For me, clang -O3 optimizes the main function into this single loop:

    LBB3_1:
        addsd	%xmm1, %xmm0
        addsd	%xmm1, %xmm0
        ucomisd	%xmm0, %xmm2
        jae	LBB3_1

%xmm0 begins at 0, %xmm1 is 0.04, and %xmm2 is 3.6525E+8.

In other words, it's measuring how long it takes to count to 3.6525E+8 by adding 0.04 repeatedly. LLVM (which both Rust and clang use as a backend) optimizes away the actual work of the program.

GCC doesn't seem to optimize things out as aggressively, so that probably explains the difference: https://godbolt.org/g/lBRIIW