Merge #3263

3263: Implement unsizing coercions using Chalk r=matklad a=flodiebold

These are coercions like `&[T; n] -> &[T]`, which are handled by the `Unsize` and `CoerceUnsized` traits. The impls for `Unsize` are all built in to the compiler and require special handling, so we need to provide them to Chalk.

This adds the following `Unsize` impls:
 - `Unsize<[T]> for [T; _]`
 - `Unsize<dyn Trait> for T where T: Trait`
 - `Unsize<dyn SuperTrait> for dyn SubTrait`

Hence we are still missing the 'unsizing the last field of a generic struct' case.

Co-authored-by: Florian Diebold <[email protected]>
Co-authored-by: Florian Diebold <[email protected]>

10 files changed, 480 insertions, 261 deletions

diff --git a/crates/ra_hir_ty/src/infer.rs b/crates/ra_hir_ty/src/infer.rs
index 76069eb9c..6e1d268de 100644
--- a/crates/ra_hir_ty/src/infer.rs
+++ b/crates/ra_hir_ty/src/infer.rs
@@ -206,12 +206,6 @@ struct InferenceContext<'a, D: HirDatabase> {
     /// closures, but currently this is the only field that will change there,
     /// so it doesn't make sense.
     return_ty: Ty,
-
-    /// Impls of `CoerceUnsized` used in coercion.
-    /// (from_ty_ctor, to_ty_ctor) => coerce_generic_index
-    // FIXME: Use trait solver for this.
-    // Chalk seems unable to work well with builtin impl of `Unsize` now.
-    coerce_unsized_map: FxHashMap<(TypeCtor, TypeCtor), usize>,
 }
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
@@ -222,7 +216,6 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             obligations: Vec::default(),
             return_ty: Ty::Unknown, // set in collect_fn_signature
             trait_env: TraitEnvironment::lower(db, &resolver),
-            coerce_unsized_map: Self::init_coerce_unsized_map(db, &resolver),
             db,
             owner,
             body: db.body(owner),
diff --git a/crates/ra_hir_ty/src/infer/coerce.rs b/crates/ra_hir_ty/src/infer/coerce.rs
index fb6a51b12..95ac3c713 100644
--- a/crates/ra_hir_ty/src/infer/coerce.rs
+++ b/crates/ra_hir_ty/src/infer/coerce.rs
@@ -4,11 +4,12 @@
 //!
 //! See: https://doc.rust-lang.org/nomicon/coercions.html
 
-use hir_def::{lang_item::LangItemTarget, resolver::Resolver, type_ref::Mutability, AdtId};
-use rustc_hash::FxHashMap;
+use hir_def::{lang_item::LangItemTarget, type_ref::Mutability};
 use test_utils::tested_by;
 
-use crate::{autoderef, db::HirDatabase, Substs, Ty, TypeCtor, TypeWalk};
+use crate::{
+    autoderef, db::HirDatabase, traits::Solution, Obligation, Substs, TraitRef, Ty, TypeCtor,
+};
 
 use super::{unify::TypeVarValue, InEnvironment, InferTy, InferenceContext};
 
@@ -39,44 +40,6 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         }
     }
 
-    pub(super) fn init_coerce_unsized_map(
-        db: &'a D,
-        resolver: &Resolver,
-    ) -> FxHashMap<(TypeCtor, TypeCtor), usize> {
-        let krate = resolver.krate().unwrap();
-        let impls = match db.lang_item(krate, "coerce_unsized".into()) {
-            Some(LangItemTarget::TraitId(trait_)) => db.impls_for_trait(krate, trait_),
-            _ => return FxHashMap::default(),
-        };
-
-        impls
-            .iter()
-            .filter_map(|&impl_id| {
-                let trait_ref = db.impl_trait(impl_id)?;
-
-                // `CoerseUnsized` has one generic parameter for the target type.
-                let cur_from_ty = trait_ref.value.substs.0.get(0)?;
-                let cur_to_ty = trait_ref.value.substs.0.get(1)?;
-
-                match (&cur_from_ty, cur_to_ty) {
-                    (ty_app!(ctor1, st1), ty_app!(ctor2, st2)) => {
-                        // FIXME: We return the first non-equal bound as the type parameter to coerce to unsized type.
-                        // This works for smart-pointer-like coercion, which covers all impls from std.
-                        st1.iter().zip(st2.iter()).enumerate().find_map(|(i, (ty1, ty2))| {
-                            match (ty1, ty2) {
-                                (Ty::Bound(idx1), Ty::Bound(idx2)) if idx1 != idx2 => {
-                                    Some(((*ctor1, *ctor2), i))
-                                }
-                                _ => None,
-                            }
-                        })
-                    }
-                    _ => None,
-                }
-            })
-            .collect()
-    }
-
     fn coerce_inner(&mut self, mut from_ty: Ty, to_ty: &Ty) -> bool {
         match (&from_ty, to_ty) {
             // Never type will make type variable to fallback to Never Type instead of Unknown.
@@ -157,154 +120,38 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     ///
     /// See: https://doc.rust-lang.org/nightly/std/marker/trait.CoerceUnsized.html
     fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty) -> Option<bool> {
-        let (ctor1, st1, ctor2, st2) = match (from_ty, to_ty) {
-            (ty_app!(ctor1, st1), ty_app!(ctor2, st2)) => (ctor1, st1, ctor2, st2),
+        let krate = self.resolver.krate().unwrap();
+        let coerce_unsized_trait = match self.db.lang_item(krate, "coerce_unsized".into()) {
+            Some(LangItemTarget::TraitId(trait_)) => trait_,
             _ => return None,
         };
 
