Function day_growth 

fn day_growth<R: Scalar>(day_count: DayCountFraction, rate: R, tau: f64) -> R

The compounded growth factor ∏(1 + r_d·tau_d) over the coupon’s days.

A day whose rate has already fixed contributes 1 + r·tau from the store; a day still to come contributes DF(start)/DF(end) off the forward curve. A run of consecutive projected days telescopes: the interior discount factors cancel and the run collapses to a single ratio across its whole span. That is not only fewer curve reads — in an active mode it is a handful of recorded statements per run instead of one per day, for a period that can be ninety of them.

A realised day breaks the run, because a realised factor is a number rather than a ratio, and it is exactly at those breaks that the telescoping has to stop. One printed day’s growth factor. Every overnight index compounds a day simply, 1 + r·τ, except one accruing on BUS/252: Brazil’s CDI, whose rate is an annual rate compounded per business day, (1 + r)^τ with τ = 1/252 (EMTA-ISDA MP1; CME Rulebook 90104). A projected run needs no such rule — it telescopes to the forward curve’s discount-factor ratio either way.