(db);
     let resolver = t.resolver(db);
     let type_ref = t.type_ref(db);
-    let substs = make_substs(&generics);
+    let substs = Substs::identity(&generics);
     let inner = Ty::from_hir(db, &resolver, &type_ref);
     inner.subst(&substs)
 }
diff --git a/crates/ra_hir/src/ty/method_resolution.rs b/crates/ra_hir/src/ty/method_resolution.rs
index ea6e0dc0f..ea7da0b62 100644
--- a/crates/ra_hir/src/ty/method_resolution.rs
+++ b/crates/ra_hir/src/ty/method_resolution.rs
@@ -16,7 +16,7 @@ use crate::{
     generics::HasGenericParams,
     ty::primitive::{UncertainIntTy, UncertainFloatTy}
 };
-use super::{TraitRef, Substs};
+use super::{TraitRef, infer::Canonical, Substs};
 
 /// This is used as a key for indexing impls.
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
@@ -183,6 +183,7 @@ impl Ty {
         name: Option<&Name>,
         mut callback: impl FnMut(&Ty, Function) -> Option<T>,
     ) -> Option<T> {
+        let krate = resolver.krate()?;
         'traits: for t in resolver.traits_in_scope() {
             let data = t.trait_data(db);
             // we'll be lazy about checking whether the type implements the
@@ -195,12 +196,19 @@ impl Ty {
                         let sig = m.signature(db);
                         if name.map_or(true, |name| sig.name() == name) && sig.has_self_param() {
                             if !known_implemented {
+                                // TODO the self type may contain type
+                                // variables, so we need to do proper
+                                // canonicalization here
                                 let trait_ref = TraitRef {
                                     trait_: t,
                                     substs: fresh_substs_for_trait(db, t, self.clone()),
                                 };
-                                let (trait_ref, _) = super::traits::canonicalize(trait_ref);
-                                if db.implements(trait_ref).is_none() {
+                                let canonical = Canonical {
+                                    num_vars: trait_ref.substs.len(),
+                                    value: trait_ref,
+                                };
+                                // FIXME cache this implements check (without solution) in a query?
+                                if super::traits::implements(db, krate, canonical).is_none() {
                                     continue 'traits;
                                 }
                             }
@@ -271,15 +279,18 @@ impl Ty {
 }
 
 /// This creates Substs for a trait with the given Self type and type variables
-/// for all other parameters. This is kind of a hack since these aren't 'real'
-/// type variables; the resulting trait reference is just used for the
-/// preliminary method candidate check.
+/// for all other parameters, to query Chalk with it.
 fn fresh_substs_for_trait(db: &impl HirDatabase, tr: Trait, self_ty: Ty) -> Substs {
     let mut substs = Vec::new();
     let generics = tr.generic_params(db);
     substs.push(self_ty);
-    substs.extend(generics.params_including_parent().into_iter().skip(1).enumerate().map(
-        |(i, _p)| Ty::Infer(super::infer::InferTy::TypeVar(super::infer::TypeVarId(i as u32))),
-    ));
+    substs.extend(
+        generics
+            .params_including_parent()
+            .into_iter()
+            .skip(1)
+            .enumerate()
+            .map(|(i, _p)| Ty::Bound(i as u32)),
+    );
     substs.into()
 }
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index c76a5012f..0aecde37c 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -2140,7 +2140,7 @@ fn test() {
 [102; 127) '{     ...d(); }': ()
 [108; 109) 'S': S<u32>(T) -> S<T>
 [108; 115) 'S(1u32)': S<u32>
-[108; 124) 'S(1u32...thod()': {unknown}
+[108; 124) 'S(1u32...thod()': u32
 [110; 114) '1u32': u32"###
     );
 }
diff --git a/crates/ra_hir/src/ty/traits.rs b/crates/ra_hir/src/ty/traits.rs
index 06f483899..367322ed2 100644
--- a/crates/ra_hir/src/ty/traits.rs
+++ b/crates/ra_hir/src/ty/traits.rs
@@ -1,47 +1,329 @@
-//! Stuff that will probably mostly replaced by Chalk.
