$ cargo +1.37.0 test --all
warning: unused manifest key: package.rust-version
    Updating crates.io index
  Downloaded thiserror v1.0.40
  Downloaded thiserror-impl v1.0.40
  Downloaded syn v2.0.2
error: failed to parse manifest at `C:\Users\jay\.cargo\registry\src\github.com-1ecc6299db9ec823\syn-2.0.2\Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  supported edition values are `2015` or `2018`, but `2021` is unknown