Dot dot is range notation: https://doc.rust-lang.org/rust-by-example/flow_control/for.h...

Here's the reference for the drain() method on vectors which takes the range argument: https://doc.rust-lang.org/rust-by-example/flow_control/for.h...

Altogether, it sounds like (I haven't read the whole thing) it's supposed to drain from the current row until the end, including every row in between.