Index: lang/rust/Makefile
===================================================================
--- lang/rust/Makefile
+++ lang/rust/Makefile
@@ -142,6 +142,17 @@
 		${WRKSRC}/src/bootstrap/install.rs
 
 do-configure:
+# Sanity check that the running kernel was built with the necessary
+# syscall compat option.
+.if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1200000
+	@${SYSCTL} -q kern.conftxt | ${SORT} -u | ${AWK} '\
+	$$2 == "COMPAT_FREEBSD11" { i++ } \
+	END { if (NR > 0 && i < 1) { \
+		print "=> Sanity check failed: kernel is missing COMPAT_FREEBSD11"; \
+		print "=> Aborting build"; \
+		exit 1 \
+	} }'
+.endif
 	@${ECHO_CMD} '[build]' > ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'vendor=true' >> ${WRKSRC}/config.toml
 	@${ECHO_CMD} 'extended=true' >> ${WRKSRC}/config.toml