fix: Disable invalid cast diagnostics for casts like &T as &dyn Trait

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

@@ -188,7 +188,28 @@ impl CastCheck {
                     }
                     _ => return Err(CastError::NonScalar),
                 },
-                _ => return Err(CastError::NonScalar),
+                _ => {
+                    // We fail to coerce to dyn Trait sometimes due to lack of some functionalities
+                    // under current chalk trait solver. (See #11847 and #18047)
+                    // So, emit no diagnostic for casts like `&T as &dyn Trait` for now,
+                    // to prevent false positive diagnositcs.
+                    // Direct cast without ref like `T as dyn Trait` is prohibited by
+                    // `InferenceDiagnostic::CastToUnsized`, so allowing this diagnostics bypass
+                    // only to `&T as &dyn Trait` is sufficient
+                    if let (TyKind::Ref(m_expr, _, _), TyKind::Ref(m_cast, _, t_cast)) =
+                        (self.expr_ty.kind(Interner), self.cast_ty.kind(Interner))
+                    {
+                        // Refuse casting `&T` into `&mut U`
+                        if m_expr > m_cast {
+                            return Err(CastError::IllegalCast);
+                        }
+                        if matches!(table.resolve_ty_shallow(t_cast).kind(Interner), TyKind::Dyn(_))
+                        {
+                            return Ok(());
+                        }
+                    }
+                    return Err(CastError::NonScalar);
+                }
             };
 
         // rustc checks whether the `expr_ty` is foreign adt with `non_exhaustive` sym

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

@@ -289,7 +289,7 @@ pub fn main() {
     fn dyn_tail_need_normalization() {
         check_diagnostics(
             r#"
-//- minicore: dispatch_from_dyn
+//- minicore: coerce_unsized, dispatch_from_dyn
 trait Trait {
     type Associated;
 }
@@ -370,7 +370,7 @@ fn main() {
     fn fat_ptr_cast() {
         check_diagnostics_with_disabled(
             r#"
-//- minicore: sized
+//- minicore: coerce_unsized, dispatch_from_dyn
 trait Foo {
     fn foo(&self) {} //~ WARN method `foo` is never used
 }
@@ -399,12 +399,12 @@ fn main() {
     let d = to_raw(a) as usize;
 }
 "#,
-            &["E0308"],
+            &["E0308", "unused_variables"],
         );
 
         check_diagnostics_with_disabled(
             r#"
-//- minicore: sized
+//- minicore: coerce_unsized, dispatch_from_dyn
 trait Trait {}
 
 struct Box<T: ?Sized>;
@@ -476,7 +476,7 @@ fn main() {
     fn ptr_to_ptr_different_regions() {
         check_diagnostics(
             r#"
-//- minicore: sized
+//- minicore: coerce_unsized, dispatch_from_dyn
 struct Foo<'a> { a: &'a () }
 
 fn extend_lifetime_very_very_safely<'a>(v: *const Foo<'a>) -> *const Foo<'static> {
@@ -503,7 +503,7 @@ fn main() {
     fn ptr_to_trait_obj_add_auto() {
         check_diagnostics(
             r#"
-//- minicore: pointee
+//- minicore: pointee, coerce_unsized, dispatch_from_dyn
 trait Trait<'a> {}
 
 fn add_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send) {
@@ -522,7 +522,7 @@ fn add_multiple_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send +
     fn ptr_to_trait_obj_add_super_auto() {
         check_diagnostics(
             r#"
-//- minicore: pointee
+//- minicore: pointee, coerce_unsized, dispatch_from_dyn
 trait Trait: Send {}
 impl Trait for () {}
 
@@ -538,7 +538,7 @@ fn main() {
     fn ptr_to_trait_obj_ok() {
         check_diagnostics(
             r#"
-//- minicore: pointee
+//- minicore: pointee, coerce_unsized, dispatch_from_dyn
 trait Trait<'a> {}
 
 fn remove_auto<'a>(x: *mut (dyn Trait<'a> + Send)) -> *mut dyn Trait<'a> {
@@ -560,7 +560,7 @@ fn unprincipled<'a, 'b>(x: *mut (dyn Send + 'a)) -> *mut (dyn Sync + 'b) {
     fn ptr_to_trait_obj_wrap_upcast() {
         check_diagnostics(
             r#"
-//- minicore: sized
+//- minicore: pointee, coerce_unsized, dispatch_from_dyn
 trait Super {}
 trait Sub: Super {}
 
@@ -1004,4 +1004,16 @@ fn _slice(bar: &[i32]) -> bool {
             &["E0308"],
         );
     }
+
+    #[test]
+    fn cast_into_dst_with_integer_fallback() {
+        check_diagnostics(
+            r#"
+//- minicore: fmt, coerce_unsized, dispatch_from_dyn
+fn test() {
+    let _ = &42 as &dyn core::fmt::Debug;
+}
+"#,
+        )
+    }
 }