-        let coerce_generic_index = *self.coerce_unsized_map.get(&(*ctor1, *ctor2))?;
-
-        // Check `Unsize` first
-        match self.check_unsize_and_coerce(
-            st1.0.get(coerce_generic_index)?,
-            st2.0.get(coerce_generic_index)?,
-            0,
-        ) {
-            Some(true) => {}
-            ret => return ret,
+        let generic_params = crate::utils::generics(self.db, coerce_unsized_trait.into());
+        if generic_params.len() != 2 {
+            // The CoerceUnsized trait should have two generic params: Self and T.
+            return None;
         }
 
-        let ret = st1
-            .iter()
-            .zip(st2.iter())
-            .enumerate()
-            .filter(|&(idx, _)| idx != coerce_generic_index)
-            .all(|(_, (ty1, ty2))| self.unify(ty1, ty2));
+        let substs = Substs::build_for_generics(&generic_params)
+            .push(from_ty.clone())
+            .push(to_ty.clone())
+            .build();
+        let trait_ref = TraitRef { trait_: coerce_unsized_trait, substs };
+        let goal = InEnvironment::new(self.trait_env.clone(), Obligation::Trait(trait_ref));
 
-        Some(ret)
-    }
+        let canonicalizer = self.canonicalizer();
+        let canonicalized = canonicalizer.canonicalize_obligation(goal);
 
-    /// Check if `from_ty: Unsize<to_ty>`, and coerce to `to_ty` if it holds.
-    ///
-    /// It should not be directly called. It is only used by `try_coerce_unsized`.
-    ///
-    /// See: https://doc.rust-lang.org/nightly/std/marker/trait.Unsize.html
-    fn check_unsize_and_coerce(&mut self, from_ty: &Ty, to_ty: &Ty, depth: usize) -> Option<bool> {
-        if depth > 1000 {
-            panic!("Infinite recursion in coercion");
-        }
-
-        match (&from_ty, &to_ty) {
-            // `[T; N]` -> `[T]`
-            (ty_app!(TypeCtor::Array, st1), ty_app!(TypeCtor::Slice, st2)) => {
-                Some(self.unify(&st1[0], &st2[0]))
-            }
+        let solution = self.db.trait_solve(krate, canonicalized.value.clone())?;
 
