BigInt foreach loop

[Steven Schveighoffer via Digitalmars-d-learn]

On 8/4/17 8:49 AM, Q. Schroll wrote:
> One can do
>    BigInt n = returnsBigInt();
>    foreach (i; BigInt(0) .. n)
>    { .. }
> How is this implemented? The code for BigInt is very large and I didn't 
> find it.
> And is it more efficient than
>    for (BigInt i = 0; i < n; ++i)
>    { .. }

Any foreach range statement like this:

foreach(var; A .. B)

is treated as if you wrote:

for(auto var = A; var < B; ++var)

So it's pretty much exactly like what you wrote, just the initializer is 
different but the result is the same.

 > as incrementing is a costly operation?

Here is increment in bigint:
https://github.com/dlang/phobos/blob/master/std/bigint.d#L563

And addOrSubInt:
https://github.com/dlang/phobos/blob/master/std/internal/math/biguintcore.d#L508

I think there is room for improvement, as incrementing or decrementing 
by 1 is probably something that can be optimized.

-Steve