Commits on Dec 6, 2020

1. Implement sound map value aliasing …

   This implementation gets rid of the unsound `ShallowCopy` trait (#74 &
   rust-lang/unsafe-code-guidelines#35 (comment)),
   and replaces it with a wrapper type around aliased values.
   
   The core mechanism is that the wrapper type holds a `MaybeUninit<T>`,
   and aliases it by doing a `ptr::read` of the whole `MaybeUninit<T>` to
   alias. It then takes care to only give out `&T`s, which is allowed to
   alias.
   
   To drop, the implementation casts between two different generic
   arguments of the new `Aliased` type, where one type causes the
   `Aliased<T>` to drop the inner `T` and the other does not. The
   documentation goes into more detail. The resulting cast _should_ be safe
   (unlike the old `ManuallyDrop` cast).
   
   The removal of `ShallowCopy` makes some of the API nicer by removing
   trait bounds, and obviates the need for `evmap-derive`. While I was
   going through, I also took the liberty of tidying up the external API of
   `evmap` a bit.
   
   The implementation passes all tests, and I _think_ it is sound (if you
   think it's not, please let me know!).
   
   Note that this does _not_ take care of #78.
   
   Fixes #74.
   Fixes #55 since `ShallowCopy` is no longer neeeded.
   Also fixes #72 for the same reason.

   

   jonhoo committed Dec 6, 2020
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