Should types that contain zero-length vectors of themselves be allowed?

[isabelmu]

In the discussion of pull request #11839, which aims to check the representability of structs and enums properly in typeck, @huonw pointed out that the semantics for types that directly contain zero-length vectors of themselves are potentially still undecided.

Consider for example (play):

struct Foo {
    recur: [Foo; 0],
}

fn main() { }

If typeck allows enum Foo { A([Foo; 0]) } or
struct Bar { x: [Bar; 0] } then there is an infinite recursion + stack overflow in trans::adt::represent_type, so I will amend #11839 to disallow these cases and add a FIXME referencing this issue.

To me, it seems more consistent to allow any zero-length vector than to allow only some, but the only use case I can think of is that it may make some macros simpler to write.

[reem]

Triage: the story here has not changed.

[steveklabnik]

triage: this is a question for @rust-lang/lang

[Mark-Simulacrum]

Currently we print the following. No discussion from lang team has happened to my knowledge. The error, at least, could be improved, since the recursive type T has zero size--not infinite size.

error[E0072]: recursive type `T` has infinite size
 --> test.rs:1:1
  |
1 | struct T([T; 0]);
  | ^^^^^^^^^^^^^^^^^ recursive type has infinite size
  |
  = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `T` representable

error: aborting due to previous error

[nikomatsakis]

I'm inclined to consider this a bug. We should check specifically for vectors that we can prove to be of zero-size.

[nikomatsakis]

The systems in question are under active rewrite by @eddyb -- but this seems like potentially a good candidate for mentorship. I think that supporting the examples given here would be relatively straight-forward.

[eddyb]

[T; 0] has the alignment of T and if there are a few types in the cycle, they should all get T's alignment, which may depend on other types in the cycle, so it's not that simple.
That said, it's plausible computing type alignment can be done separately from size/layout.

[Mark-Simulacrum]

This compiles without error today. Marking as E-needstest.