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Don't store subclass impl/private data in an Option<T> #589
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We don't know the exact memory layout of the Option in relation to T
so can't easily go from a *const T to the surrounding *const Option.
Since nightly from 2020-02-12 using both pointer types interchangeably
causes segmentation faults as their memory layouts are not actually
compatible and never were, but due to some compiler changes this
actually causes crashes at runtime now.
See rust-lang/rust#69102 for details.
As an Option<_> was only used for some paranoid runtime checks that are
not really needed, simply store T directly as impl/private data of
subclasses.
This has the side-effect of having zero-sized impl/private data types to
actually occupy zero bytes of memory, and in some other cases to save a
few bytes of the allocation.
CC @EPashkin @GuillaumeGomez