Bug 1568446 - Replace uses of mem::uninitialized with MaybeUninit. r=emilio

Cherry-picked from: https://github.com/servo/servo/pull/23824

servo/components/servo_arc/lib.rs

@@ -52,25 +52,6 @@ use std::sync::atomic;
 use std::sync::atomic::Ordering::{Acquire, Relaxed, Release};
 use std::{isize, usize};
 
-// Private macro to get the offset of a struct field in bytes from the address of the struct.
-macro_rules! offset_of {
-    ($container:path, $field:ident) => {{
-        // Make sure the field actually exists. This line ensures that a compile-time error is
-        // generated if $field is accessed through a Deref impl.
-        let $container { $field: _, .. };
-
-        // Create an (invalid) instance of the container and calculate the offset to its
-        // field. Using a null pointer might be UB if `&(*(0 as *const T)).field` is interpreted to
-        // be nullptr deref.
-        let invalid: $container = ::std::mem::uninitialized();
-        let offset = &invalid.$field as *const _ as usize - &invalid as *const _ as usize;
-
-        // Do not run destructors on the made up invalid instance.
-        ::std::mem::forget(invalid);
-        offset as isize
-    }};
-}
-
 /// A soft limit on the amount of references that may be made to an `Arc`.
 ///
 /// Going above this limit will abort your program (although not
@@ -196,6 +177,14 @@ struct ArcInner<T: ?Sized> {
 unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
 unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
 
+/// Computes the offset of the data field within ArcInner.
+fn data_offset<T>() -> usize {
+    let size = size_of::<ArcInner<()>>();
+    let align = align_of::<T>();
+    // https://github.com/rust-lang/rust/blob/1.36.0/src/libcore/alloc.rs#L187-L207
+    size.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1)
+}
+
 impl<T> Arc<T> {
     /// Construct an `Arc<T>`
     #[inline]
@@ -251,7 +240,7 @@ impl<T> Arc<T> {
     unsafe fn from_raw(ptr: *const T) -> Self {
         // To find the corresponding pointer to the `ArcInner` we need
         // to subtract the offset of the `data` field from the pointer.
-        let ptr = (ptr as *const u8).offset(-offset_of!(ArcInner<T>, data));
+        let ptr = (ptr as *const u8).sub(data_offset::<T>());
         Arc {
             p: ptr::NonNull::new_unchecked(ptr as *mut ArcInner<T>),
             phantom: PhantomData,

servo/components/style/gecko/profiler.rs

@@ -29,15 +29,15 @@ impl<'a> AutoProfilerLabel<'a> {
     /// stack.
     #[inline]
     pub unsafe fn new(
-        label: &mut structs::AutoProfilerLabel,
+        label: &mut std::mem::MaybeUninit<structs::AutoProfilerLabel>,
         label_type: ProfilerLabel,
     ) -> AutoProfilerLabel {
         let category_pair = match label_type {
             ProfilerLabel::Style => structs::JS::ProfilingCategoryPair_LAYOUT_StyleComputation,
             ProfilerLabel::Parse => structs::JS::ProfilingCategoryPair_LAYOUT_CSSParsing,
         };
-        structs::Gecko_Construct_AutoProfilerLabel(label, category_pair);
-        AutoProfilerLabel(label)
+        structs::Gecko_Construct_AutoProfilerLabel(label.as_mut_ptr(), category_pair);
+        AutoProfilerLabel(&mut *label.as_mut_ptr())
     }
 }
 

servo/components/style/gecko_string_cache/mod.rs

@@ -202,7 +202,8 @@ impl WeakAtom {
     where
         F: FnOnce(&str) -> Output,
     {
-        let mut buffer: [u8; 64] = unsafe { mem::uninitialized() };
+        let mut buffer = mem::MaybeUninit::<[u8; 64]>::uninit();
+        let buffer = unsafe { &mut *buffer.as_mut_ptr() };
 
         // The total string length in utf16 is going to be less than or equal
         // the slice length (each utf16 character is going to take at least one
@@ -271,7 +272,8 @@ impl WeakAtom {
         }
 
         let slice = self.as_slice();
-        let mut buffer: [u16; 64] = unsafe { mem::uninitialized() };
+        let mut buffer = mem::MaybeUninit::<[u16; 64]>::uninit();
+        let buffer = unsafe { &mut *buffer.as_mut_ptr() };
         let mut vec;
         let mutable_slice = if let Some(buffer_prefix) = buffer.get_mut(..slice.len()) {
             buffer_prefix.copy_from_slice(slice);

servo/components/style/macros.rs

@@ -112,8 +112,8 @@ macro_rules! define_keyword_type {
 #[macro_export]
 macro_rules! profiler_label {
     ($label_type:ident) => {
-        let mut _profiler_label: $crate::gecko_bindings::structs::AutoProfilerLabel =
-            unsafe { ::std::mem::uninitialized() };
+        let mut _profiler_label =
+            ::std::mem::MaybeUninit::<$crate::gecko_bindings::structs::AutoProfilerLabel>::uninit();
         let _profiler_label = if $crate::gecko::profiler::profiler_is_active() {
             unsafe {
                 Some($crate::gecko::profiler::AutoProfilerLabel::new(

