Recursive type with `Cow`, `'static` and slice triggers "overflow evaluating the requirement"

[m-ou-se]

The following compiles with rustc 1.7.0 and earlier, but not in 1.8.0 and later:

use std::borrow::Cow;

#[derive(Clone)]
pub struct Node {
	// ...
	pub children_a: &'static [Node], // Works
	pub children_b: Vec<Node>, // Works
	pub children_c: Cow<'static, [Node]> // Doesn't compile
}

The errors:

error[E0275]: overflow evaluating the requirement `<[Node] as std::borrow::ToOwned>::Owned`
 --> src/lib.rs:6:2
  |
6 |     pub children_a: &'static [Node],
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: required because it appears within the type `Node`
  = note: required because of the requirements on the impl of `std::borrow::ToOwned` for `[Node]`
  = note: required because it appears within the type `Node`
  = note: slice and array elements must have `Sized` type

Since it compiles fine with the first two members (reference to slice, and Vec), I'd expect a Cow of a slice to also work. Is this a regression, or is this supposed to not compile?

[sgrif]

This is related to #34260

[nikomatsakis]

triage: P-medium

@sgrif and I are working on this a bit =)

[jesskfullwood]

Is there a workaround for this? It has me completely stumped

[bkchr]

@nikomatsakis any update on this?

[Licenser]

Same, ran into this too and unsure how to resolve this. It seems it's a regression from 2016?

[kyleedwardsny]

I believe I found a workaround until this is fixed. The trick is to put the Cow inside a dummy struct:

#[derive(Clone)]
pub struct DummyHolder<T> {
    pub data: T,
}

#[derive(Clone)]
pub struct Node {
    pub children: DummyHolder<Cow<'static, [Node]>>,
}

[kyleedwardsny]

I just made an interesting discovery. The following does not compile:

#[derive(Clone)]
pub struct DummyHolder<T: ?Sized> { // <-- Triggers the error
    pub data: T,
}

#[derive(Clone)]
pub struct Node {
    pub children: DummyHolder<Cow<'static, [Node]>>,
}

So it seems that enforcing a Sized constraint somewhere in the loop makes this example compile (Cow allows the borrowed type to be ?Sized.)