Add a function to create headers from byte arrays without any transmute

[est31]

No description provided.

[est31]

After merging of this PR, I'd love to have a (minor) release.

[est31]

It's not really important though as I can use a transmute for now.

[alexcrichton]

This is a pretty large argument to be passing around, perhaps this could be switched to take a slice? We could also have it take a variable-length slice, assert it's 512 bytes, and then return a reference.

[est31]

@alexcrichton is it better now? I'll need to test this version of the function first whether it actually works...

[alexcrichton]

Yeah that looks fine by me, although could this take &[u8] instead of &[u8; 512]? I imagine the latter is very difficult to construct...

[est31]

I imagine the latter is very difficult to construct...

Indeed... My code can manage as it serializes from/to the array directly, but I'll change it to &[u8]. As slices also need to store the length, I'll have to convert the slice to a pointer.

[est31]

Please don't merge this yet, I haven't verified whether it does what I want it to. With unsafe code you never know...

[est31]

Hmm right there seems to be an issue here where we invoke UB... Let me see whether I can fix this.

[est31]

Yup, it is fixed now. r? @alexcrichton

[alexcrichton]

Thanks!