Imberflur/maybe-dangling.rs

`ManuallyDrop<T>` and `MaybeDangling<T>` semantics in stable Rust as per https://github.com/rust-lang/rfcs/pull/3336

::maybe-dangling

ManuallyDrop<T> and MaybeDangling<T> semantics in stable Rust as per rust-lang/rfcs#3336

This crates offers two types, ManuallyDrop<T>, and MaybeDangling<T>, which do not carry aliasing/dereferenceable-ity properties w.r.t. the T they each contain, which means they are allowed to:

1. have some expired value inside of them, such as T = &'expired …,
2. be fed to a function that does not inspect its value (such as ::core::mem::forget()),
3. exhibit well-defined behavior (no UB!).

References

• The RFC that shall eventually and ultimately supersede this very crate: rust-lang/rfcs#3336

• The miri PR implementing the check against this: rust-lang/miri#2985

• The soundness problem of ::ouroboros stemming from not using this: someguynamedjosh/ouroboros#88

• The soundness problem of ::yoke stemming from not using this: unicode-org/icu4x#3696

• An URLO thread on the topic, and my post exposing the intention to write this very crate