Index: head/share/mk/src.sys.mk
===================================================================
--- head/share/mk/src.sys.mk	(revision 287938)
+++ head/share/mk/src.sys.mk	(revision 287939)
@@ -1,17 +1,35 @@
 # $FreeBSD$
 
 # Note: This file is also duplicated in the sys/conf/kern.pre.mk so
 # it will always grab SRCCONF, even if it isn't being built in-tree
 # to preserve historical (and useful) behavior. Changes here need to
 # be reflected there so SRCCONF isn't included multiple times.
 
 # Allow user to configure things that only effect src tree builds.
 SRCCONF?=	/etc/src.conf
 .if (exists(${SRCCONF}) || ${SRCCONF} != "/etc/src.conf") && !target(_srcconf_included_)
+
+# Validate that the user didn't try setting an env-only variable in
+# their src.conf. This benefits from already including bsd.mkopt.mk.
+.for var in ${__ENV_ONLY_OPTIONS}
+__presrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno:Dyes}
+.endfor
+
 .sinclude "${SRCCONF}"
 _srcconf_included_:	.NOTMAIN
+
+# Validate the env-only variables.
+.for var in ${__ENV_ONLY_OPTIONS}
+__postrcconf_${var}:=	${MK_${var}:U-}${WITHOUT_${var}:Uno:Dyes}${WITH_${var}:Uno:Dyes}
+.if ${__presrcconf_${var}} != ${__postrcconf_${var}}
+.error Option ${var} may only be defined in ${SRC_ENV_CONF},  environment, or make argument, not ${SRCCONF}.
 .endif
+.undef __presrcconf_${var}
+.undef __postrcconf_${var}
+.endfor
+
+.endif # SRCCONF
 
 # tempting, but bsd.compiler.mk causes problems this early
 # probably need to remove dependence on bsd.own.mk 
 #.include "src.opts.mk"