Function z_spread_on_curve 

pub fn z_spread_on_curve<C: YieldCurve<f64>>(
    bond: &FixedRateBond,
    curve: &C,
    price: f64,
    basis: PriceBasis,
    settlement: NaiveDate,
) -> Result<f64, SpreadSolveError>

The z-spread implied by a dirty price: the flat continuously-compounded spread s for which Σ aᵢ·DF(tᵢ)·exp(−s·τᵢ) equals the quote.

A passive kernel. A spread is a quoted quantity read off a price, so this takes and returns plain numbers and the iterations record nothing — the same rule the callable OAS follows, and for the same reason: sixty bisection steps through an active view would tape sixty valuations whose derivatives nothing reads.

τ is measured by curves::discount::year_fraction, which is the function FlatSpread applies. That is not an implementation detail: it is what makes solving a z-spread and pinning it as a security spread exact inverses rather than agreeing to a tolerance. A spread that does not invert its own pin is a number with no referent.

The curve’s discount factors are read once, before iterating: they do not depend on the spread, so the loop is one exp per flow and no curve queries at all.

Errors

SpreadSolveError::Unbracketed when no spread in [−10%, 30%] reproduces the price, SpreadSolveError::Unconverged on the iteration cap, and SpreadSolveError::Curve from the discount queries.