There are applications where you cannot have a garbage collector. The software on the PLC that controls when a saw stops itself and doesn't slice your finger. This would be horrible if a GC pass happened during this. I don't think realtime rust is a particularly fun idea, either, though you could certainly give runtime limits at compile time I suspect the macro system will make debugging timings on such code difficult if you ever need to use "other people's code"... One reason why I'm unsure about crypto in rust.