1 files changed, 336 insertions, 0 deletions

diff --git a/crates/ra_hir/src/ty/infer/coerce.rs b/crates/ra_hir/src/ty/infer/coerce.rs
new file mode 100644
index 000000000..0429a9866
--- /dev/null
+++ b/crates/ra_hir/src/ty/infer/coerce.rs
@@ -0,0 +1,336 @@
+//! Coercion logic. Coercions are certain type conversions that can implicitly
+//! happen in certain places, e.g. weakening `&mut` to `&` or deref coercions
+//! like going from `&Vec<T>` to `&[T]`.
+//!
+//! See: https://doc.rust-lang.org/nomicon/coercions.html
+
+use rustc_hash::FxHashMap;
+
+use test_utils::tested_by;
+
+use super::{InferTy, InferenceContext, TypeVarValue};
+use crate::{
+    db::HirDatabase,
+    lang_item::LangItemTarget,
+    resolve::Resolver,
+    ty::{autoderef, Substs, Ty, TypeCtor, TypeWalk},
+    type_ref::Mutability,
+    Adt,
+};
+
+impl<'a, D: HirDatabase> InferenceContext<'a, D> {
+    /// Unify two types, but may coerce the first one to the second one
+    /// using "implicit coercion rules" if needed.
+    pub(super) fn coerce(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
+        let from_ty = self.resolve_ty_shallow(from_ty).into_owned();
+        let to_ty = self.resolve_ty_shallow(to_ty);
+        self.coerce_inner(from_ty, &to_ty)
+    }
+
+    /// Merge two types from different branches, with possible implicit coerce.
+    ///
+    /// Note that it is only possible that one type are coerced to another.
+    /// Coercing both types to another least upper bound type is not possible in rustc,
+    /// which will simply result in "incompatible types" error.
+    pub(super) fn coerce_merge_branch<'t>(&mut self, ty1: &Ty, ty2: &Ty) -> Ty {
+        if self.coerce(ty1, ty2) {
+            ty2.clone()
+        } else if self.coerce(ty2, ty1) {
+            ty1.clone()
+        } else {
+            tested_by!(coerce_merge_fail_fallback);
+            // For incompatible types, we use the latter one as result
+            // to be better recovery for `if` without `else`.
+            ty2.clone()
+        }
+    }
+
+    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::Trait(trait_)) => db.impls_for_trait(krate, trait_),
+            _ => return FxHashMap::default(),
+        };
+
+        impls
+            .iter()
+            .filter_map(|impl_block| {
+                // `CoerseUnsized` has one generic parameter for the target type.
+                let trait_ref = impl_block.target_trait_ref(db)?;
+                let cur_from_ty = trait_ref.substs.0.get(0)?;
+                let cur_to_ty = trait_ref.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::Param { idx: p1, .. }, Ty::Param { idx: p2, .. })
+                                    if p1 != p2 =>
+                                {
+                                    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.
