Struct ChainEntry 

pub struct ChainEntry {
    pub backend: Box<dyn SearchBackend>,
    pub prefer_deep: bool,
}

One backend and the one thing the chain needs to know about it beyond how to call it.

Fields

backend: Box<dyn SearchBackend>

prefer_deep: bool

Move this backend to the front when the caller asked for Depth::Deep.

Depth used to change only how a backend searched, never which one ran, so a research question went to whatever was cheapest and first — and a paid backend chosen precisely for hard questions was reached only when the free one came up empty. This is the other half: config says which backends are worth their price on a hard question, and the chain puts them first for exactly those.

It reorders rather than filters, deliberately. A preferred backend that is rate-limited must still fall through to the free one, and a quick query must still be able to reach the paid backend as a fallback when the free one is down — which is the arrangement that kept working through a total searxng outage.