diff --git a/mk/target.mk b/mk/target.mk
index 37181b05438..2a08b7b0465 100644
--- a/mk/target.mk
+++ b/mk/target.mk
@@ -42,6 +42,23 @@ $(foreach host,$(CFG_HOST), \
    $(foreach crate,$(CRATES), \
     $(eval $(call RUST_CRATE_FULLDEPS,$(stage),$(target),$(host),$(crate)))))))
 
+# $(1) stage
+# $(2) target
+# $(3) host
+define DEFINE_BOOTSTRAP_KEY
+BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3) := $$(CFG_BOOTSTRAP_KEY)
+ifeq ($(1),0)
+ifeq ($(3),$$(CFG_BUILD))
+BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3) := $$(CFG_BOOTSTRAP_KEY_STAGE0)
+endif
+endif
+endef
+
+$(foreach host,$(CFG_TARGET), \
+ $(foreach target,$(CFG_TARGET), \
+  $(foreach stage,$(STAGES), \
+   $(eval $(call DEFINE_BOOTSTRAP_KEY,$(stage),$(target),$(host))))))
+
 # RUST_TARGET_STAGE_N template: This defines how target artifacts are built
 # for all stage/target architecture combinations. This is one giant rule which
 # works as follows:
@@ -65,12 +82,9 @@ $(foreach host,$(CFG_HOST), \
 # $(4) is the crate name
 define RUST_TARGET_STAGE_N
 
-ifeq ($(1),0)
-$$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
-	export RUSTC_BOOTSTRAP_KEY := $$(CFG_BOOTSTRAP_KEY_STAGE0)
-endif
-
 $$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): CFG_COMPILER_HOST_TRIPLE = $(2)
+$$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
+	export RUSTC_BOOTSTRAP_KEY := $$(BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3))
 $$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
 		$$(CRATEFILE_$(4)) \
 		$$(CRATE_FULLDEPS_$(1)_T_$(2)_H_$(3)_$(4)) \
@@ -118,11 +132,8 @@ endef
 # $(4) - name of the tool being built
 define TARGET_TOOL
 
-ifeq ($(1),0)
 $$(TBIN$(1)_T_$(2)_H_$(3))/$(4)$$(X_$(2)): \
-	export RUSTC_BOOTSTRAP_KEY := $$(CFG_BOOTSTRAP_KEY_STAGE0)
-endif
-
+	export RUSTC_BOOTSTRAP_KEY := $$(BOOTSTRAP_KEY$(1)_T_$(2)_H_$(3))
 $$(TBIN$(1)_T_$(2)_H_$(3))/$(4)$$(X_$(2)): \
 		$$(TOOL_SOURCE_$(4)) \
 		$$(TOOL_INPUTS_$(4)) \