rustc_target: The differences between `target_os = "none"` and `target_os = "unknown"`, and `target_vendor = "unknown"` and `target_vendor = ""` are unclear

[petrochenkov]

The majority of targets use target_os = "none" / target_vendor = "unknown" respectively, but some targets deviate and use target_os = "unknown" / target_vendor = "".

Possible solutions:

• There's no difference between "", "none" and "unknown", normalize to the more common variants.
• There are deep reasons for the status quo, keep and document it.
• 50 / 50 of the above.

[ehuss]

For OS, none has a specific meaning of being bare-metal (no OS). unknown would mean there is an OS, but it is not specified. One way rustc cares about this is that none targets are built as no_std by default.

Generally a missing component of the triple is supposed to be interpreted as "unknown". I think it might be reasonable to use "unknown" instead of an empty string in the Target definition, though I'm not sure if it matters too much in practice. Generally people only care about the canonical target string (like x86_64-linux-kernel or aarch64-fuchsia), and the vendor string is never used (like one probably wouldn't bother doing cfg(target_vendor="unknown") or printing it anywhere).

This is sorta documented at https://doc.rust-lang.org/cargo/appendix/glossary.html#target (which is derived from https://clang.llvm.org/docs/CrossCompilation.html#target-triple, which is in turn derived from GNU AIUI). https://doc.rust-lang.org/rustc/targets/index.html should probably discuss this in more detail.