Mark HeaderValue::from_maybe_shared_unchecked() as safe.

[orium]

This method was always safe.

[seanmonstar]

Is there a very strong reason to?

I know it doesn't follow the official guidelines, but I also don't think they are always right.

[orium]

unsafe signals that UB is possible when the pre-conditions are violated. In my case it lead me to look at alternatives (for longer than I would like to admit) because I thought code that I maintain had UB. Took me a while to realize there's actually no UB and that the method is safe as far as "rust safety" is concerned (the method safety section said "src must contain valid UTF-8" which is incorrect). It's just not recommended in most cases because of the header values it allows, but that's signaled with the _unchecked method name suffix.

[seanmonstar]

We've been aware of the official meaning of unsafe, and purposefully chose that this should be anyways. While it doesn't currently allow UB to happen, unsafe characters can cause message splitting, which is also very dangerous. Also, we could conceivably in the future adjust the inner storage to be more string-like and require closer to the official definition of unsafe. So in that sense, there's nothing stopping a function from claiming to be unsafe in the name of backwards-compatibility.

[orium]

@seanmonstar can I at least fix the documentation? It currently says:

    /// ## Safety
    /// `src` must contain valid UTF-8. In a release build it is undefined
    /// behaviour to call this with `src` that is not valid UTF-8.

but there's no UB. I can keep the safety section an explain what the risk is.

[orium]

Another issue that the function rejects non-utf-8 sequences in debug mode, but not in release. That would make sense if the goal was actually to reject non-utf-8 sequences, but HeaderValue::from_bytes() accepts them, so that's not an invariant HeaderValue maintains.

I can also fix that in a follow-up PR.