With rustc 1.29 it fails in unicode-xid v.0.1.0 which i think is one of the first or the first rust target.
I have not seen another "Compiling ..." statement before it.
make[5]: Leaving directory '/<<PKGBUILDDIR>>/build-browser/security'
make[5]: Entering directory '/<<PKGBUILDDIR>>/build-browser/toolkit/library/gtest/rust'
make[5]: Nothing to be done for 'target'.
make[5]: Leaving directory '/<<PKGBUILDDIR>>/build-browser/toolkit/library/gtest/rust'
make[5]: Entering directory '/<<PKGBUILDDIR>>/build-browser/toolkit/library/rust'
force-cargo-library-build
env RUSTFLAGS='-C opt-level=2 -C debuginfo=2 ' CARGO_TARGET_DIR=/<<PKGBUILDDIR>>/build-browser/toolkit/library RUSTC=/usr/bin/rustc RUSTDOC=/usr/bin/rustdoc MOZ_SRC=/<<PKGBUILDDIR>> MOZ_DIST=/<<PKGBUILDDIR>>/build-browser/dist LIBCLANG_PATH="/usr/lib/llvm-11/lib" CLANG_PATH="/usr/lib/llvm-11/bin/clang" PKG_CONFIG_ALLOW_CROSS=1 RUST_BACKTRACE=full MOZ_TOPOBJDIR=/<<PKGBUILDDIR>>/build-browser MOZ_CARGO_WRAP_LDFLAGS="-lpthread -Wl,--as-needed -Wl,--reduce-memory-overheads -Wl,--no-keep-memory -Wl,--stats -Wl,-z,noexecstack -Wl,-z,text -Wl,-z,relro -Wl,--build-id -Wl,-rpath-link,/<<PKGBUILDDIR>>/build-browser/dist/bin -Wl,-rpath-link,/usr/lib" MOZ_CARGO_WRAP_LD=" /usr/bin/gcc -std=gnu99" CARGO_TARGET_SPARC64_UNKNOWN_LINUX_GNU_LINKER=/<<PKGBUILDDIR>>/build/cargo-linker /usr/bin/cargo rustc --release --frozen --manifest-path /<<PKGBUILDDIR>>/toolkit/library/rust/Cargo.toml --lib --target=sparc64-unknown-linux-gnu --features "servo bindgen quantum_render cubeb_pulse_rust cubeb-remoting moz_memory" -- -C lto
Compiling unicode-xid v0.1.0
error: Unrecognized option: 'json'
error: could not compile `unicode-xid`
make[5]: *** [/<<PKGBUILDDIR>>/config/
rules.mk:951: force-cargo-library-build] Error 101
make[5]: Leaving directory '/<<PKGBUILDDIR>>/build-browser/toolkit/library/rust'
make[4]: *** [/<<PKGBUILDDIR>>/config/
recurse.mk:74: toolkit/library/rust/target] Error 2
make[4]: Leaving directory '/<<PKGBUILDDIR>>/build-browser'
make[3]: *** [/<<PKGBUILDDIR>>/config/
recurse.mk:34: compile] Error 2
make[3]: Leaving directory '/<<PKGBUILDDIR>>/build-browser'
make[2]: *** [/<<PKGBUILDDIR>>/config/
rules.mk:418: default] Error 2
make[2]: Leaving directory '/<<PKGBUILDDIR>>/build-browser'
dh_auto_build: error: cd build-browser && make -j1 LD_LIBS=-Wl,--no-gc-sections _LEAKTEST_FILES=leaktest.py returned exit code 2
make[1]: *** [debian/rules:216: stamps/build-browser] Error 25
make[1]: Leaving directory '/<<PKGBUILDDIR>>'
make: *** [debian/rules:321: build-arch] Error 2
dpkg-buildpackage: error: debian/rules build-arch subprocess returned exit status 2
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Build finished at 2021-11-21T02:58:31Z