-use std::collections::HashMap;
+//! Chalk integration.
+use std::sync::{Arc, Mutex};
 
-use crate::{db::HirDatabase, generics::HasGenericParams};
-use super::{TraitRef, Substs, infer::{TypeVarId, InferTy}, Ty};
+use chalk_ir::{TypeId, TraitId, StructId, ImplId, TypeKindId, ProjectionTy, Parameter, Identifier, cast::Cast};
+use chalk_rust_ir::{AssociatedTyDatum, TraitDatum, StructDatum, ImplDatum};
 
-// Copied (and simplified) from Chalk
+use crate::{Crate, Trait, db::HirDatabase, HasGenericParams, ImplBlock};
+use super::{TraitRef, Ty, ApplicationTy, TypeCtor, Substs, infer::Canonical};
 
-#[derive(Clone, Debug, PartialEq, Eq)]
-/// A (possible) solution for a proposed goal. Usually packaged in a `Result`,
-/// where `Err` represents definite *failure* to prove a goal.
-pub enum Solution {
-    /// The goal indeed holds, and there is a unique value for all existential
-    /// variables.
-    Unique(Substs),
+#[derive(Debug, Copy, Clone)]
+struct ChalkContext<'a, DB> {
+    db: &'a DB,
+    krate: Crate,
+}
 
-    /// The goal may be provable in multiple ways, but regardless we may have some guidance
-    /// for type inference.
-    Ambig(Guidance),
+pub(crate) trait ToChalk {
+    type Chalk;
+    fn to_chalk(self, db: &impl HirDatabase) -> Self::Chalk;
+    fn from_chalk(db: &impl HirDatabase, chalk: Self::Chalk) -> Self;
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
-/// When a goal holds ambiguously (e.g., because there are multiple possible
-/// solutions), we issue a set of *guidance* back to type inference.
-pub enum Guidance {
-    /// The existential variables *must* have the given values if the goal is
-    /// ever to hold, but that alone isn't enough to guarantee the goal will
-    /// actually hold.
-    Definite(Substs),
+pub(crate) fn from_chalk<T, ChalkT>(db: &impl HirDatabase, chalk: ChalkT) -> T
+where
+    T: ToChalk<Chalk = ChalkT>,
+{
+    T::from_chalk(db, chalk)
+}
 
-    /// There are multiple plausible values for the existentials, but the ones
-    /// here are suggested as the preferred choice heuristically. These should
-    /// be used for inference fallback only.
-    Suggested(Substs),
+impl ToChalk for Ty {
+    type Chalk = chalk_ir::Ty;
+    fn to_chalk(self, db: &impl HirDatabase) -> chalk_ir::Ty {
+        match self {
+            Ty::Apply(apply_ty) => chalk_ir::Ty::Apply(apply_ty.to_chalk(db)),
+            Ty::Param { idx, .. } => {
+                chalk_ir::PlaceholderIndex { ui: chalk_ir::UniverseIndex::ROOT, idx: idx as usize }
+                    .to_ty()
+            }
+            Ty::Bound(idx) => chalk_ir::Ty::BoundVar(idx as usize),
+            Ty::Infer(_infer_ty) => panic!("uncanonicalized infer ty"),
+            Ty::Unknown => unimplemented!(), // TODO turn into placeholder?
