How do I prevent cargo fmt from changing the compiled binary?

Question

I reformatted my code base with rustfmt 0.4.1-stable and had a huge diff that is hard to check by eye.

A long time ago, I had a similar problem after cargo fmt produced a huge diff. At that time, I solved it by:

cargo build --release
strip -s target/release/mylib.so -o target/release/mylib-stripped.so
diff <(hexdump -C < target/release/mylib-stripped.so) <(hexdump -C < target/release/prev-mylib-stripped.so)

The difference was just few bytes and looked like a time stamp, but this time the difference between both hexdump / objdump outputs is huge.

The code is different, for example:

                                        push   %r13
   188d3a:      41 54                   push   %r12
   188d3c:      53                      push   %rbx
-  188d3d:      48 81 ec 48 01 00 00    sub    $0x148,%rsp
-  188d44:      48 89 d3                mov    %rdx,%rbx
...
+  188d3d:      48 81 ec 88 06 00 00    sub    $0x688,%rsp
+  188d44:      4d 89 ce                mov    %r9,%r14
+  188d47:      4c 89 85 a0 fe ff ff    mov    %r8,-0x160(%rbp)
...

How do I make builds reproducible before and after cargo fmt?