Index: Mk/Uses/cargo.mk
===================================================================
--- Mk/Uses/cargo.mk
+++ Mk/Uses/cargo.mk
@@ -247,6 +247,16 @@
 # configure hook.  Place a config file for overriding crates-io index
 # by local source directory.
 cargo-configure:
+.  if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1200000
+# Sanity check that the running kernel was built with the necessary
+# syscall compat option.
+	@${PRINTF} '#include <sys/syscall.h>\n#include <errno.h>\n#include <unistd.h>\n\
+		int main(void) { return syscall(SYS_freebsd11_mknod, "", 0, 0) < 0 && errno == ENOSYS; }' | \
+		${CC} -o ${WRKDIR}/compat11_canary -xc - && ${WRKDIR}/compat11_canary || (\
+	${ECHO_MSG} "=> Sanity check failed: kernel is missing COMPAT_FREEBSD11"; \
+	${ECHO_MSG} "=> Aborting build"; \
+	${FALSE})
+.  endif
 	@${MKDIR} ${WRKDIR}/.cargo
 	@${ECHO_CMD} "[source.cargo]" > ${WRKDIR}/.cargo/config
 	@${ECHO_CMD} "directory = '${CARGO_VENDOR_DIR}'" >> ${WRKDIR}/.cargo/config
Index: Mk/bsd.gecko.mk
===================================================================
--- Mk/bsd.gecko.mk
+++ Mk/bsd.gecko.mk
@@ -376,6 +376,18 @@
 	@${REINPLACE_CMD} 's,"files":{[^}]*},"files":{},' \
 		${MOZSRC}/third_party/rust/*/.cargo-checksum.json
 
+.if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1200000
+pre-configure-script:
+# Sanity check that the running kernel was built with the necessary
+# syscall compat option.
+	@${PRINTF} '#include <sys/syscall.h>\n#include <errno.h>\n#include <unistd.h>\n\
+		int main(void) { return syscall(SYS_freebsd11_mknod, "", 0, 0) < 0 && errno == ENOSYS; }' | \
+		${CC} -o ${WRKDIR}/compat11_canary -xc - && ${WRKDIR}/compat11_canary || (\
+	${ECHO_MSG} "=> Sanity check failed: kernel is missing COMPAT_FREEBSD11"; \
+	${ECHO_MSG} "=> Aborting build"; \
+	${FALSE})
+.endif
+
 post-install-script: gecko-create-plist
 
 gecko-create-plist:
Index: lang/rust/Makefile
===================================================================
--- lang/rust/Makefile
+++ lang/rust/Makefile
@@ -142,6 +142,16 @@
 		${WRKSRC}/src/bootstrap/install.rs
 
 do-configure:
+# Sanity check that the running kernel was built with the necessary
+# syscall compat option.
+.if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1200000
+	@${PRINTF} '#include <sys/syscall.h>\n#include <errno.h>\n#include <unistd.h>\n\
+	int main(void) { return syscall(SYS_freebsd11_mknod, "", 0, 0) < 0 && errno == ENOSYS; }' | \
+	${CC} -o ${WRKDIR}/compat11_canary -xc - && ${WRKDIR}/compat11_canary || (\
+		${ECHO_MSG} "=> Sanity check failed: kernel is missing COMPAT_FREEBSD11"; \
+		${ECHO_MSG} "=> Aborting build"; \
+		${FALSE})
+.endif
 	@${ECHO_CMD} '[build]' > ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'vendor=true' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'extended=true' >> ${WRKSRC}/config.toml