It provides IR, low level optimization passes, native code generation, and common metadata features (e.g. for debugging). Rust probably has a library somewhere

It provides IR, low level optimization passes, native code generation, and common metadata features (e.g. for debugging). Rust probably has a library somewhere that binds to it. So you would have to write linking (with help from LLVM), ASTs, high-level optimizations and validation, garbage collection (with help from LLVM), and standard library (or link an existing one). Also intermediate IRs or ASTs you plan on using (for example Rust's MIR) and their infrastructure.