Merge #1823

1823: Support type coercion r=flodiebold a=uHOOCCOOHu



Co-authored-by: uHOOCCOOHu <hooccooh1896@gmail.com>

crates/ra_hir/src/marks.rs

@@ -13,4 +13,5 @@ test_utils::marks!(
     infer_while_let
     macro_rules_from_other_crates_are_visible_with_macro_use
     prelude_is_macro_use
+    coerce_merge_fail_fallback
 );

crates/ra_hir/src/ty/infer.rs

@@ -14,7 +14,7 @@
 //! the `ena` crate, which is extracted from rustc.
 
 use std::borrow::Cow;
-use std::iter::repeat;
+use std::iter::{repeat, repeat_with};
 use std::mem;
 use std::ops::Index;
 use std::sync::Arc;
@@ -42,6 +42,7 @@ use crate::{
         RecordFieldPat, Statement, UnaryOp,
     },
     generics::{GenericParams, HasGenericParams},
+    lang_item::LangItemTarget,
     name,
     nameres::Namespace,
     path::{known, GenericArg, GenericArgs},
@@ -188,6 +189,21 @@ struct InferenceContext<'a, D: HirDatabase> {
     result: InferenceResult,
     /// The return type of the function being inferred.
     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>,
+}
+
+macro_rules! ty_app {
+    ($ctor:pat, $param:pat) => {
+        Ty::Apply(ApplicationTy { ctor: $ctor, parameters: $param })
+    };
+    ($ctor:pat) => {
+        ty_app!($ctor, _)
+    };
 }
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
@@ -198,12 +214,52 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             obligations: Vec::default(),
             return_ty: Ty::Unknown, // set in collect_fn_signature
             trait_env: lower::trait_env(db, &resolver),
+            coerce_unsized_map: Self::init_coerce_unsized_map(db, &resolver),
             db,
             body,
             resolver,
         }
     }
 
+    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 resolve_all(mut self) -> InferenceResult {
         // FIXME resolve obligations as well (use Guidance if necessary)
         let mut result = mem::replace(&mut self.result, InferenceResult::default());
@@ -278,27 +334,45 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         let ty1 = self.resolve_ty_shallow(ty1);
         let ty2 = self.resolve_ty_shallow(ty2);
         match (&*ty1, &*ty2) {
-            (Ty::Unknown, _) | (_, Ty::Unknown) => true,
             (Ty::Apply(a_ty1), Ty::Apply(a_ty2)) if a_ty1.ctor == a_ty2.ctor => {
                 self.unify_substs(&a_ty1.parameters, &a_ty2.parameters, depth + 1)
             }
+            _ => self.unify_inner_trivial(&ty1, &ty2),
+        }
+    }
+
+    fn unify_inner_trivial(&mut self, ty1: &Ty, ty2: &Ty) -> bool {
+        match (ty1, ty2) {
+            (Ty::Unknown, _) | (_, Ty::Unknown) => true,
+
             (Ty::Infer(InferTy::TypeVar(tv1)), Ty::Infer(InferTy::TypeVar(tv2)))
             | (Ty::Infer(InferTy::IntVar(tv1)), Ty::Infer(InferTy::IntVar(tv2)))
-            | (Ty::Infer(InferTy::FloatVar(tv1)), Ty::Infer(InferTy::FloatVar(tv2))) => {
+            | (Ty::Infer(InferTy::FloatVar(tv1)), Ty::Infer(InferTy::FloatVar(tv2)))
+            | (
+                Ty::Infer(InferTy::MaybeNeverTypeVar(tv1)),
+                Ty::Infer(InferTy::MaybeNeverTypeVar(tv2)),
+            ) => {
                 // both type vars are unknown since we tried to resolve them
                 self.var_unification_table.union(*tv1, *tv2);
                 true
             }
+
+            // The order of MaybeNeverTypeVar matters here.
+            // Unifying MaybeNeverTypeVar and TypeVar will let the latter become MaybeNeverTypeVar.
+            // Unifying MaybeNeverTypeVar and other concrete type will let the former become it.
             (Ty::Infer(InferTy::TypeVar(tv)), other)
             | (other, Ty::Infer(InferTy::TypeVar(tv)))
-            | (Ty::Infer(InferTy::IntVar(tv)), other)
+            | (Ty::Infer(InferTy::MaybeNeverTypeVar(tv)), other)
-            | (other, Ty::Infer(InferTy::IntVar(tv)))
+            | (other, Ty::Infer(InferTy::MaybeNeverTypeVar(tv)))
-            | (Ty::Infer(InferTy::FloatVar(tv)), other)
+            | (Ty::Infer(InferTy::IntVar(tv)), other @ ty_app!(TypeCtor::Int(_)))
-            | (other, Ty::Infer(InferTy::FloatVar(tv))) => {
+            | (other @ ty_app!(TypeCtor::Int(_)), Ty::Infer(InferTy::IntVar(tv)))
+            | (Ty::Infer(InferTy::FloatVar(tv)), other @ ty_app!(TypeCtor::Float(_)))
+            | (other @ ty_app!(TypeCtor::Float(_)), Ty::Infer(InferTy::FloatVar(tv))) => {
                 // the type var is unknown since we tried to resolve it
                 self.var_unification_table.union_value(*tv, TypeVarValue::Known(other.clone()));
                 true
             }
+
             _ => false,
         }
     }
@@ -315,6 +389,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         Ty::Infer(InferTy::FloatVar(self.var_unification_table.new_key(TypeVarValue::Unknown)))
     }
 
+    fn new_maybe_never_type_var(&mut self) -> Ty {
+        Ty::Infer(InferTy::MaybeNeverTypeVar(
+            self.var_unification_table.new_key(TypeVarValue::Unknown),
+        ))
+    }
+
     /// Replaces Ty::Unknown by a new type var, so we can maybe still infer it.
     fn insert_type_vars_shallow(&mut self, ty: Ty) -> Ty {
         match ty {
@@ -795,15 +875,302 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         ret_ty
     }
 
