[CanBeNull] on a collection field also makes its elements nullable (per-element flag on disk)

[kamronbatman]

Summary

[CanBeNull] on a collection field is also applied to the collection's elements (and dictionary keys/values), because every collection rule passes the field's full attribute list down when it resolves the element rule.

Where

ListMigrationRule, HashSetMigrationRule, SortedSetMigrationRule, ArrayMigrationRule, DictionaryMigrationRule all call SerializableMigrationRulesEngine.GenerateSerializableProperty(compilation, "…Entry", elementType, 0, attributes, parentSymbol, null) with the collection field's attributes. Element rules that honour [CanBeNull] (RawSerializableMigrationRule, PrimitiveUOTypeMigrationRule, nested collection rules) then emit per-element null handling.

Observed

[CanBeNull]
[SerializableField(2)]
private List<SpawnerEntry> _entryList;   // SpawnerEntry is [SerializationGenerator]

generates (Serialize):

if (_entryList != default)
{
    writer.Write(true);
    var _entryListCount = _entryList?.Count ?? 0;
    writer.WriteEncodedInt(_entryListCount);
    if (_entryListCount > 0)
    {
        foreach (var _entryListEntry in _entryList!)
        {
            if (_entryListEntry != default) { writer.Write(true); _entryListEntry.Serialize(writer); }
            else { writer.Write(false); }
        }
    }
}

and the matching read path allocates _entryListEntry = default into the list when the per-element flag is false. The schema records the flag twice: ["@CanBeNull", "Server.Engines.Spawners.SpawnerEntry", "RawSerializableMigrationRule", "DeserializationRequiresParent", "@CanBeNull"].

Why it matters

• One extra byte per element on disk for every [CanBeNull] collection whose element rule honours the flag.
• The read path admits null elements into a list the author never declared as holding nulls.
• There is no way to say "the collection may be null, its elements may not".

Wire-format caveat

Any existing [CanBeNull] collection with raw-serializable elements already stores the per-element flag, so changing propagation is a wire-format change for those types. In ModernUO at least PuzzleChest._guesses (Dictionary<Mobile, PuzzleChestSolutionAndTime>) is affected; List<Mobile> / HashSet<Item> / List<Item> uses go through the entity rule and are unaffected. A fix therefore needs a version bump and MigrateFrom on those consumers, landed together with the generator release.

Proposed fix

Strip [CanBeNull] (and [Tidy], which is a collection concern) from the attributes handed to element/key/value rules. If nullable elements are ever wanted, add an explicit annotation for that ([CanBeNullElements] or similar) rather than inferring it from the collection's attribute. Add a snapshot fixture with a [CanBeNull] List<NestedSerializable> pinning the single-flag output, and bump the affected ModernUO types in the same release.

Found while working on modernuo/ModernUO#2621, which avoids the annotation for now.