+            (ty_app!(TypeCtor::Never), Ty::Infer(InferTy::TypeVar(tv))) => {
+                let var = self.new_maybe_never_type_var();
+                self.var_unification_table.union_value(*tv, TypeVarValue::Known(var));
+                return true;
+            }
+            (ty_app!(TypeCtor::Never), _) => return true,
+
+            // Trivial cases, this should go after `never` check to
+            // avoid infer result type to be never
+            _ => {
+                if self.unify_inner_trivial(&from_ty, &to_ty) {
+                    return true;
+                }
+            }
+        }
+
+        // Pointer weakening and function to pointer
+        match (&mut from_ty, to_ty) {
+            // `*mut T`, `&mut T, `&T`` -> `*const T`
+            // `&mut T` -> `&T`
+            // `&mut T` -> `*mut T`
+            (ty_app!(c1@TypeCtor::RawPtr(_)), ty_app!(c2@TypeCtor::RawPtr(Mutability::Shared)))
+            | (ty_app!(c1@TypeCtor::Ref(_)), ty_app!(c2@TypeCtor::RawPtr(Mutability::Shared)))
+            | (ty_app!(c1@TypeCtor::Ref(_)), ty_app!(c2@TypeCtor::Ref(Mutability::Shared)))
+            | (ty_app!(c1@TypeCtor::Ref(Mutability::Mut)), ty_app!(c2@TypeCtor::RawPtr(_))) => {
+                *c1 = *c2;
+            }
+
+            // Illegal mutablity conversion
+            (
+                ty_app!(TypeCtor::RawPtr(Mutability::Shared)),
+                ty_app!(TypeCtor::RawPtr(Mutability::Mut)),
+            )
+            | (
+                ty_app!(TypeCtor::Ref(Mutability::Shared)),
+                ty_app!(TypeCtor::Ref(Mutability::Mut)),
+            ) => return false,
+
+            // `{function_type}` -> `fn()`
+            (ty_app!(TypeCtor::FnDef(_)), ty_app!(TypeCtor::FnPtr { .. })) => {
+                match from_ty.callable_sig(self.db) {
+                    None => return false,
+                    Some(sig) => {
+                        let num_args = sig.params_and_return.len() as u16 - 1;
+                        from_ty =
+                            Ty::apply(TypeCtor::FnPtr { num_args }, Substs(sig.params_and_return));
+                    }
+                }
+            }
+
+            _ => {}
+        }
+
+        if let Some(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
+            return ret;
+        }
+
+        // Auto Deref if cannot coerce
+        match (&from_ty, to_ty) {
+            // FIXME: DerefMut
+            (ty_app!(TypeCtor::Ref(_), st1), ty_app!(TypeCtor::Ref(_), st2)) => {
+                self.unify_autoderef_behind_ref(&st1[0], &st2[0])
+            }
+
+            // Otherwise, normal unify
+            _ => self.unify(&from_ty, to_ty),
+        }
+    }
+
+    /// Coerce a type using `from_ty: CoerceUnsized<ty_ty>`
+    ///
+    /// 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),
+            _ => 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 ret = st1
+            .iter()
+            .zip(st2.iter())
+            .enumerate()
+            .filter(|&(idx, _)| idx != coerce_generic_index)
+            .all(|(_, (ty1, ty2))| self.unify(ty1, ty2));
+
+        Some(ret)
+    }
+
+    /// 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]))
+            }
+
+            // `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(Adt::Struct(struct1)), st1),
+                ty_app!(TypeCtor::Adt(Adt::Struct(struct2)), st2),
+            ) if struct1 == struct2 => {
+                let fields = struct1.fields(self.db);
+                let (last_field, prev_fields) = fields.split_last()?;
+
+                // Get the generic parameter involved in the last field.
+                let unsize_generic_index = {
+                    let mut index = None;
+                    let mut multiple_param = false;
+                    last_field.ty(self.db).walk(&mut |ty| match ty {
+                        &Ty::Param { idx, .. } => {
+                            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;
+                prev_fields.iter().for_each(|field| {
+                    field.ty(self.db).walk(&mut |ty| match ty {
+                        &Ty::Param { 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)
+            }
+
+            _ => None,
+        }
+    }
+
+    /// Unify `from_ty` to `to_ty` with optional auto Deref
+    ///
+    /// Note that the parameters are already stripped the outer reference.
+    fn unify_autoderef_behind_ref(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
+        let canonicalized = self.canonicalizer().canonicalize_ty(from_ty.clone());
+        let to_ty = self.resolve_ty_shallow(&to_ty);
+        // FIXME: Auto DerefMut
+        for derefed_ty in
+            autoderef::autoderef(self.db, &self.resolver.clone(), canonicalized.value.clone())
+        {
+            let derefed_ty = canonicalized.decanonicalize_ty(derefed_ty.value);
+            match (&*self.resolve_ty_shallow(&derefed_ty), &*to_ty) {
+                // Stop when constructor matches.
+                (ty_app!(from_ctor, st1), ty_app!(to_ctor, st2)) if from_ctor == to_ctor => {
+                    // It will not recurse to `coerce`.
+                    return self.unify_substs(st1, st2, 0);
+                }
+                _ => {}
+            }
+        }
+
+        false
+    }
+}