From my (admittedly probably limited) experience with LLVM IR from writing my own compiler, it seems like if you were writing a program entirely in LLVM IR you

From my (admittedly probably limited) experience with LLVM IR from writing my own compiler, it seems like if you were writing a program entirely in LLVM IR you could use a subset that could be compiled and run on any fully-supported LLVM target. Of course that's a different proposition than e.g. compiling a C program to LLVM IR using Clang and then trying to compile that IR on a different target, or trying to interact with non-LLVM-IR functions that conform to the platform ABI. Of course, the resulting native code may not be the best for the target, since e.g. a native integer on one platform might become a pair of smaller integers on another platform. But it could work. With all of that said, I expect the proposal was to use the Rust compiler to compile the Rust compiler to IR, and I imagine Rust is complex enough that it must generate at least some target-specific IR. Perhaps one could take the generated IR and "normalize" it, but it's questionable whether it would be worth it.