Don't implement sizedness check via all_field_tys()

Since we don't implement it currently for perf reasons, but here we only need a struct's tail field, it will be wrong.

crates/hir-ty/src/next_solver/interner.rs

@@ -690,15 +690,10 @@ impl<'db> inherent::AdtDef<DbInterner<'db>> for AdtDef {
         interner: DbInterner<'db>,
         sizedness: SizedTraitKind,
     ) -> Option<EarlyBinder<DbInterner<'db>, Ty<'db>>> {
-        if self.is_struct() {
-            let tail_ty = self.all_field_tys(interner).skip_binder().into_iter().last()?;
-
-            let constraint_ty = sizedness_constraint_for_ty(interner, sizedness, tail_ty)?;
-
-            Some(EarlyBinder::bind(constraint_ty))
-        } else {
-            None
-        }
+        let tail_ty = self.struct_tail_ty(interner)?;
+        tail_ty
+            .map_bound(|tail_ty| sizedness_constraint_for_ty(interner, sizedness, tail_ty))
+            .transpose()
     }
 
     fn destructor(

crates/hir-ty/src/next_solver/util.rs

@@ -422,12 +422,10 @@ pub fn sizedness_constraint_for_ty<'db>(
             .next_back()
             .and_then(|ty| sizedness_constraint_for_ty(interner, sizedness, ty)),
 
-        Adt(adt, args) => {
-            let tail_ty =
-                EarlyBinder::bind(adt.all_field_tys(interner).skip_binder().into_iter().last()?)
-                    .instantiate(interner, args);
+        Adt(adt, args) => adt.struct_tail_ty(interner).and_then(|tail_ty| {
+            let tail_ty = tail_ty.instantiate(interner, args);
             sizedness_constraint_for_ty(interner, sizedness, tail_ty)
-        }
+        }),
 
         Placeholder(..) | Bound(..) | Infer(..) => {
             panic!("unexpected type `{ty:?}` in sizedness_constraint_for_ty")

crates/hir-ty/src/tests/method_resolution.rs

@@ -2229,3 +2229,32 @@ fn test(x: *mut u8) {
 "#,
     );
 }
+
+#[test]
+fn unsized_struct() {
+    check_types(
+        r#"
+//- minicore: sized, phantom_data
+use core::marker::PhantomData;
+
+const UI_DEV_CREATE: Ioctl = Ioctl(PhantomData);
+
+struct Ioctl<T: ?Sized = NoArgs>(PhantomData<T>);
+
+struct NoArgs([u8]);
+
+impl<T> Ioctl<T> {
+    fn ioctl(self) {}
+}
+
+impl Ioctl<NoArgs> {
+    fn ioctl(self) -> u32 { 0 }
+}
+
+fn main() {
+    UI_DEV_CREATE.ioctl();
+ // ^^^^^^^^^^^^^^^^^^^^^ u32
+}
+    "#,
+    );
+}