If so, then that’s correct, as rustc can’t compile C code. You need a C -> wasm compiler, and that’s emscripten.
Hopefully we’ll have built the ecosystem enough that you won’t need to rely on those libraries :)
Also, doesn't clang also have a wasm backend now? Emscripten isn't the only option.
Clang does have a wasm backend, same as the unknown target. It suffers the same problems; that is, one of the things that emscripten does is provide a runtime/support code that does things like "automatically translate opengl to webgl". That is, one of emscripten's major goals is "get code that was never assumed to be able to run on the web to run on the web with as little hassle as possible." Outside of the web, those C libraries were very much intended to work on the native platform. So there's extra complexity there.
We continue to support emscripten for when you need this use-case, but it effectively only works if you're building an application, not a library, and we want to support people writing libraries.