-    /// This is similar to unify, but it makes the first type coerce to the
+    /// Infer type of expression with possibly implicit coerce to the expected type.
-    /// second one.
+    /// Return the type after possible coercion.
-    fn coerce(&mut self, from_ty: &Ty, to_ty: &Ty) -> bool {
+    fn infer_expr_coerce(&mut self, expr: ExprId, expected: &Expectation) -> Ty {
-        if is_never(from_ty) {
+        let ty = self.infer_expr_inner(expr, &expected);
-            // ! coerces to any type
+        let ty = if !self.coerce(&ty, &expected.ty) {
-            true
+            self.result
+                .type_mismatches
+                .insert(expr, TypeMismatch { expected: expected.ty.clone(), actual: ty.clone() });
+            // Return actual type when type mismatch.
+            // This is needed for diagnostic when return type mismatch.
+            ty
+        } else if expected.ty == Ty::Unknown {
+            ty
         } else {
-            self.unify(from_ty, to_ty)
+            expected.ty.clone()
+        };
+
+        self.resolve_ty_as_possible(&mut vec![], 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.
+    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()
+        }
+    }
+
+    /// Unify two types, but may coerce the first one to the second one
+    /// using "implicit coercion rules" if needed.
+    ///
+    /// See: https://doc.rust-lang.org/nomicon/coercions.html
+    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)
+    }
+
+    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
     }
 
     fn infer_expr(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
@@ -828,15 +1195,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.infer_expr(*condition, &Expectation::has_type(Ty::simple(TypeCtor::Bool)));
 
                 let then_ty = self.infer_expr_inner(*then_branch, &expected);
-                self.coerce(&then_ty, &expected.ty);
-
                 let else_ty = match else_branch {
                     Some(else_branch) => self.infer_expr_inner(*else_branch, &expected),
                     None => Ty::unit(),
                 };
-                self.coerce(&else_ty, &expected.ty);
 
-                expected.ty.clone()
+                self.coerce_merge_branch(&then_ty, &else_ty)
             }
             Expr::Block { statements, tail } => self.infer_block(statements, *tail, expected),
             Expr::TryBlock { body } => {
@@ -932,13 +1296,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 .infer_method_call(tgt_expr, *receiver, &args, &method_name, generic_args.as_ref()),
             Expr::Match { expr, arms } => {
                 let input_ty = self.infer_expr(*expr, &Expectation::none());
-                let expected = if expected.ty == Ty::Unknown {
-                    Expectation::has_type(self.new_type_var())
-                } else {
-                    expected.clone()
-                };
 
-                let mut arm_tys = Vec::with_capacity(arms.len());
+                let mut result_ty = self.new_maybe_never_type_var();
 
                 for arm in arms {
                     for &pat in &arm.pats {
@@ -950,16 +1309,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                             &Expectation::has_type(Ty::simple(TypeCtor::Bool)),
                         );
                     }
-                    arm_tys.push(self.infer_expr_inner(arm.expr, &expected));
+
+                    let arm_ty = self.infer_expr_inner(arm.expr, &expected);
+                    result_ty = self.coerce_merge_branch(&result_ty, &arm_ty);
                 }
 
-                let lub_ty = calculate_least_upper_bound(expected.ty, &arm_tys);
+                result_ty
-
-                for arm_ty in &arm_tys {
-                    self.coerce(arm_ty, &lub_ty);
-                }
-
-                lub_ty
             }
             Expr::Path(p) => {
                 // FIXME this could be more efficient...
@@ -986,6 +1341,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     self.write_variant_resolution(tgt_expr.into(), variant);
                 }
 
+                self.unify(&ty, &expected.ty);
+
                 let substs = ty.substs().unwrap_or_else(Substs::empty);
                 for (field_idx, field) in fields.iter().enumerate() {
                     let field_ty = def_id
@@ -1001,7 +1358,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                         })
                         .map_or(Ty::Unknown, |field| field.ty(self.db))
                         .subst(&substs);
-                    self.infer_expr(field.expr, &Expectation::has_type(field_ty));
+                    self.infer_expr_coerce(field.expr, &Expectation::has_type(field_ty));
                 }
                 if let Some(expr) = spread {
                     self.infer_expr(*expr, &Expectation::has_type(ty.clone()));
@@ -1171,35 +1528,41 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 Ty::Unknown
             }
             Expr::Tuple { exprs } => {
-                let mut ty_vec = Vec::with_capacity(exprs.len());
+                let mut tys = match &expected.ty {
-                for arg in exprs.iter() {
+                    ty_app!(TypeCtor::Tuple { .. }, st) => st
-                    ty_vec.push(self.infer_expr(*arg, &Expectation::none()));
+                        .iter()
+                        .cloned()
+                        .chain(repeat_with(|| self.new_type_var()))
+                        .take(exprs.len())
+                        .collect::<Vec<_>>(),
+                    _ => (0..exprs.len()).map(|_| self.new_type_var()).collect(),
+                };
+
+                for (expr, ty) in exprs.iter().zip(tys.iter_mut()) {
+                    self.infer_expr_coerce(*expr, &Expectation::has_type(ty.clone()));
                 }
 
-                Ty::apply(
+                Ty::apply(TypeCtor::Tuple { cardinality: tys.len() as u16 }, Substs(tys.into()))
-                    TypeCtor::Tuple { cardinality: ty_vec.len() as u16 },
-                    Substs(ty_vec.into()),
-                )
             }
             Expr::Array(array) => {
                 let elem_ty = match &expected.ty {
-                    Ty::Apply(a_ty) => match a_ty.ctor {
+                    ty_app!(TypeCtor::Array, st) | ty_app!(TypeCtor::Slice, st) => {
-                        TypeCtor::Slice | TypeCtor::Array => {
+                        st.as_single().clone()
-                            Ty::clone(&a_ty.parameters.as_single())
                     }
                     _ => self.new_type_var(),
-                    },
-                    _ => self.new_type_var(),
                 };
 
                 match array {
                     Array::ElementList(items) => {
                         for expr in items.iter() {
-                            self.infer_expr(*expr, &Expectation::has_type(elem_ty.clone()));
+                            self.infer_expr_coerce(*expr, &Expectation::has_type(elem_ty.clone()));
                         }
                     }
                     Array::Repeat { initializer, repeat } => {
-                        self.infer_expr(*initializer, &Expectation::has_type(elem_ty.clone()));
+                        self.infer_expr_coerce(
+                            *initializer,
+                            &Expectation::has_type(elem_ty.clone()),
+                        );
                         self.infer_expr(
                             *repeat,
                             &Expectation::has_type(Ty::simple(TypeCtor::Int(
@@ -1246,14 +1609,19 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 Statement::Let { pat, type_ref, initializer } => {
                     let decl_ty =
                         type_ref.as_ref().map(|tr| self.make_ty(tr)).unwrap_or(Ty::Unknown);
-                    let decl_ty = self.insert_type_vars(decl_ty);
-                    let ty = if let Some(expr) = initializer {
-                        let expr_ty = self.infer_expr(*expr, &Expectation::has_type(decl_ty));
-                        expr_ty
-                    } else {
-                        decl_ty
-                    };
 
