Support custom `CoerceUnsized`

Split out tests about coercion

4 files changed, 476 insertions, 247 deletions

diff --git a/crates/ra_hir/src/ty/infer.rs b/crates/ra_hir/src/ty/infer.rs
index cbbba8b23..ba63050a9 100644
--- a/crates/ra_hir/src/ty/infer.rs
+++ b/crates/ra_hir/src/ty/infer.rs
@@ -42,6 +42,7 @@ use crate::{
         RecordFieldPat, Statement, UnaryOp,
     },
     generics::{GenericParams, HasGenericParams},
+    lang_item::LangItemTarget,
     name,
     nameres::Namespace,
     path::{known, GenericArg, GenericArgs},
@@ -188,6 +189,12 @@ 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 {
@@ -207,12 +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());
@@ -919,16 +966,8 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             _ => {}
         }
 
-        // FIXME: Solve `FromTy: CoerceUnsized<ToTy>` instead of listing common impls here.
-        match (&from_ty, &to_ty) {
-            // Mutilibity is checked above
-            (ty_app!(TypeCtor::Ref(_), st1), ty_app!(TypeCtor::Ref(_), st2))
-            | (ty_app!(TypeCtor::RawPtr(_), st1), ty_app!(TypeCtor::RawPtr(_), st2)) => {
-                if self.try_coerce_unsized(&st1[0], &st2[0], 0) {
-                    return true;
-                }
-            }
-            _ => {}
+        if let Some(ret) = self.try_coerce_unsized(&from_ty, &to_ty) {
+            return ret;
         }
 
         // Auto Deref if cannot coerce
@@ -943,10 +982,43 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         }
     }
 
-    /// Coerce a type to a DST if `FromTy: Unsize<ToTy>`
+    /// Coerce a type using `from_ty: CoerceUnsized<ty_ty>`
     ///
-    /// See: `https://doc.rust-lang.org/nightly/std/marker/trait.Unsize.html`
-    fn try_coerce_unsized(&mut self, from_ty: &Ty, to_ty: &Ty, depth: usize) -> bool {
+    /// 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");
         }
@@ -954,29 +1026,113 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         match (&from_ty, &to_ty) {
             // `[T; N]` -> `[T]`
             (ty_app!(TypeCtor::Array, st1), ty_app!(TypeCtor::Slice, st2)) => {
-                self.unify(&st1[0], &st2[0])
+                Some(self.unify(&st1[0], &st2[0]))
             }
 
             // `T` -> `dyn Trait` when `T: Trait`
             (_, Ty::Dyn(_)) => {
                 // FIXME: Check predicates
-                true
+                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 => {
-                // FIXME: Check preconditions here
-                for (ty1, ty2) in st1.iter().zip(st2.iter()) {
-                    if !self.try_coerce_unsized(ty1, ty2, depth + 1) {
-                        return false;
+                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;
                 }
-                true
+
+                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)
             }
 
-            _ => false,
+            _ => None,
         }
     }
 
@@ -1433,12 +1589,11 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                     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 {
-                        self.infer_expr_coerce(*expr, &Expectation::has_type(decl_ty))
-                    } else {
-                        decl_ty
-                    };
+                    if let Some(expr) = initializer {
+                        self.infer_expr_coerce(*expr, &Expectation::has_type(decl_ty.clone()));
+                    }
 
+                    let ty = self.resolve_ty_as_possible(&mut vec![], decl_ty);
                     self.infer_pat(*pat, &ty, BindingMode::default());
                 }
                 Statement::Expr(expr) => {
diff --git a/crates/ra_hir/src/ty/tests.rs b/crates/ra_hir/src/ty/tests.rs
index 081bfe37c..cd5a05092 100644
--- a/crates/ra_hir/src/ty/tests.rs
+++ b/crates/ra_hir/src/ty/tests.rs
@@ -21,6 +21,7 @@ use crate::{
 // update the snapshots.
 
 mod never_type;
+mod coercion;
 
