.SECONDEXPANSION:

#dir/x.foo:

#ding=$(@:.foo=.bar)
#
#%.foo: $$(info got here) /./proc/self/cwd/$$(ding)
#	echo "Prerequisites: $^"
#
#%.bar:
#	echo "We want $@"

# $(call save-and-use,/,prereqs)
#save-and-use=$(eval $@=$(if $1,$2,$(foreach p,$2,$(dir $@)$p)))$2

pr1=$(info in pr1, $$@ is $@)$(eval $@@gdeps:=$(subst %,$*,$1))$1
pr2=$(info in pr2, $$@ is $@)$(eval $@@gdeps:=$(addprefix $(dir $@),$(subst %,$*,$1)))$1

FORCE:

define define-rule
$(eval 
$1: $(if $(findstring /,$1),$$$$(call pr1,$2),$$$$(call pr2,$2)) FORCE
	$$(info +++ $$$$^ is $$^, gdeps is $$($$@@gdeps))
)
endef

$(call define-rule,%.foo,%.bar)

x/%.bar:
	$(info +++ Making $@)