Not a Rust thing per se, but rust-srpm-macros' macros.rust has:
%[0%{?_package_note_status} ? "-Clink-arg=%_package_note_flags" :
""]
in %build_rustflags, which gets exported in RUSTFLAGS.
I suppose for C and C++ packages, mingw must be filtering out that
distro flag somehow?
Thanks, I've updated the mingw macros in mingw-filesystem and now the
build succeeds!