Rewrite Views to be Miri compatible

[ibraheemdev]

Rewrite the Views type to use a manual VTable instead of fat pointer manipulation. I find this pattern to be much clearer than the existing code.

The current implementation is also aliasing Box through the OpaqueBoxDyn and Box<dyn Free> pointers, which is dubious, although Miri seemed to be pleased with the small diff:

- type OpaqueBoxDyn = [u8; std::mem::size_of::<Box<dyn CastTo<Dummy>>>()];
+ type OpaqueBoxDyn = [MaybeUninit<u8>; std::mem::size_of::<Box<dyn CastTo<Dummy>>>()];

[netlify]

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[nikomatsakis]

I gave this a quick look and it seems reasonable. I know I considered this approach and decided against it; I'm trying to remember why. I'll look it over more closely later today.

[codspeed-hq]

CodSpeed Performance Report

Merging #588 will not alter performance

_{Comparing ibraheemdev:views-vtable (bd82274) with master (a20b894)}

Summary

✅ 8 untouched benchmarks

[nikomatsakis]

Read it over again. I like it! Nice work.

[nikomatsakis]

I'd merge this but clippy is grouchy.

[MichaReiser]

I'll fix the clippy issue and merge the PR afterward.