diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -43,7 +43,11 @@
 CWARNEXTRA+=	-Wno-error=shift-negative-value
 CWARNEXTRA+=	-Wno-address-of-packed-member
 .if ${COMPILER_VERSION} >= 130000
-CWARNFLAGS+=	-Wno-error=unused-but-set-variable
+.if ${MK_SET_BUT_NOTUSED_KERNEL_WARNINGS} == "no"
+CWARNEXTRA+=	${NO_WUNUSED_BUT_SET_VARIABLE}
+.else
+CWARNEXTRA+=	-Wno-error=unused-but-set-variable
+.endif
 .endif
 .endif	# clang
 
diff --git a/sys/conf/kern.opts.mk b/sys/conf/kern.opts.mk
--- a/sys/conf/kern.opts.mk
+++ b/sys/conf/kern.opts.mk
@@ -60,7 +60,8 @@
     INIT_ALL_ZERO \
     KERNEL_RETPOLINE \
     RATELIMIT \
-    REPRODUCIBLE_BUILD
+    REPRODUCIBLE_BUILD \
+    SET_BUT_NOTUSED_KERNEL_WARNINGS
 
 # Some options are totally broken on some architectures. We disable
 # them. If you need to enable them on an experimental basis, you