Reverse that: "C can't support a platform when that platform's vendors just provide a proprietary Rust compiler and nothing else".
Reverse that: "C can't support a platform when that platform's vendors just provide a proprietary Rust compiler and nothing else".
Seems to me that that is equally true and doesn't remove any validity from the argument.
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It's unclear what point you're trying to make here.
Proprietary platforms with proprietary-only toolchains are bad, for a wide variety of reasons. Open toolchains are a good thing, for many reasons, including that they can support many different programming languages.