Objective

There's no general error for when an entity doesn't exist, and some methods are going to need one when they get Resultified. The closest thing is EntityFetchError, but that error has a slightly more specific purpose.

Solution

• Added EntityDoesNotExistError.
  • Contains Entity and EntityDoesNotExistDetails.
• Changed EntityFetchError and QueryEntityError:
  • Changed NoSuchEntity variant to wrap EntityDoesNotExistError and renamed the variant to EntityDoesNotExist.
• Renamed EntityFetchError to EntityMutableFetchError to make its purpose clearer.
• Renamed TryDespawnError to EntityDespawnError to make it more general.
• Changed World::inspect_entity to return Result<[ok], EntityDoesNotExistError> instead of panicking.
• Changed World::get_entity and WorldEntityFetch::fetch_ref to return Result<[ok], EntityDoesNotExistError> instead of Result<[ok], Entity>.
• Changed UnsafeWorldCell::get_entity to return Result<UnsafeEntityCell, EntityDoesNotExistError> instead of Option<UnsafeEntityCell>.

Migration Guide

• World::inspect_entity now returns Result<impl Iterator<Item = &ComponentInfo>, EntityDoesNotExistError> instead of impl Iterator<Item = &ComponentInfo>.
• World::get_entity now returns EntityDoesNotExistError as an error instead of Entity. You can still access the entity's ID through the error's entity field.
• UnsafeWorldCell::get_entity now returns Result<UnsafeEntityCell, EntityDoesNotExistError> instead of Option<UnsafeEntityCell>.