-            // `T` -> `dyn Trait` when `T: Trait`
-            (_, Ty::Dyn(_)) => {
-                // FIXME: Check predicates
-                Some(true)
-            }
-
-            // `(..., T)` -> `(..., U)` when `T: Unsize<U>`
-            (
-                ty_app!(TypeCtor::Tuple { cardinality: len1 }, st1),
-                ty_app!(TypeCtor::Tuple { cardinality: len2 }, st2),
-            ) => {
-                if len1 != len2 || *len1 == 0 {
-                    return None;
-                }
-
-                match self.check_unsize_and_coerce(
-                    st1.last().unwrap(),
-                    st2.last().unwrap(),
-                    depth + 1,
-                ) {
-                    Some(true) => {}
-                    ret => return ret,
-                }
-
-                let ret = st1[..st1.len() - 1]
-                    .iter()
-                    .zip(&st2[..st2.len() - 1])
-                    .all(|(ty1, ty2)| self.unify(ty1, ty2));
-
-                Some(ret)
-            }
-
-            // Foo<..., T, ...> is Unsize<Foo<..., U, ...>> if:
-            // - T: Unsize<U>
-            // - Foo is a struct
-            // - Only the last field of Foo has a type involving T
-            // - T is not part of the type of any other fields
-            // - Bar<T>: Unsize<Bar<U>>, if the last field of Foo has type Bar<T>
-            (
-                ty_app!(TypeCtor::Adt(AdtId::StructId(struct1)), st1),
-                ty_app!(TypeCtor::Adt(AdtId::StructId(struct2)), st2),
-            ) if struct1 == struct2 => {
-                let field_tys = self.db.field_types((*struct1).into());
-                let struct_data = self.db.struct_data(*struct1);
-
-                let mut fields = struct_data.variant_data.fields().iter();
-                let (last_field_id, _data) = fields.next_back()?;
-
-                // Get the generic parameter involved in the last field.
-                let unsize_generic_index = {
-                    let mut index = None;
-                    let mut multiple_param = false;
-                    field_tys[last_field_id].value.walk(&mut |ty| {
-                        if let &Ty::Bound(idx) = ty {
-                            if index.is_none() {
-                                index = Some(idx);
-                            } else if Some(idx) != index {
-                                multiple_param = true;
-                            }
-                        }
-                    });
-
-                    if multiple_param {
-                        return None;
-                    }
-                    index?
-                };
-
-                // Check other fields do not involve it.
-                let mut multiple_used = false;
-                fields.for_each(|(field_id, _data)| {
-                    field_tys[field_id].value.walk(&mut |ty| match ty {
-                        &Ty::Bound(idx) if idx == unsize_generic_index => multiple_used = true,
-                        _ => {}
-                    })
-                });
-                if multiple_used {
-                    return None;
-                }
-
-                let unsize_generic_index = unsize_generic_index as usize;
-
-                // Check `Unsize` first
-                match self.check_unsize_and_coerce(
-                    st1.get(unsize_generic_index)?,
-                    st2.get(unsize_generic_index)?,
-                    depth + 1,
-                ) {
-                    Some(true) => {}
-                    ret => return ret,
-                }
-
-                // Then unify other parameters
-                let ret = st1
-                    .iter()
-                    .zip(st2.iter())
-                    .enumerate()
-                    .filter(|&(idx, _)| idx != unsize_generic_index)
-                    .all(|(_, (ty1, ty2))| self.unify(ty1, ty2));
-
-                Some(ret)
+        match solution {
+            Solution::Unique(v) => {
+                canonicalized.apply_solution(self, v.0);
             }
+            _ => return None,
+        };
 
-            _ => None,
-        }
+        Some(true)
     }
 
     /// Unify `from_ty` to `to_ty` with optional auto Deref
diff --git a/crates/ra_hir_ty/src/infer/unify.rs b/crates/ra_hir_ty/src/infer/unify.rs
index 2d03c5c33..aed527fe5 100644
--- a/crates/ra_hir_ty/src/infer/unify.rs
+++ b/crates/ra_hir_ty/src/infer/unify.rs
@@ -7,10 +7,7 @@ use ena::unify::{InPlaceUnificationTable, NoError, UnifyKey, UnifyValue};
 use test_utils::tested_by;
 
 use super::{InferenceContext, Obligation};
-use crate::{
-    db::HirDatabase, utils::make_mut_slice, Canonical, InEnvironment, InferTy, ProjectionPredicate,
-    ProjectionTy, Substs, TraitRef, Ty, TypeCtor, TypeWalk,
-};
+use crate::{db::HirDatabase, Canonical, InEnvironment, InferTy, Substs, Ty, TypeCtor, TypeWalk};
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b, D>
@@ -50,42 +47,38 @@ where
         })
     }
 
-    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.table.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.table.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)
+    fn do_canonicalize<T: TypeWalk>(&mut self, t: T, binders: usize) -> T {
+        t.fold_binders(
+            &mut |ty, binders| 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.table.var_unification_table.inlined_probe_value(inner).known()
+                    {
+                        self.var_stack.push(inner);
+                        let result = self.do_canonicalize(known_ty.clone(), binders);
+                        self.var_stack.pop();
+                        result
+                    } else {
+                        let root = self.ctx.table.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 + binders) 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
+                _ => ty,
+            },
+            binders,
+        )
     }
 
     fn into_canonicalized<T>(self, result: T) -> Canonicalized<T> {
@@ -95,28 +88,8 @@ where
         }
     }
 
