What would I lose if I turn of garbage collection?

[termer]

Using `--mm:none` would be an unpleasant experience because you wouldn't be 
able to use managed reference types (such as anything that is or is based on a 
`ref` object) without leaking them. Just use `--mm:arc` if you don't want a GC 
at all. Objects with reference cycles will leak, but as long as you don't have 
any of those, everything will be handled for you, very similarly to how Rust 
does its memory management.