But what makes Times<Times<Newton, Meter>, Meter> the same as Times<Newton, Times<Meter, Meter>>?
Systems will use an array of unit powers, so that if the array were defined as <Joules, Seconds, Newtons, Meters>, then acceleration would be <0,-2,0,1> and watts would be <1,-1,0,0>. Addition and subtraction require that your arrays are equal, and multiplication and division are pairwise additive/subtractive.
Just represent every unit in terms of them then it's good.
One problem with the array you came up is that the units are not orthogonal, since Joules = Newtons * Meters.
We would to resolve the nesting during multiplication and division. For example:
let a: Scalar<Times<A, B>> = Scalar::new(10.0);
let b: Scalar<Times<A, Times<B, C>>> = Scalar::new(5.0);
let c: Scalar<Times<A, Times<A, Times<B, Times<C>>>> = a * b;