~/rust $ find -name '*.ml' | wc -l
  72
  ~/rust $ find -name '*.ml' | grep -v ./src/llvm | wc -l
  0

i.e. there are some OCaml files in a Rust checkout, but they're all part of LLVM, mostly part of the "OCaml Kaleidoscope": http://llvm.org/docs/tutorial/OCamlLangImpl1.html