Rust doesn’t offer a very good way to specify that (m / s) * s is actually the same type as m.

This particular example is in the works:

#![feature(generic_const_exprs)]

use std::ops::Mul;
pub struct UnitValue<T, const M: i8, const KG: i8, const S: i8, const A: i8, const K: i8>(T);
pub fn meters<T>(val: T) -> UnitValue::<T, 1, 0, 0, 0, 0> { UnitValue(val) }
impl<T, U, const M: i8, const M_RHS: i8,
   const KG: i8, const KG_RHS: i8,
   const S: i8, const S_RHS: i8,
   const A: i8, const A_RHS: i8,
   const K: i8, const K_RHS: i8>
   Mul<UnitValue<U, M_RHS, KG_RHS, S_RHS, A_RHS, K_RHS>> for UnitValue<T, M, KG, S, A, K>
where T: Mul<U>,
   UnitValue<T::Output, { M + M_RHS }, { KG + KG_RHS }, { S + S_RHS }, { A + A_RHS }, { K + K_RHS }>: ,
{
   type Output = UnitValue<T::Output, { M + M_RHS }, { KG + KG_RHS }, { S + S_RHS }, { A + A_RHS }, { K + K_RHS }>;
   fn mul(self, rhs: UnitValue<U, M_RHS, KG_RHS, S_RHS, A_RHS, K_RHS>) -> Self::Output { UnitValue(self.0 * rhs.0) }
}

(with apologies to the candela)