out of control memory use on rhombus/private/string.rkt

[samth]

If I open rhombus/private/string.rkt from the rhombus-prototype repository, DrRacket quickly completes background expansion but memory use (as reported in the bottom corner) continues growing without bound (eg from 800MB to 8GB in a couple minutes). Nothing appears to be happening while this memory usage growth happens, and it hasn't happened with other Rhombus programs that I've noticed.