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

The playground defaults to edition 2024. Fails if you select edition 2015: Rust Playground (or 2018 for what it's worth). works locally if you pick at least edition 2021: rustc +nightly --crate-name flurb --crate-type=lib --out-dir target/flurb --emit=llvm-ir t.rs --edition 2021