This one weird trick allows you to capture loop variables.

[Steven Schveighoffer]

I thought of this when someone asked the age-old question about 
[closures not capturing loop 
variables](https://issues.dlang.org/show_bug.cgi?id=2043).

https://gist.github.com/schveiguy/b6b037bdfe74997743de81f8d3f4b92b

How does it work? It works because opApply is passed a lambda 
function generated by the compiler to implement the foreach body.

But because this is a *separate* function, when you close over 
that lambda, the lambda's stack is independently allocated on the 
heap for each loop iteration.

This doesn't exactly capture all variables, any variables in the 
enclosing scope are not duplicated (so it doesn't in effect 
capture what values were outside the loop body at that point in 
time). But isn't this the point?

In any case, thought it was interesting, and wondering if 
anyone's every thought of this before.

There are some lifetime and quality of life issues, but it's 
somewhat similar to `std.parallelism` in hooking opApply for 
nifty gain.

-Steve