Passing by pointer (in C) reduced the difference a lot, but swapping the order of Add and Int in the Rust enum was enough to reduce the different to:

  cmp ecx, 1
  je .LBB0_3

vs

  cmp ecx, 2
  jne .LBB0_2

LBB0_3 and LBBO_2 were the same in both outputs (up to alpha renaming).

Oddly, both sources seemed to be quite sensitive to match switch and enum reordering, resulting in very different generated code. Possibly something to look into further.