diff --git a/common.mk b/common.mk
index e2fd5c4b279f7e..6871ad003fc33b 100644
--- a/common.mk
+++ b/common.mk
@@ -269,21 +269,15 @@ MAKE_LINK = $(MINIRUBY) -rfileutils -e "include FileUtils::Verbose" \
 # For release builds
 YJIT_RUSTC_ARGS = --crate-name=yjit \
 	$(JIT_RUST_FLAGS) \
+	$(RUSTC_FLAGS) \
 	--edition=2021 \
-	-g \
-	-C lto=thin \
-	-C opt-level=3 \
-	-C overflow-checks=on \
 	'--out-dir=$(CARGO_TARGET_DIR)/release/' \
 	'$(top_srcdir)/yjit/src/lib.rs'
 
 ZJIT_RUSTC_ARGS = --crate-name=zjit \
 	$(JIT_RUST_FLAGS) \
+	$(RUSTC_FLAGS) \
 	--edition=2024 \
-	-g \
-	-C lto=thin \
-	-C opt-level=3 \
-	-C overflow-checks=on \
 	'--out-dir=$(CARGO_TARGET_DIR)/release/' \
 	'$(top_srcdir)/zjit/src/lib.rs'
 
diff --git a/configure.ac b/configure.ac
index 6787e9b3067572..a276783b6c2f63 100644
--- a/configure.ac
+++ b/configure.ac
@@ -69,6 +69,7 @@ dnl 93(bright yellow) is copied from .github/workflows/mingw.yml
 AC_ARG_VAR([cflags], [additional CFLAGS (ignored when CFLAGS is given)])dnl
 AC_ARG_VAR([cppflags], [additional CPPFLAGS (ignored when CPPFLAGS is given)])dnl
 AC_ARG_VAR([cxxflags], [additional CXXFLAGS (ignored when CXXFLAGS is given)])dnl
+AC_ARG_VAR([rustc_flags], [additional RUSTC_FLAGS])dnl
 
 [begin]_group "environment section" && {
 HAVE_BASERUBY=yes
@@ -4049,6 +4050,11 @@ AS_CASE(["${ZJIT_SUPPORT}"],
     AC_DEFINE(USE_ZJIT, 0)
 ])
 
+RUSTC_FLAGS='-g -C lto=thin -C opt-level=3 -C overflow-checks=on'
+AS_IF([test -n "${rustc_flags}"], [
+    RUSTC_FLAGS="${RUSTC_FLAGS} ${rustc_flags}"
+])
+
 JIT_RUST_FLAGS='--crate-type=staticlib --cfg feature=\"stats_allocator\"'
 RLIB_DIR=
 AS_CASE(["$JIT_CARGO_SUPPORT:$YJIT_SUPPORT:$ZJIT_SUPPORT"],
@@ -4106,6 +4112,7 @@ AS_IF([test -n "$RUST_LIB"], [
 dnl These variables end up in ::RbConfig::CONFIG
 AC_SUBST(RUSTC)dnl Rust compiler command
 AC_SUBST(JIT_RUST_FLAGS)dnl the common rustc flags for JIT crates such as zjit
+AC_SUBST(RUSTC_FLAGS)dnl user-configurable rustc compiler flags
 AC_SUBST(CARGO)dnl Cargo command for Rust builds
 AC_SUBST(CARGO_BUILD_ARGS)dnl for selecting Rust build profiles
 AC_SUBST(YJIT_SUPPORT)dnl what flavor of YJIT the Ruby build includes
@@ -4838,6 +4845,7 @@ config_summary "strip command"       "$STRIP"
 config_summary "install doc"         "$DOCTARGETS"
 config_summary "YJIT support"        "$YJIT_SUPPORT"
 config_summary "ZJIT support"        "$ZJIT_SUPPORT"
+config_summary "RUSTC_FLAGS"         "$RUSTC_FLAGS"
 config_summary "man page type"       "$MANTYPE"
 config_summary "search path"         "$search_path"
 config_summary "static-linked-ext"   ${EXTSTATIC:+"yes"}
diff --git a/defs/jit.mk b/defs/jit.mk
index 42b56c4cd928b6..27b14e7a07255d 100644
--- a/defs/jit.mk
+++ b/defs/jit.mk
@@ -40,6 +40,7 @@ else ifneq ($(strip $(RLIB_DIR)),) # combo build
 $(RUST_LIB): $(srcdir)/ruby.rs
 	$(ECHO) 'building $(@F)'
 	$(gnumake_recursive)$(Q) $(RUSTC) --edition=2024 \
+	    $(RUSTC_FLAGS) \
 	    '-L$(@D)' \
 	    --extern=yjit \
 	    --extern=zjit \
@@ -58,6 +59,7 @@ $(JIT_RLIB):
 	$(gnumake_recursive)$(Q) $(RUSTC) --crate-name=jit \
 	    --edition=2024 \
 	    $(JIT_RUST_FLAGS) \
+	    $(RUSTC_FLAGS) \
 	    '--out-dir=$(@D)' \
 	    '$(top_srcdir)/jit/src/lib.rs'
 endif # ifneq ($(JIT_CARGO_SUPPORT),no)
diff --git a/gc/mmtk/extconf.rb b/gc/mmtk/extconf.rb
index 5f4228972bbbb8..c0e788037ecfe4 100644
--- a/gc/mmtk/extconf.rb
+++ b/gc/mmtk/extconf.rb
@@ -15,6 +15,10 @@
     MMTK_BUILD = debug
     LIBMMTK_RUBY = libmmtk_ruby.#$LIBEXT
     RUSTSRCS = #{rustsrcs.join(" \\\n\t   ")}
+
+    ifeq ($(MMTK_BUILD), debug)
+    CPPFLAGS += -DMMTK_DEBUG
+    endif
     MAKEFILE
   ]
 end
diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 042517e7d741b6..38e730a3761e4d 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -963,6 +963,18 @@ rb_gc_impl_writebarrier(void *objspace_ptr, VALUE a, VALUE b)
 
     if (SPECIAL_CONST_P(b)) return;
 
+#ifdef MMTK_DEBUG
+    if (!rb_gc_impl_pointer_to_heap_p(objspace_ptr, (void *)a)) {
+        char buff[256];
+        rb_bug("a: %s is not an object", rb_raw_obj_info(buff, 256, a));
+    }
+
+    if (!rb_gc_impl_pointer_to_heap_p(objspace_ptr, (void *)b)) {
+        char buff[256];
+        rb_bug("b: %s is not an object", rb_raw_obj_info(buff, 256, b));
+    }
+#endif
+
     mmtk_object_reference_write_post(cache->mutator, (MMTk_ObjectReference)a);
 }
 
diff --git a/template/Makefile.in b/template/Makefile.in
index 443c394cb4f84d..0b7b50e3aa3340 100644
--- a/template/Makefile.in
+++ b/template/Makefile.in
@@ -115,6 +115,7 @@ CARGO_TARGET_DIR=@abs_top_builddir@/target
 CARGO_BUILD_ARGS=@CARGO_BUILD_ARGS@
 ZJIT_TEST_FEATURES=@ZJIT_TEST_FEATURES@
 JIT_RUST_FLAGS=@JIT_RUST_FLAGS@
+RUSTC_FLAGS=@RUSTC_FLAGS@
 RLIB_DIR=@RLIB_DIR@
 RUST_LIB=@RUST_LIB@
 RUST_LIBOBJ = $(RUST_LIB:.a=.@OBJEXT@)