-    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);
+        let result = self.do_canonicalize(ty, 0);
         self.into_canonicalized(result)
     }
 
@@ -125,10 +98,8 @@ where
         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))
-            }
+            Obligation::Trait(tr) => Obligation::Trait(self.do_canonicalize(tr, 0)),
+            Obligation::Projection(pr) => Obligation::Projection(self.do_canonicalize(pr, 0)),
         };
         self.into_canonicalized(InEnvironment {
             value: result,
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 13c5e6c6b..0009c426c 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -461,6 +461,12 @@ impl<T> Binders<T> {
     }
 }
 
+impl<T: Clone> Binders<&T> {
+    pub fn cloned(&self) -> Binders<T> {
+        Binders { num_binders: self.num_binders, value: self.value.clone() }
+    }
+}
+
 impl<T: TypeWalk> Binders<T> {
     /// Substitutes all variables.
     pub fn subst(self, subst: &Substs) -> T {
@@ -661,6 +667,17 @@ impl Ty {
         }
     }
 
+    /// If this is a `dyn Trait` type, this returns the `Trait` part.
+    pub fn dyn_trait_ref(&self) -> Option<&TraitRef> {
+        match self {
+            Ty::Dyn(bounds) => bounds.get(0).and_then(|b| match b {
+                GenericPredicate::Implemented(trait_ref) => Some(trait_ref),
+                _ => None,
+            }),
+            _ => None,
+        }
+    }
+
     fn builtin_deref(&self) -> Option<Ty> {
         match self {
             Ty::Apply(a_ty) => match a_ty.ctor {
@@ -746,6 +763,20 @@ pub trait TypeWalk {
     /// variable for the self type.
     fn walk_mut_binders(&mut self, f: &mut impl FnMut(&mut Ty, usize), binders: usize);
 
+    fn fold_binders(mut self, f: &mut impl FnMut(Ty, usize) -> Ty, binders: usize) -> Self
+    where
+        Self: Sized,
+    {
+        self.walk_mut_binders(
+            &mut |ty_mut, binders| {
+                let ty = mem::replace(ty_mut, Ty::Unknown);
+                *ty_mut = f(ty, binders);
+            },
+            binders,
+        );
+        self
+    }
+
     fn fold(mut self, f: &mut impl FnMut(Ty) -> Ty) -> Self
     where
         Self: Sized,
@@ -783,13 +814,16 @@ pub trait TypeWalk {
     where
         Self: Sized,
     {
-        self.fold(&mut |ty| match ty {
-            Ty::Bound(idx) => {
-                assert!(idx as i32 >= -n);
-                Ty::Bound((idx as i32 + n) as u32)
-            }
-            ty => ty,
-        })
+        self.fold_binders(
+            &mut |ty, binders| match ty {
+                Ty::Bound(idx) if idx as usize >= binders => {
+                    assert!(idx as i32 >= -n);
+                    Ty::Bound((idx as i32 + n) as u32)
+                }
+                ty => ty,
+            },
+            0,
+        )
     }
 }
 
diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 52da34574..092977e93 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -239,7 +239,10 @@ impl Ty {
     ) -> Ty {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
-                let trait_ref = TraitRef::from_resolved_path(ctx, trait_, resolved_segment, None);
+                // if this is a bare dyn Trait, we'll directly put the required ^0 for the self type in there
+                let self_ty = if remaining_segments.len() == 0 { Some(Ty::Bound(0)) } else { None };
+                let trait_ref =
+                    TraitRef::from_resolved_path(ctx, trait_, resolved_segment, self_ty);
                 return if remaining_segments.len() == 1 {
                     let segment = remaining_segments.first().unwrap();
                     let associated_ty = associated_type_by_name_including_super_traits(
diff --git a/crates/ra_hir_ty/src/tests/coercion.rs b/crates/ra_hir_ty/src/tests/coercion.rs
index 42330b269..60ad6e9be 100644
--- a/crates/ra_hir_ty/src/tests/coercion.rs
+++ b/crates/ra_hir_ty/src/tests/coercion.rs
@@ -548,3 +548,141 @@ impl<TT> S<TT> {
     "###
     );
 }
+
+#[test]
+fn coerce_unsize_array() {
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+#[lang = "unsize"]
+pub trait Unsize<T> {}
+#[lang = "coerce_unsized"]
+pub trait CoerceUnsized<T> {}
+
+impl<T: Unsize<U>, U> CoerceUnsized<&U> for &T {}
+
+fn test() {
+    let f: &[usize] = &[1, 2, 3];
+}
+"#, true),
+        @r###"
+    [162; 199) '{     ... 3]; }': ()
+    [172; 173) 'f': &[usize]
+    [186; 196) '&[1, 2, 3]': &[usize; _]
+    [187; 196) '[1, 2, 3]': [usize; _]
+    [188; 189) '1': usize
+    [191; 192) '2': usize
+    [194; 195) '3': usize
+    "###
+    );
+}
+
+#[test]
+fn coerce_unsize_trait_object() {
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+#[lang = "unsize"]
+pub trait Unsize<T> {}
+#[lang = "coerce_unsized"]
+pub trait CoerceUnsized<T> {}
+
+impl<T: Unsize<U>, U> CoerceUnsized<&U> for &T {}
+
+trait Foo<T, U> {}
+trait Bar<U, T, X>: Foo<T, U> {}
+trait Baz<T, X>: Bar<usize, T, X> {}
+
+struct S<T, X>;
+impl<T, X> Foo<T, usize> for S<T, X> {}
+impl<T, X> Bar<usize, T, X> for S<T, X> {}
+impl<T, X> Baz<T, X> for S<T, X> {}
+
+fn test() {
+    let obj: &dyn Baz<i8, i16> = &S;
+    let obj: &dyn Bar<_, _, _> = obj;
+    let obj: &dyn Foo<_, _> = obj;
+    let obj2: &dyn Baz<i8, i16> = &S;
+    let _: &dyn Foo<_, _> = obj2;
+}
+"#, true),
+        @r###"
+    [388; 573) '{     ...bj2; }': ()
+    [398; 401) 'obj': &dyn Baz<i8, i16>
+    [423; 425) '&S': &S<i8, i16>
+    [424; 425) 'S': S<i8, i16>
+    [435; 438) 'obj': &dyn Bar<usize, i8, i16>
+    [460; 463) 'obj': &dyn Baz<i8, i16>
+    [473; 476) 'obj': &dyn Foo<i8, usize>
+    [495; 498) 'obj': &dyn Bar<usize, i8, i16>
+    [508; 512) 'obj2': &dyn Baz<i8, i16>
+    [534; 536) '&S': &S<i8, i16>
+    [535; 536) 'S': S<i8, i16>
+    [546; 547) '_': &dyn Foo<i8, usize>
+    [566; 570) 'obj2': &dyn Baz<i8, i16>
+    "###
+    );
+}
+
+#[test]
+fn coerce_unsize_super_trait_cycle() {
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+#[lang = "unsize"]
+pub trait Unsize<T> {}
+#[lang = "coerce_unsized"]
+pub trait CoerceUnsized<T> {}
+
+impl<T: Unsize<U>, U> CoerceUnsized<&U> for &T {}
+
+trait A {}
+trait B: C + A {}
+trait C: B {}
+trait D: C
+
+struct S;
+impl A for S {}
+impl B for S {}
+impl C for S {}
+impl D for S {}
+
+fn test() {
+    let obj: &dyn D = &S;
+    let obj: &dyn A = obj;
+}
+"#, true),
+        @r###"
+    [292; 348) '{     ...obj; }': ()
+    [302; 305) 'obj': &dyn D
+    [316; 318) '&S': &S
+    [317; 318) 'S': S
+    [328; 331) 'obj': &dyn A
+    [342; 345) 'obj': &dyn D
+    "###
+    );
+}
+
+#[ignore]
+#[test]
+fn coerce_unsize_generic() {
+    // FIXME: Implement this
+    // https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+#[lang = "unsize"]
+pub trait Unsize<T> {}
+#[lang = "coerce_unsized"]
+pub trait CoerceUnsized<T> {}
+
+impl<T: Unsize<U>, U> CoerceUnsized<&U> for &T {}
+
+struct Foo<T> { t: T };
+struct Bar<T>(Foo<T>);
+
+fn test() {
+    let _: &Foo<[usize]> = &Foo { t: [1, 2, 3] };
+    let _: &Bar<[usize]> = &Bar(Foo { t: [1, 2, 3] });
+}
+"#, true),
+        @r###"
+    "###
+    );
+}
diff --git a/crates/ra_hir_ty/src/traits.rs b/crates/ra_hir_ty/src/traits.rs
index e83449957..2317fcac3 100644
--- a/crates/ra_hir_ty/src/traits.rs
+++ b/crates/ra_hir_ty/src/traits.rs
@@ -335,6 +335,12 @@ pub struct ClosureFnTraitImplData {
     fn_trait: FnTrait,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct UnsizeToSuperTraitObjectData {
+    trait_: TraitId,
+    super_trait: TraitId,
+}
+
 /// An impl. Usually this comes from an impl block, but some built-in types get
 /// synthetic impls.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
@@ -343,6 +349,12 @@ pub enum Impl {
     ImplBlock(ImplId),
     /// Closure types implement the Fn traits synthetically.
     ClosureFnTraitImpl(ClosureFnTraitImplData),
+    /// [T; n]: Unsize<[T]>
+    UnsizeArray,
+    /// T: Unsize<dyn Trait> where T: Trait
+    UnsizeToTraitObject(TraitId),
+    /// dyn Trait: Unsize<dyn SuperTrait> if Trait: SuperTrait
+    UnsizeToSuperTraitObject(UnsizeToSuperTraitObjectData),
 }
 /// This exists just for Chalk, because our ImplIds are only unique per module.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
diff --git a/crates/ra_hir_ty/src/traits/builtin.rs b/crates/ra_hir_ty/src/traits/builtin.rs
index a537420a5..cc0f3eeb4 100644
--- a/crates/ra_hir_ty/src/traits/builtin.rs
+++ b/crates/ra_hir_ty/src/traits/builtin.rs
@@ -4,8 +4,12 @@ use hir_def::{expr::Expr, lang_item::LangItemTarget, TraitId, TypeAliasId};
 use hir_expand::name::name;
 use ra_db::CrateId;
 
