Rust doesn't have a GC in the first place, so there's nothing to turn on. Even then, the fundamental hurdle with GC isn't the effect on your program's runtime, it's the effect that it has on the lifetimes of your data. GC (and RC) are means of dynamic lifetime determination. Manual memory management is static lifetime determination. The latter requires you to structure your code in a specific way, which may be less convenient for the programmer depending on the application. Converting dynamic lifetime determination to static lifetime determination then requires changing the very structure of the program itself (including how APIs work) while preserving semantics, which is beyond any existing tools.