+                    // Always use the declared type when specified
+                    let mut ty = decl_ty.clone();
+
+                    if let Some(expr) = initializer {
+                        let actual_ty =
+                            self.infer_expr_coerce(*expr, &Expectation::has_type(decl_ty.clone()));
+                        if decl_ty == Ty::Unknown {
+                            ty = actual_ty;
+                        }
+                    }
+
+                    let ty = self.resolve_ty_as_possible(&mut vec![], ty);
                     self.infer_pat(*pat, &ty, BindingMode::default());
                 }
                 Statement::Expr(expr) => {
@@ -1261,9 +1629,13 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
             }
         }
-        let ty =
+
-            if let Some(expr) = tail { self.infer_expr_inner(expr, expected) } else { Ty::unit() };
+        if let Some(expr) = tail {
-        ty
+            self.infer_expr_coerce(expr, expected)
+        } else {
+            self.coerce(&Ty::unit(), &expected.ty);
+            Ty::unit()
+        }
     }
 
     fn check_call_arguments(&mut self, args: &[ExprId], param_tys: &[Ty]) {
@@ -1285,7 +1657,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
 
                 let param_ty = self.normalize_associated_types_in(param_ty);
-                self.infer_expr(arg, &Expectation::has_type(param_ty));
+                self.infer_expr_coerce(arg, &Expectation::has_type(param_ty.clone()));
             }
         }
     }
@@ -1400,12 +1772,16 @@ pub enum InferTy {
     TypeVar(TypeVarId),
     IntVar(TypeVarId),
     FloatVar(TypeVarId),
+    MaybeNeverTypeVar(TypeVarId),
 }
 
 impl InferTy {
     fn to_inner(self) -> TypeVarId {
         match self {
-            InferTy::TypeVar(ty) | InferTy::IntVar(ty) | InferTy::FloatVar(ty) => ty,
+            InferTy::TypeVar(ty)
+            | InferTy::IntVar(ty)
+            | InferTy::FloatVar(ty)
+            | InferTy::MaybeNeverTypeVar(ty) => ty,
         }
     }
 
@@ -1418,6 +1794,7 @@ impl InferTy {
             InferTy::FloatVar(..) => Ty::simple(TypeCtor::Float(
                 primitive::UncertainFloatTy::Known(primitive::FloatTy::f64()),
             )),
+            InferTy::MaybeNeverTypeVar(..) => Ty::simple(TypeCtor::Never),
         }
     }
 }
@@ -1475,37 +1852,3 @@ mod diagnostics {
         }
     }
 }
-
-fn is_never(ty: &Ty) -> bool {
-    if let Ty::Apply(ApplicationTy { ctor: TypeCtor::Never, .. }) = ty {
-        true
-    } else {
-        false
-    }
-}
-
-fn calculate_least_upper_bound(expected_ty: Ty, actual_tys: &[Ty]) -> Ty {
-    let mut all_never = true;
-    let mut last_never_ty = None;
-    let mut least_upper_bound = expected_ty;
-
-    for actual_ty in actual_tys {
-        if is_never(actual_ty) {
-            last_never_ty = Some(actual_ty.clone());
-        } else {
-            all_never = false;
-            least_upper_bound = match (actual_ty, &least_upper_bound) {
-                (_, Ty::Unknown)
-                | (Ty::Infer(_), Ty::Infer(InferTy::TypeVar(_)))
-                | (Ty::Apply(_), _) => actual_ty.clone(),
-                _ => least_upper_bound,
-            }
-        }
-    }
-
-    if all_never && last_never_ty.is_some() {
-        last_never_ty.unwrap()
-    } else {
-        least_upper_bound
-    }
-}

crates/ra_hir/src/ty/infer/unify.rs

@@ -63,6 +63,7 @@ where
                         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)

crates/ra_hir/src/ty/tests.rs

@@ -20,6 +20,9 @@ use crate::{
 // against snapshots of the expected results using insta. Use cargo-insta to
 // update the snapshots.
 
+mod never_type;
+mod coercion;
+
 #[test]
 fn infer_await() {
     let (mut db, pos) = MockDatabase::with_position(
@@ -236,17 +239,23 @@ fn test() {
     let a = 1isize;
     let b: usize = 1;
     let c = b;
+    let d: u32;
+    let e;
+    let f: i32 = e;
 }
 "#),
         @r###"
-
+    [11; 118) '{     ...= e; }': ()
-    [11; 71) '{     ...= b; }': ()
     [21; 22) 'a': isize
     [25; 31) '1isize': isize
     [41; 42) 'b': usize
     [52; 53) '1': usize
     [63; 64) 'c': usize
     [67; 68) 'b': usize
+    [78; 79) 'd': u32
+    [94; 95) 'e': i32
+    [105; 106) 'f': i32
+    [114; 115) 'e': i32
     "###
     );
 }
@@ -328,14 +337,14 @@ fn test() {
 "#),
         @r###"
     [45; 49) 'self': &[T]
-    [56; 79) '{     ...     }': !
+    [56; 79) '{     ...     }': T
     [66; 73) 'loop {}': !
     [71; 73) '{}': ()
     [133; 160) '{     ...o"); }': ()
     [139; 149) '<[_]>::foo': fn foo<u8>(&[T]) -> T
     [139; 157) '<[_]>:..."foo")': u8
     [150; 156) 'b"foo"': &[u8]
-"###
+    "###
     );
 }
 