+        }
+    }
+    fn from_chalk(db: &impl HirDatabase, chalk: chalk_ir::Ty) -> Self {
+        match chalk {
+            chalk_ir::Ty::Apply(apply_ty) => {
+                match apply_ty.name {
+                    // FIXME handle TypeKindId::Trait/Type here
+                    chalk_ir::TypeName::TypeKindId(_) => Ty::Apply(from_chalk(db, apply_ty)),
+                    chalk_ir::TypeName::AssociatedType(_) => unimplemented!(),
+                    chalk_ir::TypeName::Placeholder(idx) => {
+                        assert_eq!(idx.ui, chalk_ir::UniverseIndex::ROOT);
+                        Ty::Param { idx: idx.idx as u32, name: crate::Name::missing() }
+                    }
+                }
+            }
+            chalk_ir::Ty::Projection(_) => unimplemented!(),
+            chalk_ir::Ty::UnselectedProjection(_) => unimplemented!(),
+            chalk_ir::Ty::ForAll(_) => unimplemented!(),
+            chalk_ir::Ty::BoundVar(idx) => Ty::Bound(idx as u32),
+            chalk_ir::Ty::InferenceVar(_iv) => panic!("unexpected chalk infer ty"),
+        }
+    }
+}
 
-    /// There's no useful information to feed back to type inference
-    Unknown,
+impl ToChalk for ApplicationTy {
+    type Chalk = chalk_ir::ApplicationTy;
+
+    fn to_chalk(self: ApplicationTy, db: &impl HirDatabase) -> chalk_ir::ApplicationTy {
+        let struct_id = self.ctor.to_chalk(db);
+        let name = chalk_ir::TypeName::TypeKindId(struct_id.into());
+        let parameters = self.parameters.to_chalk(db);
+        chalk_ir::ApplicationTy { name, parameters }
+    }
+
+    fn from_chalk(db: &impl HirDatabase, apply_ty: chalk_ir::ApplicationTy) -> ApplicationTy {
+        let ctor = match apply_ty.name {
+            chalk_ir::TypeName::TypeKindId(chalk_ir::TypeKindId::StructId(struct_id)) => {
+                from_chalk(db, struct_id)
+            }
+            chalk_ir::TypeName::TypeKindId(_) => unimplemented!(),
+            chalk_ir::TypeName::Placeholder(_) => unimplemented!(),
+            chalk_ir::TypeName::AssociatedType(_) => unimplemented!(),
+        };
+        let parameters = from_chalk(db, apply_ty.parameters);
+        ApplicationTy { ctor, parameters }
+    }
+}
+
+impl ToChalk for Substs {
+    type Chalk = Vec<chalk_ir::Parameter>;
+
+    fn to_chalk(self, db: &impl HirDatabase) -> Vec<chalk_ir::Parameter> {
+        self.iter().map(|ty| ty.clone().to_chalk(db).cast()).collect()
+    }
+
+    fn from_chalk(db: &impl HirDatabase, parameters: Vec<chalk_ir::Parameter>) -> Substs {
+        parameters
+            .into_iter()
+            .map(|p| match p {
+                chalk_ir::Parameter(chalk_ir::ParameterKind::Ty(ty)) => from_chalk(db, ty),
+                chalk_ir::Parameter(chalk_ir::ParameterKind::Lifetime(_)) => unimplemented!(),
+            })
+            .collect::<Vec<_>>()
+            .into()
+    }
+}
+
+impl ToChalk for TraitRef {
+    type Chalk = chalk_ir::TraitRef;
+
+    fn to_chalk(self: TraitRef, db: &impl HirDatabase) -> chalk_ir::TraitRef {
+        let trait_id = self.trait_.to_chalk(db);
+        let parameters = self.substs.to_chalk(db);
+        chalk_ir::TraitRef { trait_id, parameters }
+    }
+
+    fn from_chalk(db: &impl HirDatabase, trait_ref: chalk_ir::TraitRef) -> Self {
+        let trait_ = from_chalk(db, trait_ref.trait_id);
+        let substs = from_chalk(db, trait_ref.parameters);
+        TraitRef { trait_, substs }
+    }
+}
+
+impl ToChalk for Trait {
+    type Chalk = TraitId;
+
+    fn to_chalk(self, _db: &impl HirDatabase) -> TraitId {
+        self.id.into()
+    }
+
+    fn from_chalk(_db: &impl HirDatabase, trait_id: TraitId) -> Trait {
+        Trait { id: trait_id.into() }
+    }
+}
+
