# Objective
The method `World::as_unsafe_world_cell_readonly` is used to create an
`UnsafeWorldCell` which is only allowed to access world data immutably.
This can be tricky to use, as the data that an `UnsafeWorldCell` is
allowed to access exists only in documentation (you could think of it as
a "doc-time abstraction" rather than a "compile-time" abstraction). It's
quite easy to forget where a particular instance came from and attempt
to use it for mutable access, leading to instant, silent undefined
behavior.
## Solution
Add a debug-mode only flag to `UnsafeWorldCell` which tracks whether or
not the instance can be used to access world data mutably. This should
catch basic improper usages of `as_unsafe_world_cell_readonly`.
## Future work
There are a few ways that you can bypass the runtime checks introduced
by this PR:
* Any world accesses done via `UnsafeWorldCell::storages` are completely
invisible to these runtime checks. Unfortunately, `storages` constitutes
most of the world accesses used in the engine itself, so this PR will
mostly benefit downstream users of bevy.
* It's possible to call `get_resource_by_id`, and then convert the
returned `Ptr` to a `PtrMut` by calling `assert_unique`. In the future
we'll probably want to add a debug-mode only flag to `Ptr` which tracks
whether or not it can be upgraded to a `PtrMut`. I didn't include this
change in this PR as those types are currently defined using macros
which makes it a bit tricky to modify their definitions.
* Any data accesses done through a mutable `UnsafeWorldCell` are
completely unchecked, meaning it's possible to unsoundly create multiple
mutable references to a single component, for example. In the future we
may want to store an `Access<>` set inside of the world's `Storages` to
add granular debug-mode runtime checks.
That said, I'd consider this PR to be a good first step towards adding
full runtime checks to `UnsafeWorldCell`.
## Testing
Added a few tests that basic invalid mutable world access result in a
panic.
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Co-authored-by: Joseph <21144246+JoJoJet@users.noreply.github.com>
Co-authored-by: Alice I Cecile <alice.i.cecile@gmail.com>
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