Rust support in pigweed is *highly* experimental. Currently only building a single host binary using the standard libraries is supported. Only GN builds are supported and building on Windows is currently unsupported.
To build the sample rust targets, you need to enable
$ gn gen out --args="pw_rust_ENABLE_EXPERIMENTAL_BUILD=true"
Once that is set, you can build and run the
$ ninja -C out host_clang_debug/obj/pw_rust/example/bin/hello $ ./out/host_clang_debug/obj/pw_rust/example/bin/hello Hello, Pigweed!