Code compiles on playground but fails when passed via stdin to rustc

MusicalNinjaDad:

and question whether CARGO_UNSTABLE_ALLOW_FEATURES should remain the more common approach given -Zallow-features now exists

It's the other way around, rustc -Zallow-features has existed for years longer than cargo -Zallow-features (but I'm pretty sure that is also years old at this point). The latter is a superset of the former, it controls both cargo and rustc unstable features, and has a much better UX for shared workspace definitions in its TOML form.