If the project is a pure C project using a build system that can't integrate Rust very easily, or it would come at a great cost, then I can see this being a tot

If the project is a pure C project using a build system that can't integrate Rust very easily, or it would come at a great cost, then I can see this being a totally valid concern. However, AOSP is massive. It is HUGE. And complicated to build. I really, really doubt this addition will make a huge deal. This is the same for many of the huge projects adopting Rust. The worst one so far is the Linux kernel IMO, because at least that is mostly C and GNU Make. And yet even there, that still isn't that bad. Also, as far as bugs at the interface, I'm pretty sure it's a wash. C and Rust are ABI compatible to the extent that matters. The "subtle bugs" are most likely going to be ones that could've existed without the additional complexity of a new language and an interface between them. It's no worse than say, JNI, which doesn't seem to be a serious issue. I'm pretty sure the benefits of Rust can outweigh this cost. As long as you can build the project, you don't really need to understand 2 toolchains to contribute unless your contribution lands across a boundary between 2 toolchains. On a lot of really large projects this may already be the case anyways.