diff --git a/bin/ebuild.sh b/bin/ebuild.sh
index 5491c4f..403a597 100755
--- a/bin/ebuild.sh
+++ b/bin/ebuild.sh
@@ -775,8 +775,8 @@
 
 	# If ${EBUILD_FORCE_TEST} == 1 and USE came from ${T}/environment
 	# then it might not have USE=test like it's supposed to here.
-	if [[ ${EBUILD_PHASE} == test && ${EBUILD_FORCE_TEST} == 1 &&
-		test =~ ${PORTAGE_IUSE} ]] && ! has test ${USE} ; then
+	if [[ ${EBUILD_PHASE} == test && ${EBUILD_FORCE_TEST} == 1 ]] &&
+		__in_portage_iuse test && ! has test ${USE} ; then
 		export USE="${USE} test"
 	fi
 	declare -r USE
diff --git a/bin/phase-helpers.sh b/bin/phase-helpers.sh
index ba3f279..62cc692 100644
--- a/bin/phase-helpers.sh
+++ b/bin/phase-helpers.sh
@@ -237,9 +237,9 @@
 	# Make sure we have this USE flag in IUSE, but exempt binary
 	# packages for API consumers like Entropy which do not require
 	# a full profile with IUSE_IMPLICIT and stuff (see bug #456830).
-	elif [[ -n $PORTAGE_IUSE && -n $EBUILD_PHASE &&
-		-n $PORTAGE_INTERNAL_CALLER ]] ; then
-		if [[ ! $u =~ $PORTAGE_IUSE ]] ; then
+	elif [[ $(type -t __in_portage_iuse) == function &&
+		-n $EBUILD_PHASE && -n $PORTAGE_INTERNAL_CALLER ]] ; then
+		if ! __in_portage_iuse "${u}"; then
 			if [[ ${EMERGE_FROM} != binary &&
 				! ${EAPI} =~ ^(0|1|2|3|4|4-python|4-slot-abi)$ ]] ; then
 				# This is only strict starting with EAPI 5, since implicit IUSE
diff --git a/lib/portage/package/ebuild/_config/special_env_vars.py b/lib/portage/package/ebuild/_config/special_env_vars.py
index d2aa14b..1c167cd 100644
--- a/lib/portage/package/ebuild/_config/special_env_vars.py
+++ b/lib/portage/package/ebuild/_config/special_env_vars.py
@@ -14,8 +14,8 @@
 # to enter the config instance from the external environment or
 # configuration files.
 env_blacklist = frozenset((
-	"A", "AA", "BDEPEND", "BROOT", "CATEGORY", "DEPEND", "DESCRIPTION",
-	"DOCS", "EAPI",
+	"A", "AA", "BASH_FUNC___in_portage_iuse%%", "BDEPEND", "BROOT",
+	"CATEGORY", "DEPEND", "DESCRIPTION", "DOCS", "EAPI",
 	"EBUILD_FORCE_TEST", "EBUILD_PHASE",
 	"EBUILD_PHASE_FUNC", "EBUILD_SKIP_MANIFEST",
 	"ED", "EMERGE_FROM", "EPREFIX", "EROOT",
@@ -42,7 +42,8 @@
 # environment in order to prevent sandbox from sourcing /etc/profile
 # in it's bashrc (causing major leakage).
 environ_whitelist += [
-	"ACCEPT_LICENSE", "BASH_ENV", "BROOT", "BUILD_PREFIX", "COLUMNS", "D",
+	"ACCEPT_LICENSE", "BASH_ENV", "BASH_FUNC___in_portage_iuse%%",
+	"BROOT", "BUILD_PREFIX", "COLUMNS", "D",
 	"DISTDIR", "DOC_SYMLINKS_DIR", "EAPI", "EBUILD",
 	"EBUILD_FORCE_TEST",
 	"EBUILD_PHASE", "EBUILD_PHASE_FUNC", "ECLASSDIR", "ECLASS_DEPTH", "ED",
diff --git a/lib/portage/package/ebuild/config.py b/lib/portage/package/ebuild/config.py
index a68df58..f04b7c0 100644
--- a/lib/portage/package/ebuild/config.py
+++ b/lib/portage/package/ebuild/config.py
@@ -1759,13 +1759,27 @@
 				portage_iuse.update(built_use)
 			self.configdict["pkg"]["IUSE_EFFECTIVE"] = \
 				" ".join(sorted(portage_iuse))
+
+			self.configdict["env"]["BASH_FUNC___in_portage_iuse%%"] = (
+				"() { "
+				"if [[ ${#PORTAGE_IUSE_HASH[@]} -lt 1 ]]; then "
+				"  declare -gAr PORTAGE_IUSE_HASH=(%s); "
+				"fi; "
+				"[[ -n ${PORTAGE_IUSE_HASH[$1]} ]]; "
+				"}" ) % " ".join('["%s"]=1' % x for x in portage_iuse)
 		else:
 			portage_iuse = self._get_implicit_iuse()
 			portage_iuse.update(explicit_iuse)
 
-		# PORTAGE_IUSE is not always needed so it's lazily evaluated.
-		self.configdict["env"].addLazySingleton(
-			"PORTAGE_IUSE", _lazy_iuse_regex, portage_iuse)
+			# The _get_implicit_iuse() returns a regular expression
+			# so we can't use the (faster) map.  Fall back to
+			# implementing __in_portage_iuse() the older/slower way.
+
+			# PORTAGE_IUSE is not always needed so it's lazily evaluated.
+			self.configdict["env"].addLazySingleton(
+				"PORTAGE_IUSE", _lazy_iuse_regex, portage_iuse)
+			self.configdict["env"]["BASH_FUNC___in_portage_iuse%%"] = \
+				"() { [[ $1 =~ ${PORTAGE_IUSE} ]]; }"
 
 		ebuild_force_test = not restrict_test and \
 			self.get("EBUILD_FORCE_TEST") == "1"