@@ -800,6 +809,130 @@ fn test2(a1: *const A, a2: *mut A) {
     );
 }
 
+#[test]
+fn infer_argument_autoderef() {
+    assert_snapshot!(
+        infer(r#"
+#[lang = "deref"]
+pub trait Deref {
+    type Target;
+    fn deref(&self) -> &Self::Target;
+}
+
+struct A<T>(T);
+
+impl<T> A<T> {
+    fn foo(&self) -> &T {
+        &self.0
+    }
+}
+
+struct B<T>(T);
+
+impl<T> Deref for B<T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+fn test() {
+    let t = A::foo(&&B(B(A(42))));
+}
+"#),
+        @r###"
+    [68; 72) 'self': &Self
+    [139; 143) 'self': &A<T>
+    [151; 174) '{     ...     }': &T
+    [161; 168) '&self.0': &T
+    [162; 166) 'self': &A<T>
+    [162; 168) 'self.0': T
+    [255; 259) 'self': &B<T>
+    [278; 301) '{     ...     }': &T
+    [288; 295) '&self.0': &T
+    [289; 293) 'self': &B<T>
+    [289; 295) 'self.0': T
+    [315; 353) '{     ...))); }': ()
+    [325; 326) 't': &i32
+    [329; 335) 'A::foo': fn foo<i32>(&A<T>) -> &T
+    [329; 350) 'A::foo...42))))': &i32
+    [336; 349) '&&B(B(A(42)))': &&B<B<A<i32>>>
+    [337; 349) '&B(B(A(42)))': &B<B<A<i32>>>
+    [338; 339) 'B': B<B<A<i32>>>(T) -> B<T>
+    [338; 349) 'B(B(A(42)))': B<B<A<i32>>>
+    [340; 341) 'B': B<A<i32>>(T) -> B<T>
+    [340; 348) 'B(A(42))': B<A<i32>>
+    [342; 343) 'A': A<i32>(T) -> A<T>
+    [342; 347) 'A(42)': A<i32>
+    [344; 346) '42': i32
+    "###
+    );
+}
+
+#[test]
+fn infer_method_argument_autoderef() {
+    assert_snapshot!(
+        infer(r#"
+#[lang = "deref"]
+pub trait Deref {
+    type Target;
+    fn deref(&self) -> &Self::Target;
+}
+
+struct A<T>(*mut T);
+
+impl<T> A<T> {
+    fn foo(&self, x: &A<T>) -> &T {
+        &*x.0
+    }
+}
+
+struct B<T>(T);
+
+impl<T> Deref for B<T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+fn test(a: A<i32>) {
+    let t = A(0 as *mut _).foo(&&B(B(a)));
+}
+"#),
+        @r###"
+    [68; 72) 'self': &Self
+    [144; 148) 'self': &A<T>
+    [150; 151) 'x': &A<T>
+    [166; 187) '{     ...     }': &T
+    [176; 181) '&*x.0': &T
+    [177; 181) '*x.0': T
+    [178; 179) 'x': &A<T>
+    [178; 181) 'x.0': *mut T
+    [268; 272) 'self': &B<T>
+    [291; 314) '{     ...     }': &T
+    [301; 308) '&self.0': &T
+    [302; 306) 'self': &B<T>
+    [302; 308) 'self.0': T
+    [326; 327) 'a': A<i32>
+    [337; 383) '{     ...))); }': ()
+    [347; 348) 't': &i32
+    [351; 352) 'A': A<i32>(*mut T) -> A<T>
+    [351; 365) 'A(0 as *mut _)': A<i32>
+    [351; 380) 'A(0 as...B(a)))': &i32
+    [353; 354) '0': i32
+    [353; 364) '0 as *mut _': *mut i32
+    [370; 379) '&&B(B(a))': &&B<B<A<i32>>>
+    [371; 379) '&B(B(a))': &B<B<A<i32>>>
+    [372; 373) 'B': B<B<A<i32>>>(T) -> B<T>
+    [372; 379) 'B(B(a))': B<B<A<i32>>>
+    [374; 375) 'B': B<A<i32>>(T) -> B<T>
+    [374; 378) 'B(a)': B<A<i32>>
+    [376; 377) 'a': A<i32>
+"###
+    );
+}
+
 #[test]
 fn bug_484() {
     assert_snapshot!(
@@ -983,14 +1116,12 @@ fn test(x: &str, y: isize) {
 
     let b = [a, ["b"]];
     let x: [u8; 0] = [];
-    let z: &[u8] = &[1, 2, 3];
 }
 "#),
         @r###"
-
     [9; 10) 'x': &str
     [18; 19) 'y': isize
-    [28; 324) '{     ... 3]; }': ()
+    [28; 293) '{     ... []; }': ()
     [38; 39) 'a': [&str;_]
     [42; 45) '[x]': [&str;_]
     [43; 44) 'x': &str
@@ -1040,12 +1171,6 @@ fn test(x: &str, y: isize) {
     [260; 263) '"b"': &str
     [275; 276) 'x': [u8;_]
     [288; 290) '[]': [u8;_]
-    [300; 301) 'z': &[u8;_]
-    [311; 321) '&[1, 2, 3]': &[u8;_]
-    [312; 321) '[1, 2, 3]': [u8;_]
-    [313; 314) '1': u8
-    [316; 317) '2': u8
-    [319; 320) '3': u8
     "###
     );
 }
@@ -1767,8 +1892,7 @@ fn test() {
 }
 "#),
         @r###"
-
+    [80; 104) '{     ...     }': Gen<T>
-    [80; 104) '{     ...     }': !
     [90; 98) 'loop { }': !
     [95; 98) '{ }': ()
     [118; 146) '{     ...e(); }': ()
@@ -1798,8 +1922,7 @@ fn test() {
 }
 "#),
         @r###"
-
+    [76; 100) '{     ...     }': Gen<T>
-    [76; 100) '{     ...     }': !
     [86; 94) 'loop { }': !
     [91; 94) '{ }': ()
     [114; 149) '{     ...e(); }': ()
@@ -1830,8 +1953,7 @@ fn test() {
 }
 "#),
         @r###"
-
+    [102; 126) '{     ...     }': Gen<u32, T>
-    [102; 126) '{     ...     }': !
     [112; 120) 'loop { }': !
     [117; 120) '{ }': ()
     [140; 180) '{     ...e(); }': ()
@@ -1973,7 +2095,6 @@ fn test() {
 }
 "#),
         @r###"
-
     [11; 48) '{     ...&y]; }': ()
     [21; 22) 'y': &{unknown}
     [25; 32) 'unknown': &{unknown}
@@ -1998,14 +2119,13 @@ fn test() {
 }
 "#),
         @r###"
-
     [11; 80) '{     ...x)]; }': ()
     [21; 22) 'x': &&{unknown}
     [25; 32) 'unknown': &&{unknown}
     [42; 43) 'y': &&{unknown}
     [46; 53) 'unknown': &&{unknown}
-    [59; 77) '[(x, y..., &x)]': [(&&{unknown}, &&{unknown});_]
+    [59; 77) '[(x, y..., &x)]': [(&&&{unknown}, &&&{unknown});_]
-    [60; 66) '(x, y)': (&&{unknown}, &&{unknown})
+    [60; 66) '(x, y)': (&&&{unknown}, &&&{unknown})
     [61; 62) 'x': &&{unknown}
     [64; 65) 'y': &&{unknown}
     [68; 76) '(&y, &x)': (&&&{unknown}, &&&{unknown})
@@ -2026,7 +2146,7 @@ fn id<T>(x: T) -> T {
 }
 
