Progress toward a GCC-based Rust compiler

There was a GCC version that had a particular proprietary C compiler noted in its documentation because that compiler had a bug in its compiling of GCC that made certain optimizations NOPs. Thus a three stage bootstrap of GCC would produce a slightly broken first stage, and an unoptimized but correct second-stage, which would break bootstrap because it wouldn't match the optimized third stage.

So it's conceivable that a bad bootstrap compiler could break the borrow checker in rustc. It'd be unlikely to carry over to next generations, though.