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

@@ -44,9 +44,9 @@ use crate::properties::computed_value_flags::*;
 use crate::properties::longhands;
 use crate::rule_tree::StrongRuleNode;
 use crate::selector_parser::PseudoElement;
-use servo_arc::{Arc, RawOffsetArc};
+use servo_arc::{Arc, RawOffsetArc, UniqueArc};
 use std::marker::PhantomData;
-use std::mem::{forget, uninitialized, zeroed, ManuallyDrop};
+use std::mem::{forget, zeroed, ManuallyDrop};
 use std::{cmp, ops, ptr};
 use crate::values::{self, CustomIdent, Either, KeyframesName, None_};
 use crate::values::computed::{Percentage, TransitionProperty};
@@ -215,19 +215,18 @@ impl ComputedValuesInner {
             Some(p) => p.pseudo_type(),
             None => structs::PseudoStyleType::NotPseudo,
         };
-        let arc = unsafe {
-            let arc: Arc<ComputedValues> = Arc::new(uninitialized());
+        unsafe {
+            let mut arc = UniqueArc::<ComputedValues>::new_uninit();
             bindings::Gecko_ComputedStyle_Init(
-                &arc.0 as *const _ as *mut _,
+                &mut (*arc.as_mut_ptr()).0,
                 &self,
                 pseudo_ty,
             );
-            // We're simulating a move by having C++ do a memcpy and then forgetting
+            // We're simulating move semantics by having C++ do a memcpy and then forgetting
             // it on this end.
             forget(self);
-            arc
-        };
-        arc
+            UniqueArc::assume_init(arc).shareable()
+        }
     }
 }
 

servo/components/style/properties/helpers/animated_properties.mako.rs

@@ -18,7 +18,7 @@ use crate::properties::LonghandId;
 use servo_arc::Arc;
 use smallvec::SmallVec;
 use std::ptr;
-use std::mem::{self, ManuallyDrop};
+use std::mem;
 use crate::hash::FxHashMap;
 use super::ComputedValues;
 use crate::values::animated::{Animate, Procedure, ToAnimatedValue, ToAnimatedZero};
@@ -252,12 +252,12 @@ impl Clone for AnimationValue {
             }
 
             unsafe {
-                let mut out = mem::uninitialized();
+                let mut out = mem::MaybeUninit::uninit();
                 ptr::write(
-                    &mut out as *mut _ as *mut CopyVariants,
+                    out.as_mut_ptr() as *mut CopyVariants,
                     *(self as *const _ as *const CopyVariants),
                 );
-                return out;
+                return out.assume_init();
             }
         }
 
@@ -269,15 +269,15 @@ impl Clone for AnimationValue {
                 ${props[0].camel_case}(value.clone())
                 % else:
                 unsafe {
-                    let mut out = ManuallyDrop::new(mem::uninitialized());
+                    let mut out = mem::MaybeUninit::uninit();
                     ptr::write(
-                        &mut out as *mut _ as *mut AnimationValueVariantRepr<${ty}>,
+                        out.as_mut_ptr() as *mut AnimationValueVariantRepr<${ty}>,
                         AnimationValueVariantRepr {
                             tag: *(self as *const _ as *const u16),
                             value: value.clone(),
                         },
                     );
-                    ManuallyDrop::into_inner(out)
+                    out.assume_init()
                 }
                 % endif
             }
@@ -356,15 +356,15 @@ impl AnimationValue {
                 PropertyDeclaration::${props[0].camel_case}(value)
                 % else:
                 unsafe {
-                    let mut out = mem::uninitialized();
+                    let mut out = mem::MaybeUninit::uninit();
                     ptr::write(
-                        &mut out as *mut _ as *mut PropertyDeclarationVariantRepr<${specified}>,
+                        out.as_mut_ptr() as *mut PropertyDeclarationVariantRepr<${specified}>,
                         PropertyDeclarationVariantRepr {
                             tag: *(self as *const _ as *const u16),
                             value,
                         },
                     );
-                    out
+                    out.assume_init()
                 }
                 % endif
             }
@@ -424,15 +424,15 @@ impl AnimationValue {
                 % endif
 
                 unsafe {
-                    let mut out = mem::uninitialized();
+                    let mut out = mem::MaybeUninit::uninit();
                     ptr::write(
-                        &mut out as *mut _ as *mut AnimationValueVariantRepr<${ty}>,
+                        out.as_mut_ptr() as *mut AnimationValueVariantRepr<${ty}>,
                         AnimationValueVariantRepr {
                             tag: longhand_id.to_physical(context.builder.writing_mode) as u16,
                             value,
                         },
                     );
-                    out
+                    out.assume_init()
                 }
             }
             % endfor
@@ -579,15 +579,15 @@ impl Animate for AnimationValue {
                     let value = this.animate(&other_repr.value, procedure)?;
                     % endif
 
