That describes how to get set up with Rust. Long story short, cargo depends on a linker and brings it along, but doesn’t expose it to other programs via PATH. If there are build scripts that call out to rustc or a linker directly then you need to explicitly install one. This is intentional, but yeah it can be surprising when you get an error about not finding a linker.

You also need libiconv on macOS for reasons, but all of this is described in the link above.