Bug 1595674 - Make the rust build respect -j1. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D54060

--HG--
extra : moz-landing-system : lando

config/makefiles/rust.mk

@@ -41,6 +41,12 @@ cargo_build_flags += --color=always
 endif
 endif
 
+# Without -j > 1, make will not pass jobserver info down to cargo. Force
+# one job when requested as a special case.
+ifeq (1,$(MOZ_PARALLEL_BUILD))
+cargo_build_flags += -j1
+endif
+
 rustflags_lto = -Clto
 # Disable LTO when linking gkrust_gtest.
 ifneq (,$(findstring gkrust_gtest,$(RUST_LIBRARY_FILE)))