Change detection with interior mutability

[alice-i-cecile]

What problem does this solve or what need does it fill?

Because we rely on DerefMut to detect changes, we can't

What solution would you like?

Best practices and advice are clearly documented for end users.

What alternative(s) have you considered?

Yup not much we can do about this short of making it easy for developers to trigger a change manually. In cases where interior mutability is needed, deref_mut() can't be called anyway (because interior mutability enables mutations from read-only contexts). Maybe adding a commands.set_changed::<T>(entity) command to make it easy for people to queue up changes in read only contexts without actually needing to mutate the component?.

• @cart in comments of [Merged by Bors] - Reliable change detection #1471.

Additional context

First brought up in the context of #1471. This behavior exists both before and after the changes in that PR.

[Ratysz]

I posit that anyone understanding what "interior mutability" is also realises that it will not set a "this was accessed mutably" flag. More documentation is good, but just a "this only works on full mutable access, so interior mutability will not trigger it" should do here.

A form of commands.set_changed::<T>(entity) is not an alternative: it's a new feature that should probably be a thing anyway.