 fn clone<T>(x: &T) -> T {
-    x
+    *x
 }
 
 fn test() {
@@ -2037,26 +2157,26 @@ fn test() {
 }
 "#),
         @r###"
-
     [10; 11) 'x': T
     [21; 30) '{     x }': T
     [27; 28) 'x': T
     [44; 45) 'x': &T
-    [56; 65) '{     x }': &T
+    [56; 66) '{     *x }': T
-    [62; 63) 'x': &T
+    [62; 64) '*x': T
-    [77; 157) '{     ...(1); }': ()
+    [63; 64) 'x': &T
-    [87; 88) 'y': u32
+    [78; 158) '{     ...(1); }': ()
-    [91; 96) '10u32': u32
+    [88; 89) 'y': u32
-    [102; 104) 'id': fn id<u32>(T) -> T
+    [92; 97) '10u32': u32
-    [102; 107) 'id(y)': u32
+    [103; 105) 'id': fn id<u32>(T) -> T
-    [105; 106) 'y': u32
+    [103; 108) 'id(y)': u32
-    [117; 118) 'x': bool
+    [106; 107) 'y': u32
-    [127; 132) 'clone': fn clone<bool>(&T) -> T
+    [118; 119) 'x': bool
-    [127; 135) 'clone(z)': bool
+    [128; 133) 'clone': fn clone<bool>(&T) -> T
-    [133; 134) 'z': &bool
+    [128; 136) 'clone(z)': bool
-    [141; 151) 'id::<i128>': fn id<i128>(T) -> T
+    [134; 135) 'z': &bool
-    [141; 154) 'id::<i128>(1)': i128
+    [142; 152) 'id::<i128>': fn id<i128>(T) -> T
-    [152; 153) '1': i128
+    [142; 155) 'id::<i128>(1)': i128
+    [153; 154) '1': i128
     "###
     );
 }
@@ -2181,7 +2301,6 @@ fn extra_compiler_flags() {
 }
 "#),
         @r###"
-
     [27; 323) '{     ...   } }': ()
     [33; 321) 'for co...     }': ()
     [37; 44) 'content': &{unknown}
@@ -2195,8 +2314,8 @@ fn extra_compiler_flags() {
     [135; 167) '{     ...     }': &&{unknown}
     [149; 157) '&content': &&{unknown}
     [150; 157) 'content': &{unknown}
-    [182; 189) 'content': &&{unknown}
+    [182; 189) 'content': &{unknown}
-    [192; 314) 'if ICE...     }': &&{unknown}
+    [192; 314) 'if ICE...     }': &{unknown}
     [195; 232) 'ICE_RE..._VALUE': {unknown}
     [195; 248) 'ICE_RE...&name)': bool
     [242; 247) '&name': &&&{unknown}
@@ -3282,7 +3401,7 @@ impl S {
 }
 
 fn test(s: Arc<S>) {
-    (*s, s.foo())<|>
+    (*s, s.foo())<|>;
 }
 "#,
     );
@@ -3356,7 +3475,7 @@ trait Deref {
 }
 
 struct Arc<T>;
-impl<T: ?Sized> Deref for Arc<T> {
+impl<T> Deref for Arc<T> {
     type Target = T;
 }
 
@@ -3366,7 +3485,7 @@ impl S {
 }
 
 fn test(s: Arc<S>) {
-    (*s, s.foo())<|>
+    (*s, s.foo())<|>;
 }
 "#,
     );
@@ -4406,121 +4525,3 @@ fn no_such_field_diagnostics() {
     "###
     );
 }
-
-mod branching_with_never_tests {
-    use super::type_at;
-
-    #[test]
-    fn if_never() {
-        let t = type_at(
-            r#"
-//- /main.rs
-fn test() {
-    let i = if true {
-        loop {}
-    } else {
-        3.0
-    };
-    i<|>
-    ()
-}
-"#,
-        );
-        assert_eq!(t, "f64");
-    }
-
-    #[test]
-    fn if_else_never() {
-        let t = type_at(
-            r#"
-//- /main.rs
-fn test(input: bool) {
-    let i = if input {
-        2.0
-    } else {
-        return
-    };
-    i<|>
-    ()
-}
-"#,
-        );
-        assert_eq!(t, "f64");
-    }
-
-    #[test]
-    fn match_first_arm_never() {
-        let t = type_at(
-            r#"
-//- /main.rs
-fn test(a: i32) {
-    let i = match a {
-        1 => return,
-        2 => 2.0,
-        3 => loop {},
-        _ => 3.0,
-    };
-    i<|>
-    ()
-}
-"#,
-        );
-        assert_eq!(t, "f64");
-    }
-
-    #[test]
-    fn match_second_arm_never() {
-        let t = type_at(
-            r#"
-//- /main.rs
-fn test(a: i32) {
-    let i = match a {
-        1 => 3.0,
-        2 => loop {},
-        3 => 3.0,
-        _ => return,
-    };
-    i<|>
-    ()
-}
-"#,
-        );
-        assert_eq!(t, "f64");
-    }
-
-    #[test]
-    fn match_all_arms_never() {
-        let t = type_at(
-            r#"
-//- /main.rs
-fn test(a: i32) {
-    let i = match a {
-        2 => return,
-        _ => loop {},
-    };
-    i<|>
-    ()
-}
-"#,
-        );
-        assert_eq!(t, "!");
-    }
-
-    #[test]
-    fn match_no_never_arms() {
-        let t = type_at(
-            r#"
-//- /main.rs
-fn test(a: i32) {
-    let i = match a {
-        2 => 2.0,
-        _ => 3.0,
-    };
-    i<|>
-    ()
-}
-"#,
-        );
-        assert_eq!(t, "f64");
-    }
-}

