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

It does. It used to be conditional, but that was 5 years ago.

github.com/rust-lang/cargo

src/cargo/core/compiler/mod.rs

master

    1261.     if unit.show_warnings(bcx.gctx) {


    1262.         warn(msg)


    1263.     } else {


    1264.         Ok(())


    1265.     }


    1266. };


    1267.

    1268. cmd.arg("--crate-name").arg(&unit.target.crate_name());


    1269.

    1270. let edition = unit.target.edition();


    1271. edition.cmd_edition_arg(cmd);


    1272.

    1273. add_path_args(bcx.ws, unit, cmd);


    1274. add_error_format_and_color(build_runner, cmd);


    1275. add_allow_features(build_runner, cmd);


    1276.

    1277. let mut contains_dy_lib = false;


    1278. if !test {


    1279.     for crate_type in &unit.target.rustc_crate_types() {


    1280.         cmd.arg("--crate-type").arg(crate_type.as_str());


    1281.         contains_dy_lib |= crate_type == &CrateType::Dylib;