Allow single-value enum types to have size 0

[mbrubeck]

enum Foo { Foo }
std::mem::size_of::<Foo>() // evaluates to 1

Since type Foo has only one value, it could have size zero. (This is already the case for single-valued struct types like struct Foo;.)

Open question: If this change is made, should Foo as uint still evaluate to 0 in the example above?

Note: It would still be possible to force a non-zero size using [#repr(...)] or enum Foo { Foo = 1 }.

[mbrubeck]

I'm filing this just as a possibility for future optimization. It doesn't have to be a high priority, since any use cases for this could probably be fulfilled by an empty struct instead.

[steveklabnik]

Triage: no change.

[SimonSapin]

#37649 (comment)

frewsxcv and misdreavus on IRC point out that the discriminant is already eliminated in some cases:

playbot: enum Foo { SingleVariant(i32) } ::std::mem::size_of::<Foo>()
4
playbot: enum Foo { SingleVariant(()) } ::std::mem::size_of::<Foo>()
0
playbot: enum Foo { SingleVariant{} } ::std::mem::size_of::<Foo>()
1

[Mark-Simulacrum]

I'm nominating for the lang team to decide -- we should confirm that we want this before compiler team implements.