+impl ToChalk for TypeCtor {
+    type Chalk = chalk_ir::StructId;
+
+    fn to_chalk(self, db: &impl HirDatabase) -> chalk_ir::StructId {
+        db.intern_type_ctor(self).into()
+    }
+
+    fn from_chalk(db: &impl HirDatabase, struct_id: chalk_ir::StructId) -> TypeCtor {
+        db.lookup_intern_type_ctor(struct_id.into())
+    }
+}
+
+impl ToChalk for ImplBlock {
+    type Chalk = chalk_ir::ImplId;
+
+    fn to_chalk(self, db: &impl HirDatabase) -> chalk_ir::ImplId {
+        db.intern_impl_block(self).into()
+    }
+
+    fn from_chalk(db: &impl HirDatabase, impl_id: chalk_ir::ImplId) -> ImplBlock {
+        db.lookup_intern_impl_block(impl_id.into())
+    }
+}
+
+fn make_binders<T>(value: T, num_vars: usize) -> chalk_ir::Binders<T> {
+    chalk_ir::Binders {
+        value,
+        binders: std::iter::repeat(chalk_ir::ParameterKind::Ty(())).take(num_vars).collect(),
+    }
+}
+
+impl<'a, DB> chalk_solve::RustIrDatabase for ChalkContext<'a, DB>
+where
+    DB: HirDatabase,
+{
+    fn associated_ty_data(&self, _ty: TypeId) -> Arc<AssociatedTyDatum> {
+        unimplemented!()
+    }
+    fn trait_datum(&self, trait_id: TraitId) -> Arc<TraitDatum> {
+        eprintln!("trait_datum {:?}", trait_id);
+        let trait_: Trait = from_chalk(self.db, trait_id);
+        let generic_params = trait_.generic_params(self.db);
+        let bound_vars = Substs::bound_vars(&generic_params);
+        let trait_ref = trait_.trait_ref(self.db).subst(&bound_vars).to_chalk(self.db);
+        let flags = chalk_rust_ir::TraitFlags {
+            // FIXME set these flags correctly
+            auto: false,
+            marker: false,
+            upstream: trait_.module(self.db).krate(self.db) != Some(self.krate),
+            fundamental: false,
+        };
+        let where_clauses = Vec::new(); // FIXME add where clauses
+        let trait_datum_bound = chalk_rust_ir::TraitDatumBound { trait_ref, where_clauses, flags };
+        let trait_datum = TraitDatum { binders: make_binders(trait_datum_bound, bound_vars.len()) };
+        Arc::new(trait_datum)
+    }
+    fn struct_datum(&self, struct_id: StructId) -> Arc<StructDatum> {
+        eprintln!("struct_datum {:?}", struct_id);
+        let type_ctor = from_chalk(self.db, struct_id);
+        // TODO might be nicer if we can create a fake GenericParams for the TypeCtor
+        let (num_params, upstream) = match type_ctor {
+            TypeCtor::Bool
+            | TypeCtor::Char
+            | TypeCtor::Int(_)
+            | TypeCtor::Float(_)
+            | TypeCtor::Never
+            | TypeCtor::Str => (0, true),
+            TypeCtor::Slice | TypeCtor::Array | TypeCtor::RawPtr(_) | TypeCtor::Ref(_) => (1, true),
+            TypeCtor::FnPtr | TypeCtor::Tuple => unimplemented!(), // FIXME tuples and FnPtr are currently variadic... we need to make the parameter number explicit
+            TypeCtor::FnDef(_) => unimplemented!(),
+            TypeCtor::Adt(adt) => {
+                let generic_params = adt.generic_params(self.db);
+                (
+                    generic_params.count_params_including_parent(),
+                    adt.krate(self.db) != Some(self.krate),
+                )
+            }
+        };
+        let flags = chalk_rust_ir::StructFlags {
+            upstream,
+            // FIXME set fundamental flag correctly
+            fundamental: false,
+        };
+        let where_clauses = Vec::new(); // FIXME add where clauses
+        let ty = ApplicationTy {
+            ctor: type_ctor,
+            parameters: (0..num_params).map(|i| Ty::Bound(i as u32)).collect::<Vec<_>>().into(),
+        };
+        let struct_datum_bound = chalk_rust_ir::StructDatumBound {
