Move unsafe code behind a (enabled by default) feature flag

[notgull]

I am using this crate in a crate that is forbid(unsafe_code), where I would like to reduce the amount of unsafe code in my dependency tree. The unsafe code currently in this crate could be eliminated.

I propose an unchecked feature. When enabled, the get_unchecked and other methods that require unsafe are exposed. When disabled, this crate becomes forbid(unsafe_code). This feature would be enabled by default to prevent breaking changes.

I am willing to implement this feature myself.

[taiki-e]

features should be additive and we cannot use features to disable APIs.

[taiki-e]

Oh, sorry, you seem to be suggesting an additive feature that is enabled by default. However, note that for users using slab with default-features = false the approach you suggest is also a breaking change.

[notgull]

Hmm, that's a pickle. I'd rather not introduce a breaking change unless there's already an upcoming breaking change planned. I guess I could just write the PR and wait for version 0.5 for it to be merged.

[taiki-e]

(At this time there are no plans for 0.5...)

[Darksonn]

We are unlikely to have a breaking release any time soon, so your PR would probably sit around for at least a year. Additionally, I don't really think there's any value in a feature like this. None of the internals use unsafe besides the methods that are unsafe to call, so if you don't call those, you aren't using any unsafe code.