crates/ra_hir/src/ty/tests/coercion.rs

@@ -0,0 +1,369 @@
+use insta::assert_snapshot;
+use test_utils::covers;
+
+// Infer with some common definitions and impls.
+fn infer(source: &str) -> String {
+    let defs = r#"
+        #[lang = "sized"]
+        pub trait Sized {}
+        #[lang = "unsize"]
+        pub trait Unsize<T: ?Sized> {}
+        #[lang = "coerce_unsized"]
+        pub trait CoerceUnsized<T> {}
+
+        impl<'a, 'b: 'a, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
+        impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {}
+    "#;
+
+    // Append to the end to keep positions unchanged.
+    super::infer(&format!("{}{}", source, defs))
+}
+
+#[test]
+fn infer_block_expr_type_mismatch() {
+    assert_snapshot!(
+        infer(r#"
+fn test() {
+    let a: i32 = { 1i64 };
+}
+"#),
+        @r###"
+    [11; 41) '{     ...4 }; }': ()
+    [21; 22) 'a': i32
+    [30; 38) '{ 1i64 }': i64
+    [32; 36) '1i64': i64
+    "###);
+}
+
+#[test]
+fn coerce_places() {
+    assert_snapshot!(
+        infer(r#"
+struct S<T> { a: T }
+
+fn f<T>(_: &[T]) -> T { loop {} }
+fn g<T>(_: S<&[T]>) -> T { loop {} }
+
+fn gen<T>() -> *mut [T; 2] { loop {} }
+fn test1<U>() -> *mut [U] {
+    gen()
+}
+
+fn test2() {
+    let arr: &[u8; 1] = &[1];
+
+    let a: &[_] = arr;
+    let b = f(arr);
+    let c: &[_] = { arr };
+    let d = g(S { a: arr });
+    let e: [&[_]; 1] = [arr];
+    let f: [&[_]; 2] = [arr; 2];
+    let g: (&[_], &[_]) = (arr, arr);
+}
+"#),
+        @r###"
+    [31; 32) '_': &[T]
+    [45; 56) '{ loop {} }': T
+    [47; 54) 'loop {}': !
+    [52; 54) '{}': ()
+    [65; 66) '_': S<&[T]>
+    [82; 93) '{ loop {} }': T
+    [84; 91) 'loop {}': !
+    [89; 91) '{}': ()
+    [122; 133) '{ loop {} }': *mut [T;_]
+    [124; 131) 'loop {}': !
+    [129; 131) '{}': ()
+    [160; 173) '{     gen() }': *mut [U]
+    [166; 169) 'gen': fn gen<U>() -> *mut [T;_]
+    [166; 171) 'gen()': *mut [U;_]
+    [186; 420) '{     ...rr); }': ()
+    [196; 199) 'arr': &[u8;_]
+    [212; 216) '&[1]': &[u8;_]
+    [213; 216) '[1]': [u8;_]
+    [214; 215) '1': u8
+    [227; 228) 'a': &[u8]
+    [237; 240) 'arr': &[u8;_]
+    [250; 251) 'b': u8
+    [254; 255) 'f': fn f<u8>(&[T]) -> T
+    [254; 260) 'f(arr)': u8
+    [256; 259) 'arr': &[u8;_]
+    [270; 271) 'c': &[u8]
+    [280; 287) '{ arr }': &[u8]
+    [282; 285) 'arr': &[u8;_]
+    [297; 298) 'd': u8
+    [301; 302) 'g': fn g<u8>(S<&[T]>) -> T
+    [301; 316) 'g(S { a: arr })': u8
+    [303; 315) 'S { a: arr }': S<&[u8]>
+    [310; 313) 'arr': &[u8;_]
+    [326; 327) 'e': [&[u8];_]
+    [341; 346) '[arr]': [&[u8];_]
+    [342; 345) 'arr': &[u8;_]
+    [356; 357) 'f': [&[u8];_]
+    [371; 379) '[arr; 2]': [&[u8];_]
+    [372; 375) 'arr': &[u8;_]
+    [377; 378) '2': usize
+    [389; 390) 'g': (&[u8], &[u8])
+    [407; 417) '(arr, arr)': (&[u8], &[u8])
+    [408; 411) 'arr': &[u8;_]
+    [413; 416) 'arr': &[u8;_]
+    "###
+    );
+}
+
+#[test]
+fn infer_let_stmt_coerce() {
+    assert_snapshot!(
+        infer(r#"
+fn test() {
+    let x: &[i32] = &[1];
+}
+"#),
+        @r###"
+    [11; 40) '{     ...[1]; }': ()
+    [21; 22) 'x': &[i32]
+    [33; 37) '&[1]': &[i32;_]
+    [34; 37) '[1]': [i32;_]
+    [35; 36) '1': i32
+    "###);
+}
+
+#[test]
+fn infer_custom_coerce_unsized() {
+    assert_snapshot!(
+        infer(r#"
+struct A<T: ?Sized>(*const T);
+struct B<T: ?Sized>(*const T);
+struct C<T: ?Sized> { inner: *const T }
+
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<B<U>> for B<T> {}
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<C<U>> for C<T> {}
+
+fn foo1<T>(x: A<[T]>) -> A<[T]> { x }
+fn foo2<T>(x: B<[T]>) -> B<[T]> { x }
+fn foo3<T>(x: C<[T]>) -> C<[T]> { x }
+
+fn test(a: A<[u8; 2]>, b: B<[u8; 2]>, c: C<[u8; 2]>) {
+    let d = foo1(a);
+    let e = foo2(b);
+    let f = foo3(c);
+}
+"#),
+        @r###"
+    [258; 259) 'x': A<[T]>
+    [279; 284) '{ x }': A<[T]>
+    [281; 282) 'x': A<[T]>
+    [296; 297) 'x': B<[T]>
+    [317; 322) '{ x }': B<[T]>
+    [319; 320) 'x': B<[T]>
+    [334; 335) 'x': C<[T]>
+    [355; 360) '{ x }': C<[T]>
+    [357; 358) 'x': C<[T]>
+    [370; 371) 'a': A<[u8;_]>
+    [385; 386) 'b': B<[u8;_]>
+    [400; 401) 'c': C<[u8;_]>
+    [415; 481) '{     ...(c); }': ()
+    [425; 426) 'd': A<[{unknown}]>
+    [429; 433) 'foo1': fn foo1<{unknown}>(A<[T]>) -> A<[T]>
+    [429; 436) 'foo1(a)': A<[{unknown}]>
+    [434; 435) 'a': A<[u8;_]>
+    [446; 447) 'e': B<[u8]>
+    [450; 454) 'foo2': fn foo2<u8>(B<[T]>) -> B<[T]>
+    [450; 457) 'foo2(b)': B<[u8]>
+    [455; 456) 'b': B<[u8;_]>