+            self_ty: ty.to_chalk(self.db),
+            fields: Vec::new(), // FIXME add fields (only relevant for auto traits)
+            where_clauses,
+            flags,
+        };
+        let struct_datum = StructDatum { binders: make_binders(struct_datum_bound, num_params) };
+        Arc::new(struct_datum)
+    }
+    fn impl_datum(&self, impl_id: ImplId) -> Arc<ImplDatum> {
+        eprintln!("impl_datum {:?}", impl_id);
+        let impl_block: ImplBlock = from_chalk(self.db, impl_id);
+        let generic_params = impl_block.generic_params(self.db);
+        let bound_vars = Substs::bound_vars(&generic_params);
+        let trait_ref = impl_block
+            .target_trait_ref(self.db)
+            .expect("FIXME handle unresolved impl block trait ref")
+            .subst(&bound_vars);
+        let impl_type = if impl_block.module().krate(self.db) == Some(self.krate) {
+            chalk_rust_ir::ImplType::Local
+        } else {
+            chalk_rust_ir::ImplType::External
+        };
+        let impl_datum_bound = chalk_rust_ir::ImplDatumBound {
+            // FIXME handle negative impls (impl !Sync for Foo)
+            trait_ref: chalk_rust_ir::PolarizedTraitRef::Positive(trait_ref.to_chalk(self.db)),
+            where_clauses: Vec::new(),        // FIXME add where clauses
+            associated_ty_values: Vec::new(), // FIXME add associated type values
+            impl_type,
+        };
+        let impl_datum = ImplDatum { binders: make_binders(impl_datum_bound, bound_vars.len()) };
+        Arc::new(impl_datum)
+    }
+    fn impls_for_trait(&self, trait_id: TraitId) -> Vec<ImplId> {
+        eprintln!("impls_for_trait {:?}", trait_id);
+        let trait_ = from_chalk(self.db, trait_id);
+        self.db
+            .impls_for_trait(self.krate, trait_)
+            .iter()
+            // FIXME temporary hack -- as long as we're not lowering where clauses
+            // correctly, ignore impls with them completely so as to not treat
+            // impl<T> Trait for T where T: ... as a blanket impl on all types
+            .filter(|impl_block| impl_block.generic_params(self.db).where_predicates.is_empty())
+            .map(|impl_block| impl_block.to_chalk(self.db))
+            .collect()
+    }
+    fn impl_provided_for(&self, auto_trait_id: TraitId, struct_id: StructId) -> bool {
+        eprintln!("impl_provided_for {:?}, {:?}", auto_trait_id, struct_id);
+        false // FIXME
+    }
+    fn type_name(&self, _id: TypeKindId) -> Identifier {
+        unimplemented!()
+    }
+    fn split_projection<'p>(
+        &self,
+        projection: &'p ProjectionTy,
+    ) -> (Arc<AssociatedTyDatum>, &'p [Parameter], &'p [Parameter]) {
+        eprintln!("split_projection {:?}", projection);
+        unimplemented!()
+    }
+}
+
+pub(crate) fn solver(_db: &impl HirDatabase, _krate: Crate) -> Arc<Mutex<chalk_solve::Solver>> {
+    // krate parameter is just so we cache a unique solver per crate
+    let solver_choice = chalk_solve::SolverChoice::SLG { max_size: 10 };
+    Arc::new(Mutex::new(solver_choice.into_solver()))
+}
+
+/// Collects impls for the given trait in the whole dependency tree of `krate`.
+pub(crate) fn impls_for_trait(
+    db: &impl HirDatabase,
+    krate: Crate,
+    trait_: Trait,
+) -> Arc<[ImplBlock]> {
+    let mut impls = Vec::new();
+    // We call the query recursively here. On the one hand, this means we can
+    // reuse results from queries for different crates; on the other hand, this
+    // will only ever get called for a few crates near the root of the tree (the
+    // ones the user is editing), so this may actually be a waste of memory. I'm
+    // doing it like this mainly for simplicity for now.
