I looked into this once;
I found a Rust compiler written in OCaml on the web.
There should be a path from that compiler to the current version of Rust.
The problem lies in the fact that the entire "1.9 build 1.10 builds 1.11" spiel only become the official policy of the Rust project recently.
disclaimer: the following is how I interpreted my communications with
the rust community. My apologies to anyone involved for any inaccuracies and/or misrepresentations.
This means that you could be looking at individual commits, as before this, I believe nightlies (snapshots) were used to compile the Rust compiler.