> At that scale, you can't just give everyone the source and tell them to do a world compile. Stable ABIs fix that. Also, you can't coordinate between all of the people involved other than via stable ABIs. So stable ABIs save both individual build time and reduce cognitive load.

Rust supports ABI compatibility if everyone is on the same compiler version.

That means you can have a distributed caching architecture for your billion line monorepo where everyone can compile world at all times because they share artifacts. Google pioneered this for C++ and doesn't need to care about ABI as a result.

What Rust does not support is a team deciding they don't want to upgrade their toolchains and still interoperate with those that do. Or random copy and pasting of `.so` files you don't know the provenance of. Everyone must be in sync.

In my opinion, this is a reasonable constraint. It allows Rust to swap out HashMap implementations. In contrast, C++ map types are terrible for performance because they cannot be updated for stability reasons.