Nightly Rust support -Z 'mir-opt-level' option, as
rustc -Z help said:
-Z mir-opt-level=val -- MIR optimization level (0-4; default: 1 in non optimized builds and 2 in optimized builds)
But actually it set values above 4 (such as 5,10,100 ... ), I'm not sure it's intentional or a compiler bug? And what's that mean ..