Bug 1353810 - add a --enable-rust-debug option; r=chmanchester

For people working on Rust code, compiling in debug mode (Cargo's "dev"
profile) is convenient: debug assertions are turned on, optimization is
turned off, and parallel compilation inside of rustc itself can be
used.  These things make the build faster and the debugging experience
more pleasant.

To obtain that currently, one needs to --enable-debug at the Gecko
toplevel, which turns on debug assertions for the entire browser, which
makes things run unreasonably slowly.  So it would be desirable to be
able to turn *off* debug mode for the entirety of the browser, but turn
on debug mode for the Rust code only.

Hence this added switch, --enable-rust-debug, which does what it
suggests and defaults to the value of --enable-debug.  For our own
sanity and because we judge it a non-existent use case, we do not
support --enable-debug --disable-rust-debug.

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)'

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)
 {

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.

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)
 

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)