-use super::{AssocTyValue, Impl};
-use crate::{db::HirDatabase, ApplicationTy, Substs, TraitRef, Ty, TypeCtor};
+use super::{AssocTyValue, Impl, UnsizeToSuperTraitObjectData};
+use crate::{
+    db::HirDatabase,
+    utils::{all_super_traits, generics},
+    ApplicationTy, Binders, GenericPredicate, Substs, TraitRef, Ty, TypeCtor,
+};
 
 pub(super) struct BuiltinImplData {
     pub num_vars: usize,
@@ -25,6 +29,8 @@ pub(super) fn get_builtin_impls(
     db: &impl HirDatabase,
     krate: CrateId,
     ty: &Ty,
+    // The first argument for the trait, if present
+    arg: &Option<Ty>,
     trait_: TraitId,
     mut callback: impl FnMut(Impl),
 ) {
@@ -43,12 +49,60 @@ pub(super) fn get_builtin_impls(
             }
         }
     }
+
+    let unsize_trait = get_unsize_trait(db, krate);
+    if let Some(actual_trait) = unsize_trait {
+        if trait_ == actual_trait {
+            get_builtin_unsize_impls(db, krate, ty, arg, callback);
+        }
+    }
+}
+
+fn get_builtin_unsize_impls(
+    db: &impl HirDatabase,
+    krate: CrateId,
+    ty: &Ty,
+    // The first argument for the trait, if present
+    arg: &Option<Ty>,
+    mut callback: impl FnMut(Impl),
+) {
+    if !check_unsize_impl_prerequisites(db, krate) {
+        return;
+    }
+
+    if let Ty::Apply(ApplicationTy { ctor: TypeCtor::Array, .. }) = ty {
+        callback(Impl::UnsizeArray);
+        return; // array is unsized, the rest of the impls shouldn't apply
+    }
+
+    if let Some(target_trait) = arg.as_ref().and_then(|t| t.dyn_trait_ref()) {
+        // FIXME what about more complicated dyn tys with marker traits?
+        if let Some(trait_ref) = ty.dyn_trait_ref() {
+            if trait_ref.trait_ != target_trait.trait_ {
+                let super_traits = all_super_traits(db, trait_ref.trait_);
+                if super_traits.contains(&target_trait.trait_) {
+                    callback(Impl::UnsizeToSuperTraitObject(UnsizeToSuperTraitObjectData {
+                        trait_: trait_ref.trait_,
+                        super_trait: target_trait.trait_,
+                    }));
+                }
+            }
+        } else {
+            // FIXME only for sized types
+            callback(Impl::UnsizeToTraitObject(target_trait.trait_));
+        }
+    }
 }
 
 pub(super) fn impl_datum(db: &impl HirDatabase, krate: CrateId, impl_: Impl) -> BuiltinImplData {
     match impl_ {
         Impl::ImplBlock(_) => unreachable!(),
         Impl::ClosureFnTraitImpl(data) => closure_fn_trait_impl_datum(db, krate, data),
+        Impl::UnsizeArray => array_unsize_impl_datum(db, krate),
+        Impl::UnsizeToTraitObject(trait_) => trait_object_unsize_impl_datum(db, krate, trait_),
+        Impl::UnsizeToSuperTraitObject(data) => {
+            super_trait_object_unsize_impl_datum(db, krate, data)
+        }
     }
 }
 
