JuliaLang/julia — comment on commit 8929516eb7c61dbee70d69eca7b5932df9018b7b
File:  line 0
By JeffBezanson:

It's not used yet, and may not be. Its purpose is to avoid making an excessive number of gensyms by sharing them among non-overlapping contexts. Currently gensyms are actually interned and so not GC'd. It's not too big a deal though.

On a related note, it's ok to use non-gensym symbols in generated code if you add an `expr(:local, vars)` to the top of the block. This is needed for the `out` variable in printf-generated code to avoid capturing.