style: Rejigger a bit rust features so that rusttests still link.

We cannot compile with just feature(gecko + debug_assertions), since that's how
debug rusttests get compiled and they don't have the refcount logging stuff.

We were getting away with it for the pre-existing usage of the style crate,
because it wasn't used during any test and presumably the linker didn't
complain. But servo_arc is definitely used in tests.

Differential Revision: https://phabricator.services.mozilla.com/D32691

components/style/Cargo.toml

@@ -22,6 +22,7 @@ servo = ["serde", "style_traits/servo", "servo_atoms", "servo_config", "html5eve
          "cssparser/serde", "encoding_rs", "malloc_size_of/servo", "arrayvec/use_union",
          "servo_url", "string_cache", "crossbeam-channel", "to_shmem/servo", "servo_arc/servo"]
 gecko_debug = []
+gecko_refcount_logging = []
 gecko_profiler = []
 
 [dependencies]

components/style/rule_tree/mod.rs

@@ -755,7 +755,7 @@ unsafe impl Sync for RuleTree {}
 unsafe impl Send for RuleTree {}
 
 // On Gecko builds, hook into the leak checking machinery.
-#[cfg(all(feature = "gecko", debug_assertions))]
+#[cfg(feature = "gecko_refcount_logging")]
 mod gecko_leak_checking {
     use super::RuleNode;
     use std::mem::size_of;
@@ -788,13 +788,13 @@ mod gecko_leak_checking {
 
 #[inline(always)]
 fn log_new(_ptr: *const RuleNode) {
-    #[cfg(all(feature = "gecko", debug_assertions))]
+    #[cfg(feature = "gecko_refcount_logging")]
     gecko_leak_checking::log_ctor(_ptr);
 }
 
 #[inline(always)]
 fn log_drop(_ptr: *const RuleNode) {
-    #[cfg(all(feature = "gecko", debug_assertions))]
+    #[cfg(feature = "gecko_refcount_logging")]
     gecko_leak_checking::log_dtor(_ptr);
 }