 #[test]
 fn infer_await() {
@@ -926,196 +927,6 @@ fn test(a: A<i32>) {
 }
 
 #[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 {} }': !
-    [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 {} }': !
-    [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 {} }': !
-    [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 {} }': !
-    [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
-    "###
-    );
-}
-
-#[test]
 fn bug_484() {
     assert_snapshot!(
         infer(r#"
@@ -1302,7 +1113,6 @@ fn test(x: &str, y: isize) {
 }
 "#),
         @r###"
-
     [9; 10) 'x': &str
     [18; 19) 'y': isize
     [28; 324) '{     ... 3]; }': ()
@@ -1355,7 +1165,7 @@ fn test(x: &str, y: isize) {
     [260; 263) '"b"': &str
     [275; 276) 'x': [u8;_]
     [288; 290) '[]': [u8;_]
-    [300; 301) 'z': &[u8;_]
+    [300; 301) 'z': &[u8]
     [311; 321) '&[1, 2, 3]': &[u8;_]
     [312; 321) '[1, 2, 3]': [u8;_]
     [313; 314) '1': u8
@@ -2288,10 +2098,9 @@ fn test() {
 }
 "#),
         @r###"
-
     [11; 48) '{     ...&y]; }': ()
     [21; 22) 'y': &{unknown}
-    [25; 32) 'unknown': &{unknown}
+    [25; 32) 'unknown': &&{unknown}
     [38; 45) '[y, &y]': [&&{unknown};_]
     [39; 40) 'y': &{unknown}
     [42; 44) '&y': &&{unknown}
@@ -2313,12 +2122,11 @@ fn test() {
 }
 "#),
         @r###"
-
     [11; 80) '{     ...x)]; }': ()
     [21; 22) 'x': &&{unknown}
-    [25; 32) 'unknown': &&{unknown}
+    [25; 32) 'unknown': &&&{unknown}
     [42; 43) 'y': &&{unknown}
-    [46; 53) 'unknown': &&{unknown}
+    [46; 53) 'unknown': &&&{unknown}
     [59; 77) '[(x, y..., &x)]': [(&&{unknown}, &&{unknown});_]
     [60; 66) '(x, y)': (&&{unknown}, &&{unknown})
     [61; 62) 'x': &&{unknown}
diff --git a/crates/ra_hir/src/ty/tests/coercion.rs b/crates/ra_hir/src/ty/tests/coercion.rs
new file mode 100644
index 000000000..d80d3fb6f
--- /dev/null
+++ b/crates/ra_hir/src/ty/tests/coercion.rs
@@ -0,0 +1,278 @@
+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_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 {} }': !
+    [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 {} }': !
+    [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 {} }': !
+    [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 {} }': !
+    [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
+    "###
+    );
+}
diff --git a/crates/ra_hir/src/ty/tests/never_type.rs b/crates/ra_hir/src/ty/tests/never_type.rs
index b9af918e9..c202f545a 100644
--- a/crates/ra_hir/src/ty/tests/never_type.rs
+++ b/crates/ra_hir/src/ty/tests/never_type.rs
@@ -19,12 +19,10 @@ fn infer_never2() {
     let t = type_at(
         r#"
 //- /main.rs
-trait Foo { fn gen() -> Self; }
-impl Foo for ! { fn gen() -> Self { loop {} } }
-impl Foo for () { fn gen() -> Self { loop {} } }
+fn gen<T>() -> T { loop {} }
 
 fn test() {
-    let a = Foo::gen();
+    let a = gen();
     if false { a } else { loop {} };
     a<|>;
 }
@@ -38,12 +36,10 @@ fn infer_never3() {
     let t = type_at(
         r#"
 //- /main.rs
-trait Foo { fn gen() -> Self; }
-impl Foo for ! { fn gen() -> Self { loop {} } }
-impl Foo for () { fn gen() -> Self { loop {} } }
+fn gen<T>() -> T { loop {} }
 
 fn test() {
-    let a = Foo::gen();
+    let a = gen();
     if false { loop {} } else { a };
     a<|>;
 }
@@ -73,12 +69,10 @@ fn never_type_can_be_reinferred1() {
     let t = type_at(
         r#"
 //- /main.rs
-trait Foo { fn gen() -> Self; }
-impl Foo for ! { fn gen() -> Self { loop {} } }
-impl Foo for () { fn gen() -> Self { loop {} } }
+fn gen<T>() -> T { loop {} }
 
 fn test() {
-    let a = Foo::gen();
+    let a = gen();
     if false { loop {} } else { a };
     a<|>;
     if false { a };
@@ -154,8 +148,7 @@ fn test() {
     } else {
         3.0
     };
-    i<|>
-    ()
+    i<|>;
 }
 "#,
     );
@@ -173,8 +166,7 @@ fn test(input: bool) {
     } else {
         return
     };
-    i<|>
-    ()
+    i<|>;
 }
 "#,
     );
@@ -193,8 +185,7 @@ fn test(a: i32) {
         3 => loop {},
         _ => 3.0,
     };
-    i<|>
-    ()
+    i<|>;
 }
 "#,
     );
@@ -213,8 +204,7 @@ fn test(a: i32) {
         3 => 3.0,
         _ => return,
     };
-    i<|>
-    ()
+    i<|>;
 }
 "#,
     );
@@ -231,8 +221,7 @@ fn test(a: i32) {
         2 => return,
         _ => loop {},
     };
-    i<|>
-    ()
+    i<|>;
 }
 "#,
     );
@@ -249,8 +238,7 @@ fn test(a: i32) {
         2 => 2.0,
         _ => 3.0,
     };
-    i<|>
-    ()
+    i<|>;
 }
 "#,
     );