add MutUntyped::map_unchanged

[Serverator]

Adopted #6430

Objective

MutUntyped is the untyped variant of Mut<T> that stores a PtrMut instead of a &mut T. Working with a MutUntyped is a bit annoying, because as soon you want to use the ptr e.g. as a &mut dyn Reflect you cannot use a type like Mut<dyn Reflect> but instead need to carry around a &mut dyn Reflect and a impl FnMut() to mark the value as changed.

Solution

* Provide a method `map_unchanged` to turn a `MutUntyped` into a `Mut<T>` by mapping the `PtrMut<'a>` to a `&'a mut T`
  This can be used like this:

// SAFETY: ptr is of type `u8`
let val: Mut<u8> = mut_untyped.map_unchanged(|ptr| unsafe { ptr.deref_mut::<u8>() });

// SAFETY: from the context it is known that `ReflectFromPtr` was made for the type of the `MutUntyped`
let val: Mut<dyn Reflect> = mut_untyped.map_unchanged(|ptr| unsafe { reflect_from_ptr.as_reflect_ptr_mut(ptr) });

Note that nothing prevents you from doing

mut_untyped.map_unchanged(|ptr| &mut ());

or using any other mutable reference you can get, but IMO that is fine since that will only result in a Mut that will dereference to that value and mark the original value as changed. The lifetimes here prevent anything bad from happening.

Alternatives

1. Make Ticks public and provide a method to get construct a Mut from Ticks and &mut T. More powerful and more easy to misuse.
2. Do nothing. People can still do everything they want, but they need to pass (&mut dyn Reflect, impl FnMut() + '_) around instead of Mut<dyn Reflect>

Changelog

• add MutUntyped::map_unchanged to turn a MutUntyped into its typed counterpart

[github-actions]

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[JoJoJet]

Thank you for adopting this!

[alice-i-cecile]

I agree with the doc suggestions but won't block on them.

[Serverator]

Uh huh... For some reason CI is failing now, but I don't really understand why, I only committed the doc suggestions

[JoJoJet]

One of my suggestions must have had an extra space or something -- you should be able to just run cargo fmt to fix it.

[mockersf]

Miri failure is due to a but in rust nightly, it should be fixed tomorrow