~/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