1 files changed, 162 insertions, 0 deletions

diff --git a/crates/ra_hir_ty/src/infer/unify.rs b/crates/ra_hir_ty/src/infer/unify.rs
new file mode 100644
index 000000000..f3a875678
--- /dev/null
+++ b/crates/ra_hir_ty/src/infer/unify.rs
@@ -0,0 +1,162 @@
+//! Unification and canonicalization logic.
+
+use super::{InferenceContext, Obligation};
+use crate::{
+    db::HirDatabase, utils::make_mut_slice, Canonical, InEnvironment, InferTy, ProjectionPredicate,
+    ProjectionTy, Substs, TraitRef, Ty, TypeWalk,
+};
+
+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(), var_stack: Vec::new() }
+    }
+}
+
+pub(super) struct Canonicalizer<'a, 'b, D: HirDatabase>
+where
+    'a: 'b,
+{
+    ctx: &'b mut InferenceContext<'a, D>,
+    free_vars: Vec<InferTy>,
+    /// A stack of type variables that is used to detect recursive types (which
+    /// are an error, but we need to protect against them to avoid stack
+    /// overflows).
+    var_stack: Vec<super::TypeVarId>,
+}
+
+pub(super) struct Canonicalized<T> {
+    pub value: Canonical<T>,
+    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
+        })
+    }
+
+    fn do_canonicalize_ty(&mut self, ty: Ty) -> Ty {
+        ty.fold(&mut |ty| match ty {
+            Ty::Infer(tv) => {
+                let inner = tv.to_inner();
+                if self.var_stack.contains(&inner) {
+                    // recursive type
+                    return tv.fallback_value();
+                }
+                if let Some(known_ty) =
+                    self.ctx.var_unification_table.inlined_probe_value(inner).known()
+                {
+                    self.var_stack.push(inner);
+                    let result = self.do_canonicalize_ty(known_ty.clone());
+                    self.var_stack.pop();
+                    result
+                } else {
+                    let root = self.ctx.var_unification_table.find(inner);
+                    let free_var = match tv {
+                        InferTy::TypeVar(_) => InferTy::TypeVar(root),
+                        InferTy::IntVar(_) => InferTy::IntVar(root),
+                        InferTy::FloatVar(_) => InferTy::FloatVar(root),
+                        InferTy::MaybeNeverTypeVar(_) => InferTy::MaybeNeverTypeVar(root),
+                    };
+                    let position = self.add(free_var);
+                    Ty::Bound(position as u32)
+                }
+            }
+            _ => ty,
+        })
+    }
+
+    fn do_canonicalize_trait_ref(&mut self, mut trait_ref: TraitRef) -> TraitRef {
+        for ty in make_mut_slice(&mut trait_ref.substs.0) {
+            *ty = self.do_canonicalize_ty(ty.clone());
+        }
+        trait_ref
+    }
+
+    fn into_canonicalized<T>(self, result: T) -> Canonicalized<T> {
+        Canonicalized {
+            value: Canonical { value: result, num_vars: self.free_vars.len() },
+            free_vars: self.free_vars,
+        }
+    }
+
+    fn do_canonicalize_projection_ty(&mut self, mut projection_ty: ProjectionTy) -> ProjectionTy {
+        for ty in make_mut_slice(&mut projection_ty.parameters.0) {
+            *ty = self.do_canonicalize_ty(ty.clone());
+        }
+        projection_ty
+    }
+
+    fn do_canonicalize_projection_predicate(
+        &mut self,
+        projection: ProjectionPredicate,
+    ) -> ProjectionPredicate {
+        let ty = self.do_canonicalize_ty(projection.ty);
+        let projection_ty = self.do_canonicalize_projection_ty(projection.projection_ty);
+
+        ProjectionPredicate { ty, projection_ty }
+    }
+
+    // FIXME: add some point, we need to introduce a `Fold` trait that abstracts
+    // over all the things that can be canonicalized (like Chalk and rustc have)
+
+    pub(crate) fn canonicalize_ty(mut self, ty: Ty) -> Canonicalized<Ty> {
+        let result = self.do_canonicalize_ty(ty);
+        self.into_canonicalized(result)
+    }
+
+    pub(crate) fn canonicalize_obligation(
+        mut self,
+        obligation: InEnvironment<Obligation>,
+    ) -> Canonicalized<InEnvironment<Obligation>> {
+        let result = match obligation.value {
+            Obligation::Trait(tr) => Obligation::Trait(self.do_canonicalize_trait_ref(tr)),
+            Obligation::Projection(pr) => {
+                Obligation::Projection(self.do_canonicalize_projection_predicate(pr))
+            }
+        };
+        self.into_canonicalized(InEnvironment {
+            value: result,
+            environment: obligation.environment,
+        })
+    }
+}
+
+impl<T> Canonicalized<T> {
+    pub fn decanonicalize_ty(&self, mut ty: Ty) -> Ty {
+        ty.walk_mut_binders(
+            &mut |ty, binders| match ty {
+                &mut Ty::Bound(idx) => {
+                    if idx as usize >= binders && (idx as usize - binders) < self.free_vars.len() {
+                        *ty = Ty::Infer(self.free_vars[idx as usize - binders]);
+                    }
+                }
+                _ => {}
+            },
+            0,
+        );
+        ty
+    }
+
+    pub fn apply_solution(
+        &self,
+        ctx: &mut InferenceContext<'_, impl HirDatabase>,
+        solution: Canonical<Vec<Ty>>,
+    ) {
+        // the solution may contain new variables, which we need to convert to new inference vars
+        let new_vars = Substs((0..solution.num_vars).map(|_| ctx.new_type_var()).collect());
+        for (i, ty) in solution.value.into_iter().enumerate() {
+            let var = self.free_vars[i];
+            ctx.unify(&Ty::Infer(var), &ty.subst_bound_vars(&new_vars));
+        }
+    }
+}