weird condition "if obligation.recursion_depth >= 0"

[matthiaskrgr]

As far as I can tell from src/librustc/traits/mod.rs:132:

#[derive(Clone, PartialEq, Eq, Hash)]
pub struct Obligation<'tcx, T> {
    /// The reason we have to prove this thing.
    pub cause: ObligationCause<'tcx>,

    /// The environment in which we should prove this thing.
    pub param_env: ty::ParamEnv<'tcx>,

    /// The thing we are trying to prove.
    pub predicate: T,

    /// If we started proving this as a result of trying to prove
    /// something else, track the total depth to ensure termination.
    /// If this goes over a certain threshold, we abort compilation --
    /// in such cases, we can not say whether or not the predicate
    /// holds for certain. Stupid halting problem; such a drag.
    pub recursion_depth: usize,
}

recursion_depth is usize, thus the condition if obligation.recursion_depth >= 0 at src/librustc/traits/select.rs:3770:12 is always true

        // NOTE(flaper87): As of now, it keeps track of the whole error
        // chain. Ideally, we should have a way to configure this either
        // by using -Z verbose or just a CLI argument.
        if obligation.recursion_depth >= 0 {
            let derived_cause = DerivedObligationCause {
                parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
                parent_code: Rc::new(obligation.cause.code.clone()),
            };
            let derived_code = variant(derived_cause);
            ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
        } else {
            obligation.cause.clone()
        }

Does this point out some kind of logic error (or should the unreachable code just be deleted?).

Found via clippy.