There's never been a C-based compiler. Rust started out as OCaml. A true bootstrap would take weeks of non-stop compiling. You'd end up compiling rustc itself about 900 times, ignoring all the different LLVM builds, I think. Rust bootstraps from a binary snapshot of itself, and so you would have to work through the list of every snapshot ever (about 300, but you need to build rustc 3 times for each snapshot to properly bootstrap that copy). The snapshots are listed here: https://github.com/rust-lang/rust/blob/master/src/snapshots....

In the past the snapshots were taken almost weekly as massive language churn dictated. Now they're quite rare.