Enum FallthroughReason 

pub enum FallthroughReason<'a> {
    Skipped(&'static str),
    EmptyCompletion,
    Error(&'a RunnerError),
}

Why a tier was passed over.

Variants

Skipped(&'static str)

FallbackObserver::before_attempt vetoed the tier; the observer’s own reason string.

EmptyCompletion

An Ok that carried nothing deliverable.

Error(&'a RunnerError)

An error the FallThrough policy classified as the provider’s fault.