enable jemalloc debugging in unoptimized builds

The performance hit from these checks is significant, but unoptimized
builds are already incredibly slow. Enabling these checks results in
better test coverage since there are bots doing unoptimized builds, and
the cost is relatively small in the context of an unoptimized build.
This also allows using `JEMALLOC_FLAGS` to override the default
configure flags.

Makefile.in

@@ -65,6 +65,7 @@
 #
 #   * `VERBOSE=1` - Print all commands. Use this to see what's going on.
 #   * `RUSTFLAGS=...` - Add compiler flags to all `rustc` invocations
+#   * `JEMALLOC_FLAGS=...` - Pass flags to jemalloc's configure script
 #
 #   * `TESTNAME=...` - Specify the name of tests to run
 #   * `CHECK_IGNORED=1` - Run normally-ignored tests

mk/main.mk

@@ -90,15 +90,19 @@ endif
 CFG_RUSTC_FLAGS := $(RUSTFLAGS)
 CFG_GCCISH_CFLAGS :=
 CFG_GCCISH_LINK_FLAGS :=
+CFG_JEMALLOC_FLAGS :=
 
 ifdef CFG_DISABLE_OPTIMIZE
   $(info cfg: disabling rustc optimization (CFG_DISABLE_OPTIMIZE))
   CFG_RUSTC_FLAGS +=
+  CFG_JEMALLOC_FLAGS += --enable-debug
 else
   # The rtopt cfg turns off runtime sanity checks
   CFG_RUSTC_FLAGS += -O --cfg rtopt
 endif
 
+CFG_JEMALLOC_FLAGS += $(JEMALLOC_FLAGS)
+
 ifdef CFG_DISABLE_DEBUG
   CFG_RUSTC_FLAGS += --cfg ndebug
   CFG_GCCISH_CFLAGS += -DRUST_NDEBUG

mk/rt.mk

@@ -301,7 +301,7 @@ JEMALLOC_LOCAL_$(1) := $$(JEMALLOC_BUILD_DIR_$(1))/lib/$$(JEMALLOC_REAL_NAME_$(1
 $$(JEMALLOC_LOCAL_$(1)): $$(JEMALLOC_DEPS) $$(MKFILE_DEPS)
 	@$$(call E, make: jemalloc)
 	cd "$$(JEMALLOC_BUILD_DIR_$(1))"; "$(S)src/jemalloc/configure" \
-		$$(JEMALLOC_ARGS_$(1)) --with-jemalloc-prefix=je_ \
+		$$(JEMALLOC_ARGS_$(1)) --with-jemalloc-prefix=je_ $(CFG_JEMALLOC_FLAGS) \
 		--build=$(CFG_BUILD) --host=$(1) \
 		CC="$$(CC_$(1))" \
 		AR="$$(AR_$(1))" \