+    for dep in krate.dependencies(db) {
+        impls.extend(db.impls_for_trait(dep.krate, trait_).iter());
+    }
+    let crate_impl_blocks = db.impls_in_crate(krate);
+    impls.extend(crate_impl_blocks.lookup_impl_blocks_for_trait(&trait_));
+    impls.into()
+}
+
+fn solve(
+    db: &impl HirDatabase,
+    krate: Crate,
+    goal: &chalk_ir::UCanonical<chalk_ir::InEnvironment<chalk_ir::Goal>>,
+) -> Option<chalk_solve::Solution> {
+    let context = ChalkContext { db, krate };
+    let solver = db.chalk_solver(krate);
+    let solution = solver.lock().unwrap().solve(&context, goal);
+    eprintln!("solve({:?}) => {:?}", goal, solution);
+    solution
 }
 
 /// Something that needs to be proven (by Chalk) during type checking, e.g. that
 /// a certain type implements a certain trait. Proving the Obligation might
 /// result in additional information about inference variables.
-///
-/// This might be handled by Chalk when we integrate it?
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum Obligation {
     /// Prove that a certain type implements a trait (the type is the `Self` type
@@ -49,67 +331,98 @@ pub enum Obligation {
     Trait(TraitRef),
 }
 
-/// Rudimentary check whether an impl exists for a given type and trait; this
-/// will actually be done by chalk.
-pub(crate) fn implements(db: &impl HirDatabase, trait_ref: TraitRef) -> Option<Solution> {
-    // FIXME use all trait impls in the whole crate graph
-    let krate = trait_ref.trait_.module(db).krate(db);
-    let krate = match krate {
-        Some(krate) => krate,
-        None => return None,
-    };
-    let crate_impl_blocks = db.impls_in_crate(krate);
-    let mut impl_blocks = crate_impl_blocks
-        .lookup_impl_blocks_for_trait(&trait_ref.trait_)
-        // we don't handle where clauses at all, waiting for Chalk for that
-        .filter(|impl_block| impl_block.generic_params(db).where_predicates.is_empty());
-    impl_blocks
-        .find_map(|impl_block| unify_trait_refs(&trait_ref, &impl_block.target_trait_ref(db)?))
-}
-
-pub(super) fn canonicalize(trait_ref: TraitRef) -> (TraitRef, Vec<TypeVarId>) {
-    let mut canonical = HashMap::new(); // mapping uncanonical -> canonical
-    let mut uncanonical = Vec::new(); // mapping canonical -> uncanonical (which is dense)
-    let mut substs = trait_ref.substs.0.to_vec();
-    for ty in &mut substs {
-        ty.walk_mut(&mut |ty| match ty {
-            Ty::Infer(InferTy::TypeVar(tv)) => {
-                let tv: &mut TypeVarId = tv;
-                *tv = *canonical.entry(*tv).or_insert_with(|| {
-                    let i = uncanonical.len();
-                    uncanonical.push(*tv);
-                    TypeVarId(i as u32)
-                });
-            }
-            _ => {}
-        });
-    }
-    (TraitRef { substs: substs.into(), ..trait_ref }, uncanonical)
+/// Check using Chalk whether trait is implemented for given parameters including `Self` type.
+pub(crate) fn implements(
+    db: &impl HirDatabase,
+    krate: Crate,
+    trait_ref: Canonical<TraitRef>,
+) -> Option<Solution> {
+    let goal: chalk_ir::Goal = trait_ref.value.to_chalk(db).cast();
+    eprintln!("goal: {:?}", goal);
+    let env = chalk_ir::Environment::new();
+    let in_env = chalk_ir::InEnvironment::new(&env, goal);
+    let parameter = chalk_ir::ParameterKind::Ty(chalk_ir::UniverseIndex::ROOT);
+    let canonical =
+        chalk_ir::Canonical { value: in_env, binders: vec![parameter; trait_ref.num_vars] };
+    // We currently don't deal with universes (I think / hope they're not yet
+    // relevant for our use cases?)
