From the ABI Stability manifesto: "The value witness table stores whether a type is trivial and/or bitwise movable, whether there are extra inhabitants and if so how to store and retrieve them, etc. For enums, the value witness table will also provide functionality for interacting with the discriminator. There may be more efficient ways of representing enums that simplify this functionality (or provide a fast path), and that's under investigation"

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This is done
#12697