To use pass the name of a llvm ir file as the first argument.

In rust you can get these by adding --emit=llvm-ir to rust command line.
They show up in target/release/deps/*.ll

This can usually be done with `RUSTFLAGS='-g --emit=llvm-ir' cargo build --release`
This will generate .ll files for all dependencies as well. I've run into issues with
that not working: rust-lang/rust#56290. If you run into this
you can also do `RUSTFLAGS='-g' cargo build --release -v` and get the command line and
just append `--emit=llvm-ir`

The bitcode files should work too, but have not been tested.

You can demangle the names using rustfilt. (cargo install rustfilt)