Lets say I want to expose a Zig library to Rust or vice versa, now I must call the Zig compiler from Cargo, or Cargo from the Zig build process.

With a transpiled library such a project is "clean", it only needs the Rust or Zig toolchain. The transpiling process only needs to happen once, and this can happen automatically on a CI service when the base library is updated and then made available automatically through the language's package repository.

Rust probably won't get the ability to transpile to Zig, and Zig probably won't be able to transpile to Rust. Some sort of Lingua Franca makes sense in a multi-language world, and C (or even a subset of C) makes sense for that because it already is the Lingua France, and has a very small feature set to consider. I'm aware that this is a controversial opinion though ;)

If we were going to create a Lingua Franca for such a purpose from scratch, it would probably look very little like C. It would probably look like a much reduced subset of Some compiler IR, or Haskell. We would want something that is typesafe and lowers trivially to an AST. As the barest of table stakes, a language with any undefined or implementation defined behavior whatsoever is obviously unsuitable for this purpose. After all, we need a wide variety of compilers to be able to easily generate their own IR from it, and for them all to guarantee exactly the same behavior when they do. C seems close to the worst choice possible.

Edit: Haha, I realized I invented WASM again. This happens frequently...