Making a function `pub` can cause it to fail to compile

[Diggsey]

I don't know if this is necessarily a bug, but it's certainly surprising:

trait Test {
    type Result;
}

impl Test for u32 {
    type Result = Foo<u32>;
}

struct Foo<T: Test> {
    foo: Option<T::Result>,
    bar: T,
}

fn make<T: Default + Test>() -> Foo<T> {
    Foo {
        foo: None,
        bar: Default::default(),
    }
}

/*pub */fn test() {
    make::<u32>();
}

Playground link

Uncommenting the pub keyword will cause it to fail to compile with the error:

error[E0391]: cycle detected when computing layout of `Foo<u32>`
  |
  = note: ...which requires computing layout of `std::option::Option<Foo<u32>>`...
  = note: ...which again requires computing layout of `Foo<u32>`, completing the cycle

[jonas-schievink]

It also happens when renaming the function to main, so visiblity is not the issue. Instead, mentioning the invalid type in any place where it needs to be code-gen'd will cause the error. I believe monomorphization errors due to conditionally-infinite types are expected though.

[RalfJung]

Yeah, private unused functions (except for main of a bin crate) are not codegen'd, so you won't get any monomorphization-time errors.

[jonas-schievink]

Closing as expected behavior.