Yeah exactly, imo the way the Go version requires you to declare an interface, in this case with a no-op method essentially acting as a tag, and declare conform

Yeah exactly, imo the way the Go version requires you to declare an interface, in this case with a no-op method essentially acting as a tag, and declare conformance on your types is quite noisy and feels like a hack. Also I prefer with Rust-style sum types that the declaration happens in one place. In the Go version your Shape types could be scattered around all over the place, which makes it harder to parse and understand if you're reading unfamiliar code.