diff --git a/Makefile.in b/Makefile.in
index dbd98a31c00..9ab91c40a0f 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -57,6 +57,13 @@ else
   CFG_STDLIB_DEFAULT = $(2)/$(CFG_STDLIB)
 endif
 
+# Uncomment this to temporarily use the snapshot's runtime for stage1.
+# This is useful when making incompatible changes to the parts of the
+# runtime used directly by the compiler -- the binaries built by the
+# snapshot won't know about the changes yet. Don't leave this on. Turn
+# it on, shapshot, and turn it off again.
+# CFG_USE_SNAP_LIBS_FOR_STAGE1 = 1
+
 # version-string calculation
 CFG_GIT_DIR := $(CFG_SRC_DIR).git
 CFG_VERSION = prerelease
diff --git a/mk/stageN.mk b/mk/stageN.mk
index 171b954d9a7..dc3778968ea 100644
--- a/mk/stageN.mk
+++ b/mk/stageN.mk
@@ -8,6 +8,15 @@
 # new rustrt in stage0/lib/.
 
 define LIBGEN
+
+ifdef CFG_USE_SNAP_LIBS_FOR_STAGE1
+ifeq ($(2), 0)
+RUNTIME_SOURCE = stage0/$(CFG_RUNTIME)
+stage$(2)/lib/$$(CFG_STDLIB): stage0/$$(CFG_STDLIB)
+	@$$(call E, cp: $$@)
+	$$(Q)cp $$< $$@
+else
+RUNTIME_SOURCE = rt/$(CFG_RUNTIME)
 stage$(2)/lib/$$(CFG_STDLIB): $$(STDLIB_CRATE) $$(STDLIB_INPUTS) \
                               stage$(2)/rustc$$(X)               \
                               stage$(2)/$$(CFG_RUNTIME)          \
@@ -16,6 +25,8 @@ stage$(2)/lib/$$(CFG_STDLIB): $$(STDLIB_CRATE) $$(STDLIB_INPUTS) \
                               $$(SREQ$(1))
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(2))  --lib -o $$@ $$<
+endif
+endif
 
 stage$(2)/lib/libstd.rlib: $$(STDLIB_CRATE) $$(STDLIB_INPUTS) \
                            stage$(2)/rustc$$(X)               \
@@ -26,7 +37,7 @@ stage$(2)/lib/libstd.rlib: $$(STDLIB_CRATE) $$(STDLIB_INPUTS) \
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(2)) --lib --static -o $$@ $$<
 
-stage$(2)/lib/$$(CFG_RUNTIME): rt/$$(CFG_RUNTIME)
+stage$(2)/lib/$$(CFG_RUNTIME): $$(RUNTIME_SOURCE)
 	@$$(call E, cp: $$@)
 	$$(Q)cp $$< $$@
 
@@ -58,11 +69,20 @@ stage$(2)/rustc$$(X): $$(COMPILER_CRATE) $$(COMPILER_INPUTS)          \
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(1)) -L stage$(2) -o $$@ $$<
 
-stage$(2)/$$(CFG_RUNTIME): rt/$$(CFG_RUNTIME)
+RUNTIME_SOURCE = rt/$(CFG_RUNTIME)
+STDLIB_SOURCE = stage$(1)/lib/$$(CFG_STDLIB)
+ifdef CFG_USE_SNAP_LIBS_FOR_STAGE1
+  ifeq ($(2), 1)
+    RUNTIME_SOURCE = stage0/$(CFG_RUNTIME)
+    STDLIB_SOURCE = stage0/$(CFG_STDLIB)
+  endif
+endif
+
+stage$(2)/$$(CFG_RUNTIME): $$(RUNTIME_SOURCE)
 	@$$(call E, cp: $$@)
 	$$(Q)cp $$< $$@
 
-stage$(2)/$$(CFG_STDLIB): stage$(1)/lib/$$(CFG_STDLIB)
+stage$(2)/$$(CFG_STDLIB): $$(STDLIB_SOURCE)
 	@$$(call E, cp: $$@)
 	$$(Q)cp $$< $$@