fix perf issue in macro parser
fix perf issue in macro parser

For a fuller description of the performance issue fixed by this:

https://github.com/rust-lang/rust/issues/51754#issuecomment-403242159

GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust