diff --git a/config/rules.mk b/config/rules.mk
index 6f2ddb65146e..2702ad58bb1e 100644
--- a/config/rules.mk
+++ b/config/rules.mk
@@ -906,7 +906,7 @@ cargo_target_flag := --target=$(RUST_TARGET)
 
 # Permit users to pass flags to cargo from their mozconfigs (e.g. --color=always).
 cargo_build_flags = $(CARGOFLAGS)
-ifndef MOZ_DEBUG
+ifndef MOZ_DEBUG_RUST
 cargo_build_flags += --release
 endif
 cargo_build_flags += --frozen
@@ -936,8 +936,8 @@ endif
 ifndef MOZ_OPTIMIZE
 rustflags = -C opt-level=0
 # Unfortunately, -C opt-level=0 implies -C debug-assertions, so we need
-# to explicitly disable them when MOZ_DEBUG is not set.
-ifndef MOZ_DEBUG
+# to explicitly disable them when MOZ_DEBUG_RUST is not set.
+ifndef MOZ_DEBUG_RUST
 rustflags += -C debug-assertions=no
 endif
 rustflags_override = RUSTFLAGS='$(rustflags)'
diff --git a/layout/style/ServoBindings.cpp b/layout/style/ServoBindings.cpp
index d235784f75d7..afe61cb3bfb1 100644
--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -88,7 +88,7 @@ Gecko_IsInDocument(RawGeckoNodeBorrowed aNode)
   return aNode->IsInComposedDoc();
 }
 
-#ifdef DEBUG
+#ifdef MOZ_DEBUG_RUST
 bool
 Gecko_FlattenedTreeParentIsParent(RawGeckoNodeBorrowed aNode)
 {
diff --git a/moz.configure b/moz.configure
index 42d9b9573b97..9e241dfbec91 100644
--- a/moz.configure
+++ b/moz.configure
@@ -103,6 +103,20 @@ js_option('--enable-debug',
 add_old_configure_assignment('MOZ_DEBUG',
                              depends('--enable-debug')(lambda v: bool(v)))
 
+imply_option('--enable-rust-debug',
+             depends('--enable-debug')(lambda v: bool(v) or None))
+
+js_option('--enable-rust-debug',
+          help='Build Rust code with debug assertions turned on.')
+
+@depends('--enable-rust-debug')
+def debug_rust(debug):
+    if debug:
+        return True
+
+set_config('MOZ_DEBUG_RUST', debug_rust)
+set_define('MOZ_DEBUG_RUST', debug_rust)
+
 include('build/moz.configure/pkg.configure')
 # Make this assignment here rather than in pkg.configure to avoid
 # requiring this file in unit tests.
diff --git a/python/mozbuild/mozbuild/frontend/data.py b/python/mozbuild/mozbuild/frontend/data.py
index 5b07dafdddcc..347f48ae9674 100644
--- a/python/mozbuild/mozbuild/frontend/data.py
+++ b/python/mozbuild/mozbuild/frontend/data.py
@@ -425,7 +425,7 @@ def cargo_output_directory(context, target_var):
     # in those directories.  The directory structure depends not only
     # on the target, but also what sort of build we are doing.
     rust_build_kind = 'release'
-    if context.config.substs.get('MOZ_DEBUG'):
+    if context.config.substs.get('MOZ_DEBUG_RUST'):
         rust_build_kind = 'debug'
     return mozpath.join(context.config.substs[target_var], rust_build_kind)
 
diff --git a/xpcom/string/nsSubstring.cpp b/xpcom/string/nsSubstring.cpp
index 3a89e50c3857..a244a791568e 100644
--- a/xpcom/string/nsSubstring.cpp
+++ b/xpcom/string/nsSubstring.cpp
@@ -378,6 +378,10 @@ void Gecko_IncrementStringAdoptCount(void* aData)
 {
   MOZ_LOG_CTOR(aData, "StringAdopt", 1);
 }
+#elif defined(MOZ_DEBUG_RUST)
+void Gecko_IncrementStringAdoptCount(void *aData)
+{
+}
 #endif
 
 void Gecko_FinalizeCString(nsACString* aThis)