@@ -65,6 +119,8 @@ pub(super) fn associated_ty_value(
     }
 }
 
+// Closure Fn trait impls
+
 fn check_closure_fn_trait_impl_prerequisites(
     db: &impl HirDatabase,
     krate: CrateId,
@@ -165,6 +221,129 @@ fn closure_fn_trait_output_assoc_ty_value(
     }
 }
 
+// Array unsizing
+
+fn check_unsize_impl_prerequisites(db: &impl HirDatabase, krate: CrateId) -> bool {
+    // the Unsize trait needs to exist and have two type parameters (Self and T)
+    let unsize_trait = match get_unsize_trait(db, krate) {
+        Some(t) => t,
+        None => return false,
+    };
+    let generic_params = generics(db, unsize_trait.into());
+    generic_params.len() == 2
+}
+
+fn array_unsize_impl_datum(db: &impl HirDatabase, krate: CrateId) -> BuiltinImplData {
+    // impl<T> Unsize<[T]> for [T; _]
+    // (this can be a single impl because we don't distinguish array sizes currently)
+
+    let trait_ = get_unsize_trait(db, krate) // get unsize trait
+        // the existence of the Unsize trait has been checked before
+        .expect("Unsize trait missing");
+
+    let var = Ty::Bound(0);
+    let substs = Substs::builder(2)
+        .push(Ty::apply_one(TypeCtor::Array, var.clone()))
+        .push(Ty::apply_one(TypeCtor::Slice, var))
+        .build();
+
+    let trait_ref = TraitRef { trait_, substs };
+
+    BuiltinImplData {
+        num_vars: 1,
+        trait_ref,
+        where_clauses: Vec::new(),
+        assoc_ty_values: Vec::new(),
+    }
+}
+
+// Trait object unsizing
+
+fn trait_object_unsize_impl_datum(
+    db: &impl HirDatabase,
+    krate: CrateId,
+    trait_: TraitId,
+) -> BuiltinImplData {
+    // impl<T, T1, ...> Unsize<dyn Trait<T1, ...>> for T where T: Trait<T1, ...>
+
+    let unsize_trait = get_unsize_trait(db, krate) // get unsize trait
+        // the existence of the Unsize trait has been checked before
+        .expect("Unsize trait missing");
+
+    let self_ty = Ty::Bound(0);
+
+    let target_substs = Substs::build_for_def(db, trait_)
+        .push(Ty::Bound(0))
+        // starting from ^2 because we want to start with ^1 outside of the
+        // `dyn`, which is ^2 inside
+        .fill_with_bound_vars(2)
+        .build();
+    let num_vars = target_substs.len();
+    let target_trait_ref = TraitRef { trait_, substs: target_substs };
+    let target_bounds = vec![GenericPredicate::Implemented(target_trait_ref)];
+
+    let self_substs = Substs::build_for_def(db, trait_).fill_with_bound_vars(0).build();
+    let self_trait_ref = TraitRef { trait_, substs: self_substs };
+    let where_clauses = vec![GenericPredicate::Implemented(self_trait_ref)];
+
+    let impl_substs =
+        Substs::builder(2).push(self_ty).push(Ty::Dyn(target_bounds.clone().into())).build();
+
+    let trait_ref = TraitRef { trait_: unsize_trait, substs: impl_substs };
+
+    BuiltinImplData { num_vars, trait_ref, where_clauses, assoc_ty_values: Vec::new() }
+}
+
+fn super_trait_object_unsize_impl_datum(
+    db: &impl HirDatabase,
+    krate: CrateId,
+    data: UnsizeToSuperTraitObjectData,
+) -> BuiltinImplData {
+    // impl<T1, ...> Unsize<dyn SuperTrait> for dyn Trait<T1, ...>
+
+    let unsize_trait = get_unsize_trait(db, krate) // get unsize trait
+        // the existence of the Unsize trait has been checked before
+        .expect("Unsize trait missing");
+
+    let self_substs = Substs::build_for_def(db, data.trait_).fill_with_bound_vars(0).build();
+
+    let num_vars = self_substs.len() - 1;
+
+    let self_trait_ref = TraitRef { trait_: data.trait_, substs: self_substs.clone() };
+    let self_bounds = vec![GenericPredicate::Implemented(self_trait_ref.clone())];
+
+    // we need to go from our trait to the super trait, substituting type parameters
+    let path = crate::utils::find_super_trait_path(db, data.trait_, data.super_trait);
+
+    let mut current_trait_ref = self_trait_ref;
+    for t in path.into_iter().skip(1) {
+        let bounds = db.generic_predicates(current_trait_ref.trait_.into());
+        let super_trait_ref = bounds
+            .iter()
+            .find_map(|b| match &b.value {
+                GenericPredicate::Implemented(tr)
+                    if tr.trait_ == t && tr.substs[0] == Ty::Bound(0) =>
+                {
+                    Some(Binders { value: tr, num_binders: b.num_binders })
+                }
+                _ => None,
+            })
+            .expect("trait bound for known super trait not found");
+        current_trait_ref = super_trait_ref.cloned().subst(&current_trait_ref.substs);
+    }
+
+    let super_bounds = vec![GenericPredicate::Implemented(current_trait_ref)];
+
+    let substs = Substs::builder(2)
+        .push(Ty::Dyn(self_bounds.into()))
+        .push(Ty::Dyn(super_bounds.into()))
+        .build();
+
+    let trait_ref = TraitRef { trait_: unsize_trait, substs };
+
+    BuiltinImplData { num_vars, trait_ref, where_clauses: Vec::new(), assoc_ty_values: Vec::new() }
+}
+
 fn get_fn_trait(
     db: &impl HirDatabase,
     krate: CrateId,
@@ -176,3 +355,11 @@ fn get_fn_trait(
         _ => None,
     }
 }
+
+fn get_unsize_trait(db: &impl HirDatabase, krate: CrateId) -> Option<TraitId> {
+    let target = db.lang_item(krate, "unsize".into())?;
+    match target {
+        LangItemTarget::TraitId(t) => Some(t),
+        _ => None,
+    }
+}
diff --git a/crates/ra_hir_ty/src/traits/chalk.rs b/crates/ra_hir_ty/src/traits/chalk.rs
index 1bdf13e48..e1e430aeb 100644
--- a/crates/ra_hir_ty/src/traits/chalk.rs
+++ b/crates/ra_hir_ty/src/traits/chalk.rs
@@ -572,8 +572,10 @@ where
             .collect();
 
