Here's an overview:

1. A stage3 for riscv64gc needs to contain both dev-lang/rust and dev-lang/go:

• dev-lang/go: Unless this is available, it needs to be manually bootstrapped from another platform by the user if they wish to use go
• dev-lang/rust: Unless this is available, the user needs to download a third-party binary (dev-lang/rust-bin). Choosing not to have rust installed is not a viable option, anymore, due the the many packages requiring it. Requiring Funtoo users to download a third party binary to have a functioning system is not a good practice imho. I would argue that dev-lang/rust should be installed in all profiles by default (same as sys-devel/gcc), but that's a different issue.

2. When trying to build a new stage3 for riscv64gc with metro, it won't build dev-lang/go and dev-lang/rust, because even when they're available in the initial stage3 seed, they aren't in packages.build and thus not available in the stage1 and stage2 built by metro.