Trait unexpectedly not implemented with recursive where clause, with poor error message

[emilio]

This is an issue we came across in Firefox. Note that the Animate impls are derived (including the where clause), which made it a bit more subtle. This is a reduced example of what's going on:

trait Animate : Sized {
    fn animate(&self) {}
}

struct GenericFoo<T> {
    something: i32,
    fallback: T,
}
enum GenericBar<T> {
    None,
    TheThing(T),
    Foo(Box<GenericFoo<Self>>),
}

impl Animate for i32 {}
impl Animate for f32 {}
impl<T: Animate> Animate for GenericFoo<T> {}

impl<T: Animate> Animate for GenericBar<T>
where
    GenericFoo<Self>: Animate,
{}

fn main() {
    let bar = GenericBar::<f32>::None;
    bar.animate();
}

I expect this code to work (like it does if you remove the where GenericFoo<Self>: Animate), or at least provide a more descriptive error message (something about the where clause being cyclic or something, which is what I assume is going on, as GenericFoo<Self>: Animate requires Self: Animate?)

Instead, the compiler silently doesn't implement the trait for GenericBar<f32>, and there's nothing in the error output which would point you to the culprit (the where clause).

Meta

rustc --version --verbose:

rustc 1.89.0-nightly (16d2276fa 2025-05-16)

But happens also in stable.