         let ty: Ty = from_chalk(self.db, parameters[0].assert_ty_ref().clone());
+        let arg: Option<Ty> =
+            parameters.get(1).map(|p| from_chalk(self.db, p.assert_ty_ref().clone()));
 
-        builtin::get_builtin_impls(self.db, self.krate, &ty, trait_, |i| {
+        builtin::get_builtin_impls(self.db, self.krate, &ty, &arg, trait_, |i| {
             result.push(i.to_chalk(self.db))
         });
 
diff --git a/crates/ra_hir_ty/src/utils.rs b/crates/ra_hir_ty/src/utils.rs
index 508ae9046..463fd65b4 100644
--- a/crates/ra_hir_ty/src/utils.rs
+++ b/crates/ra_hir_ty/src/utils.rs
@@ -62,6 +62,38 @@ pub(super) fn all_super_traits(db: &impl DefDatabase, trait_: TraitId) -> Vec<Tr
     result
 }
 
+/// Finds a path from a trait to one of its super traits. Returns an empty
+/// vector if there is no path.
+pub(super) fn find_super_trait_path(
+    db: &impl DefDatabase,
+    trait_: TraitId,
+    super_trait: TraitId,
+) -> Vec<TraitId> {
+    let mut result = Vec::with_capacity(2);
+    result.push(trait_);
+    return if go(db, super_trait, &mut result) { result } else { Vec::new() };
+
+    fn go(db: &impl DefDatabase, super_trait: TraitId, path: &mut Vec<TraitId>) -> bool {
+        let trait_ = *path.last().unwrap();
+        if trait_ == super_trait {
+            return true;
+        }
+
+        for tt in direct_super_traits(db, trait_) {
+            if path.contains(&tt) {
+                continue;
+            }
+            path.push(tt);
+            if go(db, super_trait, path) {
+                return true;
+            } else {
+                path.pop();
+            }
+        }
+        false
+    }
+}
+
 pub(super) fn associated_type_by_name_including_super_traits(
     db: &impl DefDatabase,
     trait_: TraitId,