mk: If NO_REBUILD is set then don't rebuild core/std before testing

Can make turnaround of testing changes to core/std/syntax much faster.

Author: brson

mk/tests.mk

@@ -244,21 +244,29 @@ $(foreach host,$(CFG_HOST_TRIPLES), \
 
 define TEST_RUNNER
 
+# If NO_REBUILD is set then break the dependencies on std so we can
+# test crates without rebuilding core and std first
+ifeq ($(NO_REBUILD),)
+STDTESTDEP_$(1)_$(2)_$(3) = $$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_STDLIB_$(2))
+else
+STDTESTDEP_$(1)_$(2)_$(3) =
+endif
+
 $(3)/test/coretest.stage$(1)-$(2)$$(X_$(2)):			\
 		$$(CORELIB_CRATE) $$(CORELIB_INPUTS)	\
-		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_STDLIB_$(2))
+		$$(STDTESTDEP_$(1)_$(2)_$(3))
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(1)_T_$(2)_H_$(3)) -o $$@ $$< --test
 
 $(3)/test/stdtest.stage$(1)-$(2)$$(X_$(2)):			\
 		$$(STDLIB_CRATE) $$(STDLIB_INPUTS)	\
-		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_STDLIB_$(2))
+		$$(STDTESTDEP_$(1)_$(2)_$(3))
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(1)_T_$(2)_H_$(3)) -o $$@ $$< --test
 
 $(3)/test/syntaxtest.stage$(1)-$(2)$$(X_$(2)):			\
 		$$(LIBSYNTAX_CRATE) $$(LIBSYNTAX_INPUTS)	\
-		$$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_STDLIB_$(2))
+		$$(STDTESTDEP_$(1)_$(2)_$(3))
 	@$$(call E, compile_and_link: $$@)
 	$$(STAGE$(1)_T_$(2)_H_$(3)) -o $$@ $$< --test