+    [467; 468) 'f': C<[u8]>
+    [471; 475) 'foo3': fn foo3<u8>(C<[T]>) -> C<[T]>
+    [471; 478) 'foo3(c)': C<[u8]>
+    [476; 477) 'c': C<[u8;_]>
+    "###
+    );
+}
+
+#[test]
+fn infer_if_coerce() {
+    assert_snapshot!(
+        infer(r#"
+fn foo<T>(x: &[T]) -> &[T] { loop {} }
+fn test() {
+    let x = if true {
+        foo(&[1])
+    } else {
+        &[1]
+    };
+}
+"#),
+        @r###"
+    [11; 12) 'x': &[T]
+    [28; 39) '{ loop {} }': &[T]
+    [30; 37) 'loop {}': !
+    [35; 37) '{}': ()
+    [50; 126) '{     ...  }; }': ()
+    [60; 61) 'x': &[i32]
+    [64; 123) 'if tru...     }': &[i32]
+    [67; 71) 'true': bool
+    [72; 97) '{     ...     }': &[i32]
+    [82; 85) 'foo': fn foo<i32>(&[T]) -> &[T]
+    [82; 91) 'foo(&[1])': &[i32]
+    [86; 90) '&[1]': &[i32;_]
+    [87; 90) '[1]': [i32;_]
+    [88; 89) '1': i32
+    [103; 123) '{     ...     }': &[i32;_]
+    [113; 117) '&[1]': &[i32;_]
+    [114; 117) '[1]': [i32;_]
+    [115; 116) '1': i32
+    "###
+    );
+}
+
+#[test]
+fn infer_if_else_coerce() {
+    assert_snapshot!(
+        infer(r#"
+fn foo<T>(x: &[T]) -> &[T] { loop {} }
+fn test() {
+    let x = if true {
+        &[1]
+    } else {
+        foo(&[1])
+    };
+}
+"#),
+        @r###"
+    [11; 12) 'x': &[T]
+    [28; 39) '{ loop {} }': &[T]
+    [30; 37) 'loop {}': !
+    [35; 37) '{}': ()
+    [50; 126) '{     ...  }; }': ()
+    [60; 61) 'x': &[i32]
+    [64; 123) 'if tru...     }': &[i32]
+    [67; 71) 'true': bool
+    [72; 92) '{     ...     }': &[i32;_]
+    [82; 86) '&[1]': &[i32;_]
+    [83; 86) '[1]': [i32;_]
+    [84; 85) '1': i32
+    [98; 123) '{     ...     }': &[i32]
+    [108; 111) 'foo': fn foo<i32>(&[T]) -> &[T]
+    [108; 117) 'foo(&[1])': &[i32]
+    [112; 116) '&[1]': &[i32;_]
+    [113; 116) '[1]': [i32;_]
+    [114; 115) '1': i32
+    "###
+    );
+}
+
+#[test]
+fn infer_match_first_coerce() {
+    assert_snapshot!(
+        infer(r#"
+fn foo<T>(x: &[T]) -> &[T] { loop {} }
+fn test(i: i32) {
+    let x = match i {
+        2 => foo(&[2]),
+        1 => &[1],
+        _ => &[3],
+    };
+}
+"#),
+        @r###"
+    [11; 12) 'x': &[T]
+    [28; 39) '{ loop {} }': &[T]
+    [30; 37) 'loop {}': !
+    [35; 37) '{}': ()
+    [48; 49) 'i': i32
+    [56; 150) '{     ...  }; }': ()
+    [66; 67) 'x': &[i32]
+    [70; 147) 'match ...     }': &[i32]
+    [76; 77) 'i': i32
+    [88; 89) '2': i32
+    [93; 96) 'foo': fn foo<i32>(&[T]) -> &[T]
+    [93; 102) 'foo(&[2])': &[i32]
+    [97; 101) '&[2]': &[i32;_]
+    [98; 101) '[2]': [i32;_]
+    [99; 100) '2': i32
+    [112; 113) '1': i32
+    [117; 121) '&[1]': &[i32;_]
+    [118; 121) '[1]': [i32;_]
+    [119; 120) '1': i32
+    [131; 132) '_': i32
+    [136; 140) '&[3]': &[i32;_]
+    [137; 140) '[3]': [i32;_]
+    [138; 139) '3': i32
+    "###
+    );
+}
+
+#[test]
+fn infer_match_second_coerce() {
+    assert_snapshot!(
+        infer(r#"
+fn foo<T>(x: &[T]) -> &[T] { loop {} }
+fn test(i: i32) {
+    let x = match i {
+        1 => &[1],
+        2 => foo(&[2]),
+        _ => &[3],
+    };
+}
+"#),
+        @r###"
+    [11; 12) 'x': &[T]
+    [28; 39) '{ loop {} }': &[T]
+    [30; 37) 'loop {}': !
+    [35; 37) '{}': ()
+    [48; 49) 'i': i32
+    [56; 150) '{     ...  }; }': ()
+    [66; 67) 'x': &[i32]
+    [70; 147) 'match ...     }': &[i32]
+    [76; 77) 'i': i32
+    [88; 89) '1': i32
+    [93; 97) '&[1]': &[i32;_]
+    [94; 97) '[1]': [i32;_]
+    [95; 96) '1': i32
+    [107; 108) '2': i32
+    [112; 115) 'foo': fn foo<i32>(&[T]) -> &[T]
+    [112; 121) 'foo(&[2])': &[i32]
+    [116; 120) '&[2]': &[i32;_]
+    [117; 120) '[2]': [i32;_]
+    [118; 119) '2': i32
+    [131; 132) '_': i32
+    [136; 140) '&[3]': &[i32;_]
+    [137; 140) '[3]': [i32;_]
+    [138; 139) '3': i32
+    "###
+    );
+}
+
+#[test]
+fn coerce_merge_one_by_one1() {
+    covers!(coerce_merge_fail_fallback);
+
+    assert_snapshot!(
+        infer(r#"
+fn test() {
+    let t = &mut 1;
+    let x = match 1 {
+        1 => t as *mut i32,
+        2 => t as &i32,
+        _ => t as *const i32,
+    };
+}
+"#),
+        @r###"
+    [11; 145) '{     ...  }; }': ()
+    [21; 22) 't': &mut i32
+    [25; 31) '&mut 1': &mut i32
+    [30; 31) '1': i32
+    [41; 42) 'x': *const i32
+    [45; 142) 'match ...     }': *const i32
+    [51; 52) '1': i32
+    [63; 64) '1': i32
+    [68; 69) 't': &mut i32
+    [68; 81) 't as *mut i32': *mut i32
+    [91; 92) '2': i32
+    [96; 97) 't': &mut i32
+    [96; 105) 't as &i32': &i32
+    [115; 116) '_': i32
+    [120; 121) 't': &mut i32
+    [120; 135) 't as *const i32': *const i32
+    "###
+    );
+}

