container_of / offset_of

[diwic]

There is some uncertainty about how to create a Rust version of container_of that is never UB, which is currently needed in order to create Arc::from_raw here: rust-lang/rust#37192

Since there is a warning in ptr.offset saying this:

Both the starting and resulting pointer must be either in bounds or one byte past the end of an 
allocated object. If either pointer is out of bounds or arithmetic overflow occurs then any further
use of the returned value will result in undefined behavior.

...the idea has come up to create a temporary object like this:

let t: T = mem::uninitialized(); // or mem::zeroed()
// do pointer arithmetic on t 
mem::forget(t);

...but this seems risky; in case do pointer arithmetic on t would panic for some unforeseen reason,
we'll try to drop an uninitialized object.

The other version is to start off with a null pointer, like this: https://github.com/alexcrichton/ctest/blob/2839e49847a6adca6e96cc81c46a1f03f8562ac0/src/lib.rs#L731 - but I'm not sure if that's better.

[Amanieu]

Originally I used a null pointer in my offset_of! macro, but after some discussion with @ubsan we decided that this can trigger UB because it creates a null reference.

[diwic]

Yeah, this seems hard to get right, that's why I wanted to bring it up in this context as well.
It would be good to have some "blessed" version of doing it - either in std or in some rust-lang-nursery crate.

[RalfJung]

Closing as duplicate of #19, rust-lang/unsafe-code-guidelines#243.