Bug 1593951 - Use MaybeUninit in style struct clone impls / constructors. r=xidorn

Differential Revision: https://phabricator.services.mozilla.com/D51788

--HG--
extra : moz-landing-system : lando

layout/style/nsStyleStruct.cpp

@@ -2688,8 +2688,10 @@ nsStyleDisplay::nsStyleDisplay(const Document& aDocument)
       mOverscrollBehaviorX(StyleOverscrollBehavior::Auto),
       mOverscrollBehaviorY(StyleOverscrollBehavior::Auto),
       mOverflowAnchor(StyleOverflowAnchor::Auto),
-      mScrollSnapType(
-          {StyleScrollSnapAxis::Both, StyleScrollSnapStrictness::None}),
+      mScrollSnapAlign{StyleScrollSnapAlignKeyword::None,
+                       StyleScrollSnapAlignKeyword::None},
+      mScrollSnapType{StyleScrollSnapAxis::Both,
+                      StyleScrollSnapStrictness::None},
       mLineClamp(0),
       mRotate(StyleRotate::None()),
       mTranslate(StyleTranslate::None()),
@@ -2753,6 +2755,8 @@ nsStyleDisplay::nsStyleDisplay(const nsStyleDisplay& aSource)
       mScrollBehavior(aSource.mScrollBehavior),
       mOverscrollBehaviorX(aSource.mOverscrollBehaviorX),
       mOverscrollBehaviorY(aSource.mOverscrollBehaviorY),
+      mOverflowAnchor(aSource.mOverflowAnchor),
+      mScrollSnapAlign(aSource.mScrollSnapAlign),
       mScrollSnapType(aSource.mScrollSnapType),
       mLineClamp(aSource.mLineClamp),
       mTransform(aSource.mTransform),

servo/components/style/properties/gecko.mako.rs

@@ -46,7 +46,7 @@ use crate::rule_tree::StrongRuleNode;
 use crate::selector_parser::PseudoElement;
 use servo_arc::{Arc, RawOffsetArc, UniqueArc};
 use std::marker::PhantomData;
-use std::mem::{forget, zeroed, ManuallyDrop};
+use std::mem::{forget, MaybeUninit};
 use std::{cmp, ops, ptr};
 use crate::values::{self, CustomIdent, Either, KeyframesName, None_};
 use crate::values::computed::{Percentage, TransitionProperty};
@@ -218,7 +218,7 @@ impl ComputedValuesInner {
         unsafe {
             let mut arc = UniqueArc::<ComputedValues>::new_uninit();
             bindings::Gecko_ComputedStyle_Init(
-                &mut (*arc.as_mut_ptr()).0,
+                arc.as_mut_ptr() as *mut _,
                 &self,
                 pseudo_ty,
             );
@@ -621,14 +621,14 @@ def set_gecko_property(ffi_name, expr):
 impl ${style_struct.gecko_struct_name} {
     #[allow(dead_code, unused_variables)]
     pub fn default(document: &structs::Document) -> Arc<Self> {
-        let mut result = Arc::new(${style_struct.gecko_struct_name} { gecko: ManuallyDrop::new(unsafe { zeroed() }) });
         unsafe {
+            let mut result = UniqueArc::<Self>::new_uninit();
             Gecko_Construct_Default_${style_struct.gecko_ffi_name}(
-                &mut *Arc::get_mut(&mut result).unwrap().gecko,
+                result.as_mut_ptr() as *mut _,
                 document,
             );
+            UniqueArc::assume_init(result).shareable()
         }
-        result
     }
 }
 impl Drop for ${style_struct.gecko_struct_name} {
@@ -641,9 +641,9 @@ impl Drop for ${style_struct.gecko_struct_name} {
 impl Clone for ${style_struct.gecko_struct_name} {
     fn clone(&self) -> Self {
         unsafe {
-            let mut result = ${style_struct.gecko_struct_name} { gecko: ManuallyDrop::new(zeroed()) };
-            Gecko_CopyConstruct_${style_struct.gecko_ffi_name}(&mut *result.gecko, &*self.gecko);
-            result
+            let mut result = MaybeUninit::<Self>::uninit();
+            Gecko_CopyConstruct_${style_struct.gecko_ffi_name}(result.as_mut_ptr() as *mut _, &*self.gecko);
+            result.assume_init()
         }
     }
 }