7.3.0 MinGW-based builds of GCC have been available from multiple sources for a long time, and even 8.1.0 is now available: MinGW-w64 GCC 8.1.0
They all work fine with Rust and are fully capable of building it from source (have tested it several times), and solve several issues present in 6.4. Not really a nightly build if it’s using a now rather outdated GCC as the foundation, is it?
And yes, I know the MSVC-based toolchain is recommended on Windows. The thing is, due to the objectively Linux-focused nature of Rust as a whole, there’s just way too many crates that you simply cannot compile or use with the MSVC toolchain due to them having various hardcoded Linux-isms and linking against Linux-only libraries. Based on my personal requirements, at least, I have no choice but to use the MinGW toolchain.