Can’t use `-Z macro-backtrace` unstable option with `cargo`

-Z macro-backtrace is a rustc flag, not a cargo flag. You should be able to pass it to rustc using cargo +nightly rustc -- -Z macro-backtrace. (The +nightly in the command line is optional if you already switched to the nightly compiler as the default.)

Alternatively, you can set the RUSTFLAGS environment variable:

export RUSTFLAGS="-Z macro-backtrace"