Let me explain the order of operations I went through:

1. Run cargo miri test. Realise there are no backtraces.
2. I try again with env RUST_BACKTRACE=1 cargo miri test.
3. This still doesn't work. I double-check the output and I get this little nugget:

   note: in Miri, you may have to set `-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect
   

4. env RUST_BACKTRACE=1 cargo miri test -Zmiri-env-forward=RUST_BACKTRACE

5. error: unknown `-Z` flag specified: miri-env-forward
   
   For available unstable features, see https://doc.rust-lang.org/nightly/cargo/reference/unstable.html
   If you intended to use an unstable rustc feature, try setting `RUSTFLAGS="-Zmiri-env-forward"
   

6. env RUST_BACKTRACE=1 RUSTFLAGS=-Zmiri-env-forward=RUST_BACKTRACE cargo miri test
7. Still doesn't work. At this point, I have to search for -Zmiri-env-forward and ultimately stumble on the readme for this repo mentioning I should use MIRIFLAGS, not RUSTFLAGS.

This is… very frustrating. Not to mention I have a particularly large test suite I was running this on, and miri was running all tests by default, and it was quite slow between each step. It would be nice if any of the following applied:

1. RUST_BACKTRACE was just special and always passed to miri, at least via cargo miri.
2. cargo miri accepted -Z flags to be passed directly to miri.
3. The comment mentioning RUST_BACKTRACE=1 just explicitly said RUST_BACKTRACE=1 MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE.

This could potentially also be a cargo change, but since this message is being generated by miri itself, I figured I'd start here first.