Fix missing extern for identity crate

[yonas]

[yonas]

Posted this too early. I'm now getting a cant' find crate error when running make oss, but compiling hgmain works when I go into it's directory (eden/scm/exec/hgmain) and run cargo build --release.

[bolinfest]

I'm not sure what issue you're hitting. Possibilities:

• Do you need to make clean and then make oss again?
• Are you on an M1? For our prebuilt binaries, we cross-compile from Intel Mac, so building on M1 directly isn't tested quite as well.

[quark-zju]

Are you using an old rustc? extern crate became unnecessary since Rust edition 2018: https://doc.rust-lang.org/edition-guide/rust-2018/path-changes.html#no-more-extern-crate and we're targeting Rust edition 2021 now. I'm closing this since upgrading rustc seems to solve this issue.