Skip checks for cast to dyn traits

crates/hir-ty/src/infer/cast.rs

@@ -120,6 +120,13 @@ impl CastCheck {
             });
         }
 
+        // Chalk doesn't support trait upcasting and fails to solve some obvious goals
+        // when the trait environment contains some recursive traits (See issue #18047)
+        // We skip cast checks for such cases for now, until the next-gen solver.
+        if contains_dyn_trait(&self.cast_ty) {
+            return Ok(());
+        }
+
         if let Ok((adj, _)) = table.coerce(&self.expr_ty, &self.cast_ty) {
             apply_adjustments(self.source_expr, adj);
             set_coercion_cast(self.source_expr);
@@ -410,3 +417,35 @@ fn pointer_kind(ty: &Ty, table: &mut InferenceTable<'_>) -> Result<Option<Pointe
         }
     }
 }
+
+fn contains_dyn_trait(ty: &Ty) -> bool {
+    use std::ops::ControlFlow;
+
+    use chalk_ir::{
+        visit::{TypeSuperVisitable, TypeVisitable, TypeVisitor},
+        DebruijnIndex,
+    };
+
+    struct DynTraitVisitor;
+
+    impl TypeVisitor<Interner> for DynTraitVisitor {
+        type BreakTy = ();
+
+        fn as_dyn(&mut self) -> &mut dyn TypeVisitor<Interner, BreakTy = Self::BreakTy> {
+            self
+        }
+
+        fn interner(&self) -> Interner {
+            Interner
+        }
+
+        fn visit_ty(&mut self, ty: &Ty, outer_binder: DebruijnIndex) -> ControlFlow<Self::BreakTy> {
+            match ty.kind(Interner) {
+                TyKind::Dyn(_) => ControlFlow::Break(()),
+                _ => ty.super_visit_with(self.as_dyn(), outer_binder),
+            }
+        }
+    }
+
+    ty.visit_with(DynTraitVisitor.as_dyn(), DebruijnIndex::INNERMOST).is_break()
+}

crates/hir-ty/src/infer/unify.rs

@@ -902,6 +902,10 @@ impl<'a> InferenceTable<'a> {
 
     /// Check if given type is `Sized` or not
     pub(crate) fn is_sized(&mut self, ty: &Ty) -> bool {
+        // Early return for some obvious types
+        if matches!(ty.kind(Interner), TyKind::Scalar(..) | TyKind::Ref(..) | TyKind::Raw(..)) {
+            return true;
+        }
         if let Some((AdtId::StructId(id), subst)) = ty.as_adt() {
             let struct_data = self.db.struct_data(id);
             if let Some((last_field, _)) = struct_data.variant_data.fields().iter().last() {

crates/ide-diagnostics/src/handlers/invalid_cast.rs

@@ -556,6 +556,7 @@ fn unprincipled<'a, 'b>(x: *mut (dyn Send + 'a)) -> *mut (dyn Sync + 'b) {
         );
     }
 
+    #[ignore = "issue #18047"]
     #[test]
     fn ptr_to_trait_obj_wrap_upcast() {
         check_diagnostics(
@@ -1004,4 +1005,99 @@ fn _slice(bar: &[i32]) -> bool {
             &["E0308"],
         );
     }
+
+    #[test]
+    fn trait_upcasting() {
+        check_diagnostics(
+            r#"
+//- minicore: coerce_unsized, dispatch_from_dyn
+#![feature(trait_upcasting)]
+trait Foo {}
+trait Bar: Foo {}
+
+impl dyn Bar {
+    fn bar(&self) {
+        _ = self as &dyn Foo;
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn issue_18047() {
+        check_diagnostics(
+            r#"
+//- minicore: coerce_unsized, dispatch_from_dyn
+trait LocalFrom<T> {
+    fn from(_: T) -> Self;
+}
+trait LocalInto<T> {
+    fn into(self) -> T;
+}
+
+impl<T, U> LocalInto<U> for T
+where
+    U: LocalFrom<T>,
+{
+    fn into(self) -> U {
+        U::from(self)
+    }
+}
+
+impl<T> LocalFrom<T> for T {
+    fn from(t: T) -> T {
+        t
+    }
+}
+
+trait Foo {
+    type ErrorType;
+    type Assoc;
+}
+
+trait Bar {
+    type ErrorType;
+}
+
+struct ErrorLike;
+
+impl<E> LocalFrom<E> for ErrorLike
+where
+    E: Trait + 'static,
+{
+    fn from(_: E) -> Self {
+        loop {}
+    }
+}
+
+trait Baz {
+    type Assoc: Bar;
+    type Error: LocalInto<ErrorLike>;
+}
+
+impl<T, U> Baz for T
+where
+    T: Foo<Assoc = U>,
+    T::ErrorType: LocalInto<ErrorLike>,
+    U: Bar,
+    <U as Bar>::ErrorType: LocalInto<ErrorLike>,
+{
+    type Assoc = U;
+    type Error = T::ErrorType;
+}
+struct S;
+trait Trait {}
+impl Trait for S {}
+
+fn test<T>()
+where
+    T: Baz,
+    T::Assoc: 'static,
+{
+    let _ = &S as &dyn Trait;
+}
+"#,
+        );
+    }
 }