crates/ra_hir/src/ty/tests/never_type.rs

@@ -0,0 +1,246 @@
+use super::type_at;
+
+#[test]
+fn infer_never1() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test() {
+    let t = return;
+    t<|>;
+}
+"#,
+    );
+    assert_eq!(t, "!");
+}
+
+#[test]
+fn infer_never2() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn gen<T>() -> T { loop {} }
+
+fn test() {
+    let a = gen();
+    if false { a } else { loop {} };
+    a<|>;
+}
+"#,
+    );
+    assert_eq!(t, "!");
+}
+
+#[test]
+fn infer_never3() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn gen<T>() -> T { loop {} }
+
+fn test() {
+    let a = gen();
+    if false { loop {} } else { a };
+    a<|>;
+}
+"#,
+    );
+    assert_eq!(t, "!");
+}
+
+#[test]
+fn never_type_in_generic_args() {
+    let t = type_at(
+        r#"
+//- /main.rs
+enum Option<T> { None, Some(T) }
+
+fn test() {
+    let a = if true { Option::None } else { Option::Some(return) };
+    a<|>;
+}
+"#,
+    );
+    assert_eq!(t, "Option<!>");
+}
+
+#[test]
+fn never_type_can_be_reinferred1() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn gen<T>() -> T { loop {} }
+
+fn test() {
+    let a = gen();
+    if false { loop {} } else { a };
+    a<|>;
+    if false { a };
+}
+"#,
+    );
+    assert_eq!(t, "()");
+}
+
+#[test]
+fn never_type_can_be_reinferred2() {
+    let t = type_at(
+        r#"
+//- /main.rs
+enum Option<T> { None, Some(T) }
+
+fn test() {
+    let a = if true { Option::None } else { Option::Some(return) };
+    a<|>;
+    match 42 {
+        42 => a,
+        _ => Option::Some(42),
+    };
+}
+"#,
+    );
+    assert_eq!(t, "Option<i32>");
+}
+#[test]
+fn never_type_can_be_reinferred3() {
+    let t = type_at(
+        r#"
+//- /main.rs
+enum Option<T> { None, Some(T) }
+
+fn test() {
+    let a = if true { Option::None } else { Option::Some(return) };
+    a<|>;
+    match 42 {
+        42 => a,
+        _ => Option::Some("str"),
+    };
+}
+"#,
+    );
+    assert_eq!(t, "Option<&str>");
+}
+
+#[test]
+fn match_no_arm() {
+    let t = type_at(
+        r#"
+//- /main.rs
+enum Void {}
+
+fn test(a: Void) {
+    let t = match a {};
+    t<|>;
+}
+"#,
+    );
+    assert_eq!(t, "!");
+}
+
+#[test]
+fn if_never() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test() {
+    let i = if true {
+        loop {}
+    } else {
+        3.0
+    };
+    i<|>;
+}
+"#,
+    );
+    assert_eq!(t, "f64");
+}
+
+#[test]
+fn if_else_never() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test(input: bool) {
+    let i = if input {
+        2.0
+    } else {
+        return
+    };
+    i<|>;
+}
+"#,
+    );
+    assert_eq!(t, "f64");
+}
+
+#[test]
+fn match_first_arm_never() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test(a: i32) {
+    let i = match a {
+        1 => return,
+        2 => 2.0,
+        3 => loop {},
+        _ => 3.0,
+    };
+    i<|>;
+}
+"#,
+    );
+    assert_eq!(t, "f64");
+}
+
+#[test]
+fn match_second_arm_never() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test(a: i32) {
+    let i = match a {
+        1 => 3.0,
+        2 => loop {},
+        3 => 3.0,
+        _ => return,
+    };
+    i<|>;
+}
+"#,
+    );
+    assert_eq!(t, "f64");
+}
+
+#[test]
+fn match_all_arms_never() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test(a: i32) {
+    let i = match a {
+        2 => return,
+        _ => loop {},
+    };
+    i<|>;
+}
+"#,
+    );
+    assert_eq!(t, "!");
+}
+
+#[test]
+fn match_no_never_arms() {
+    let t = type_at(
+        r#"
+//- /main.rs
+fn test(a: i32) {
+    let i = match a {
+        2 => 2.0,
+        _ => 3.0,
+    };
+    i<|>;
+}
+"#,
+    );
+    assert_eq!(t, "f64");
+}