That seems perfectly reasonable for unstable software in heavy development, which Rust might very well be. Unfortunately that also means it shouldn't be used as

That seems perfectly reasonable for unstable software in heavy development, which Rust might very well be. Unfortunately that also means it shouldn't be used as part of anything stable, which makes using it in Firefox a terrible idea. Think of it as LLVM declaring it has a hard dependency on the Linux kernel from last week (that isn't in a release yet).