-                    let mut out = mem::uninitialized();
+                    let mut out = mem::MaybeUninit::uninit();
                     ptr::write(
-                        &mut out as *mut _ as *mut AnimationValueVariantRepr<${ty}>,
+                        out.as_mut_ptr() as *mut AnimationValueVariantRepr<${ty}>,
                         AnimationValueVariantRepr {
                             tag: this_tag,
                             value,
                         },
                     );
-                    out
+                    out.assume_init()
                 }
                 % endfor
                 ${" |\n".join("{}(void)".format(prop.camel_case) for prop in unanimated)} => {

servo/components/style/properties/longhands/font.mako.rs

@@ -367,15 +367,16 @@ ${helpers.predefined_type(
                     % endfor
                 };
 
-                let mut system: nsFont = unsafe { mem::uninitialized() };
-                unsafe {
+                let mut system = mem::MaybeUninit::<nsFont>::uninit();
+                let system = unsafe {
                     bindings::Gecko_nsFont_InitSystem(
-                        &mut system,
+                        system.as_mut_ptr(),
                         id as i32,
                         cx.style().get_font().gecko(),
                         cx.device().document()
-                    )
-                }
+                    );
+                    &mut *system.as_mut_ptr()
+                };
                 let font_weight = longhands::font_weight::computed_value::T::from_gecko_weight(system.weight);
                 let font_stretch = FontStretch(NonNegative(Percentage(unsafe {
                     bindings::Gecko_FontStretch_ToFloat(system.stretch)
@@ -413,7 +414,7 @@ ${helpers.predefined_type(
                     system_font: *self,
                     default_font_type: system.fontlist.mDefaultFontType,
                 };
-                unsafe { bindings::Gecko_nsFont_Destroy(&mut system); }
+                unsafe { bindings::Gecko_nsFont_Destroy(system); }
                 ret
             }
 

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

@@ -17,7 +17,7 @@ use servo_arc::{Arc, UniqueArc};
 use std::borrow::Cow;
 use std::{ops, ptr};
 use std::fmt::{self, Write};
-use std::mem::{self, ManuallyDrop};
+use std::mem;
 
 use cssparser::{Parser, RGBA, TokenSerializationType};
 use cssparser::ParserInput;
@@ -294,12 +294,12 @@ impl Clone for PropertyDeclaration {
             }
 
             unsafe {
-                let mut out = mem::uninitialized();
+                let mut out = mem::MaybeUninit::uninit();
                 ptr::write(
-                    &mut out as *mut _ as *mut CopyVariants,
+                    out.as_mut_ptr() as *mut CopyVariants,
                     *(self as *const _ as *const CopyVariants),
                 );
-                return out;
+                return out.assume_init();
             }
         }
 
@@ -333,15 +333,15 @@ impl Clone for PropertyDeclaration {
             % else:
             ${" |\n".join("{}(ref value)".format(v["name"]) for v in vs)} => {
                 unsafe {
-                    let mut out = ManuallyDrop::new(mem::uninitialized());
+                    let mut out = mem::MaybeUninit::uninit();
                     ptr::write(
-                        &mut out as *mut _ as *mut PropertyDeclarationVariantRepr<${ty}>,
+                        out.as_mut_ptr() as *mut PropertyDeclarationVariantRepr<${ty}>,
                         PropertyDeclarationVariantRepr {
                             tag: *(self as *const _ as *const u16),
                             value: value.clone(),
                         },
                     );
-                    ManuallyDrop::into_inner(out)
+                    out.assume_init()
                 }
             }
             % endif

servo/components/to_shmem/lib.rs

@@ -412,9 +412,10 @@ impl<T: ToShmem, A: Array<Item = T>> ToShmem for SmallVec<A> {
                 SmallVec::from_raw_parts(dest, self.len(), self.len())
             } else {
                 // Place the items inline.
-                let mut inline: A = mem::uninitialized();
-                to_shmem_slice_ptr(self.iter(), inline.ptr_mut(), builder);
-                SmallVec::from_buf_and_len(inline, self.len())
+                let mut s = SmallVec::new();
+                to_shmem_slice_ptr(self.iter(), s.as_mut_ptr(), builder);
+                s.set_len(self.len());
+                s
             }
         };