Allow the last field of a struct to be a DST

[nrc]

blocks #12938
assigned to me

[nrc]

This will interact with inherited fields for virtual structs. I propose we order the fields in structs with the base struct first and the concrete struct last. Then we only allow a DST field in non-virtual structs, which means they would actually be the last field.

[brson]

@nick29581 Is this done with 696f16e merged?

[pnkfelix]

Something that came up in an IRC discussion (which botbot.me unfortunately failed to capture completely): when the last field of a struct is unsized, is it always supposed to be one of the type parameters to the struct, as is shown in the examples on niko's blog? Or is one allowed to have an unsized field where the struct could never be instantiated to a sized version of itself?

I.e. compare these two variants:

struct Foo<Sized? T> { x: int, d: T }
struct Bar{ x: int, d: [int] }

I assume the way one would create an instance of the latter, if it is allowed at all, would be via transmute, as some level, even if only e.g. within the Allocator.

(update: deleted bogus example that tried to have unsized local variables. And it also tried to transmute a Foo to a Bar, which seemed sane (though incorrect in the end) when they were unsized locals, but became insane when I turned them into &Foo and &Bar.)

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Or perhaps it would be legal to do let b : &Bar = &Bar { x: 0, d: [1,2,3] as [int] }; ?

[nikomatsakis]

@pnkfelix I believe both are allowed but the only way to create the Bar example is via unsafe code

[nrc]

Fixed