Rust uses LLVM because it's pretty great, not because you couldn't implement LLVM in Rust.
Maybe cranelift will eventually surpass LLVM, but there isn't currently much reason to push for that.
If anything, making cranelift an LLVM replacement would likely go counter to its stated goals of being a simple and fast code generator.
Thus Rust cannot really replace C++ when its reference toolchain depends on it.
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