Infinitely recursive struct definitions compile successfully when behind fixed-size vector

[isabelmu]

There's no ICE or anything. The only problem is that the 'snippet 1' struct definitions are allowed, unlike those from snippet 2.

// Snippet 1

// No compiler error
struct Foo { p: ~[Foo, ..1] }
struct Bar { p: ~[Bar] }

fn main() {
  let x: Option<Foo> = None;
  let y: Option<Bar> = None;
}

Without the vector-ness:

// Snippet 2

// error: this type cannot be instantiated without an
// instance of itself; consider using `Option<Baz>`
struct Baz { p: ~Baz }

---

My understanding of what's going on: Foo and Bar from 'snippet 1' pass typeck::check::check_instantiable, because ty::is_instantiable::subtype_requires always returns false when matching sty variants ty_vec(_, _) and ty_unboxed_vec(_).

Here's the relevant RUST_LOG output for Foo:

subtypes_require(Foo, Foo)?
type_requires(Foo, ~[Foo, .. 1])?
subtypes_require(Foo, ~[Foo, .. 1])?
type_requires(Foo, [Foo, .. 1])?
subtypes_require(Foo, [Foo, .. 1])?
subtypes_require(Foo, [Foo, .. 1])? false
type_requires(Foo, [Foo, .. 1])? false
subtypes_require(Foo, ~[Foo, .. 1])? false
type_requires(Foo, ~[Foo, .. 1])? false
subtypes_require(Foo, Foo)? false

And for Bar:

subtypes_require(Bar, Bar)?
type_requires(Bar, ~[Bar])?
subtypes_require(Bar, ~[Bar])?
subtypes_require(Bar, ~[Bar])? false
type_requires(Bar, ~[Bar])? false
subtypes_require(Bar, Bar)? false

---

#3779 is the most similar issue I could find. This bug is distinct because the struct isn't infinitely sized. If I understand the terms correctly, issue #3779 describes a failure to verify struct representability, whereas this issue describes a bug in verifying struct instantiability.

[huonw]

Nice catch!

Foo is certainly invalid, since it's fundamentally the same as Baz. However, Bar is ok, since Bar { p: ~[] } is a base case for the recursion.

[isabelmu]

Oooh of course.

[huonw]

(I'm testing a fix for this now.)

[huonw]

#11661 (your understanding was correct, just needed to detect & handle fixed length vectors specifically).