+    let u_canonical = chalk_ir::UCanonical { canonical, universes: 1 };
+    let solution = solve(db, krate, &u_canonical);
+    solution_from_chalk(db, solution)
 }
 
-fn unify_trait_refs(tr1: &TraitRef, tr2: &TraitRef) -> Option<Solution> {
-    if tr1.trait_ != tr2.trait_ {
-        return None;
-    }
-    let mut solution_substs = Vec::new();
-    for (t1, t2) in tr1.substs.0.iter().zip(tr2.substs.0.iter()) {
-        // this is very bad / hacky 'unification' logic, just enough to make the simple tests pass
-        match (t1, t2) {
-            (_, Ty::Infer(InferTy::TypeVar(_))) | (_, Ty::Unknown) | (_, Ty::Param { .. }) => {
-                // type variable (or similar) in the impl, we just assume it works
-            }
-            (Ty::Infer(InferTy::TypeVar(v1)), _) => {
-                // type variable in the query and fixed type in the impl, record its value
-                solution_substs.resize_with(v1.0 as usize + 1, || Ty::Unknown);
-                solution_substs[v1.0 as usize] = t2.clone();
-            }
-            _ => {
-                // check that they're equal (actually we'd have to recurse etc.)
-                if t1 != t2 {
-                    return None;
-                }
-            }
+fn solution_from_chalk(
+    db: &impl HirDatabase,
+    solution: Option<chalk_solve::Solution>,
+) -> Option<Solution> {
+    let convert_subst = |subst: chalk_ir::Canonical<chalk_ir::Substitution>| {
+        let value = subst
+            .value
+            .parameters
+            .into_iter()
+            .map(|p| {
+                let ty = match p {
+                    chalk_ir::Parameter(chalk_ir::ParameterKind::Ty(ty)) => from_chalk(db, ty),
+                    chalk_ir::Parameter(chalk_ir::ParameterKind::Lifetime(_)) => unimplemented!(),
+                };
+                ty
+            })
+            .collect();
+        let result = Canonical { value, num_vars: subst.binders.len() };
+        SolutionVariables(result)
+    };
+    match solution {
+        Some(chalk_solve::Solution::Unique(constr_subst)) => {
+            let subst = chalk_ir::Canonical {
+                value: constr_subst.value.subst,
+                binders: constr_subst.binders,
+            };
+            Some(Solution::Unique(convert_subst(subst)))
+        }
+        Some(chalk_solve::Solution::Ambig(chalk_solve::Guidance::Definite(subst))) => {
+            Some(Solution::Ambig(Guidance::Definite(convert_subst(subst))))
+        }
+        Some(chalk_solve::Solution::Ambig(chalk_solve::Guidance::Suggested(subst))) => {
+            Some(Solution::Ambig(Guidance::Suggested(convert_subst(subst))))
         }
+        Some(chalk_solve::Solution::Ambig(chalk_solve::Guidance::Unknown)) => {
+            Some(Solution::Ambig(Guidance::Unknown))
+        }
+        None => None,
     }
-    Some(Solution::Unique(solution_substs.into()))
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(crate) struct SolutionVariables(pub Canonical<Vec<Ty>>);
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+/// A (possible) solution for a proposed goal.
+pub(crate) enum Solution {
+    /// The goal indeed holds, and there is a unique value for all existential
+    /// variables.
+    Unique(SolutionVariables),
+
+    /// The goal may be provable in multiple ways, but regardless we may have some guidance
+    /// for type inference. In this case, we don't return any lifetime
+    /// constraints, since we have not "committed" to any particular solution
+    /// yet.
+    Ambig(Guidance),
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+/// When a goal holds ambiguously (e.g., because there are multiple possible
+/// solutions), we issue a set of *guidance* back to type inference.
+pub(crate) enum Guidance {
+    /// The existential variables *must* have the given values if the goal is
+    /// ever to hold, but that alone isn't enough to guarantee the goal will
+    /// actually hold.
+    Definite(SolutionVariables),
+
+    /// There are multiple plausible values for the existentials, but the ones
+    /// here are suggested as the preferred choice heuristically. These should
+    /// be used for inference fallback only.
+    Suggested(SolutionVariables),
+
+    /// There's no useful information to feed back to type inference
+    Unknown,
 }