Introduce an unsafe HasBaseFlow trait for base()/base_mut() casts.

components/layout/block.rs

@@ -492,8 +492,12 @@ pub enum FormattingContextType {
     Other,
 }
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for BlockFlow {}
+
 // A block formatting context.
 #[derive(Serialize)]
+#[repr(C)]
 pub struct BlockFlow {
     /// Data common to all flows.
     pub base: BaseFlow,

components/layout/flex.rs

@@ -328,8 +328,12 @@ impl FlexLine {
     }
 }
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for FlexFlow {}
+
 /// A block with the CSS `display` property equal to `flex`.
 #[derive(Debug, Serialize)]
+#[repr(C)]
 pub struct FlexFlow {
     /// Data common to all block flows.
     block_flow: BlockFlow,

components/layout/flow.rs

@@ -65,11 +65,19 @@ use table_rowgroup::TableRowGroupFlow;
 use table_wrapper::TableWrapperFlow;
 use webrender_api::ClipAndScrollInfo;
 
+/// This marker trait indicates that a type is a struct with `#[repr(C)]` whose first field
+/// is of type `BaseFlow` or some type that also implements this trait.
+///
+/// In other words, the memory representation of `BaseFlow` must be a prefix
+/// of the memory representation of types implementing `HasBaseFlow`.
+#[allow(unsafe_code)]
+pub unsafe trait HasBaseFlow {}
+
 /// Virtual methods that make up a float context.
 ///
 /// Note that virtual methods have a cost; we should not overuse them in Servo. Consider adding
 /// methods to `ImmutableFlowUtils` or `MutableFlowUtils` before adding more methods here.
-pub trait Flow: fmt::Debug + Sync + Send + 'static {
+pub trait Flow: HasBaseFlow + fmt::Debug + Sync + Send + 'static {
     // RTTI
     //
     // TODO(pcwalton): Use Rust's RTTI, once that works.
@@ -451,7 +459,7 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
 
 #[inline(always)]
 #[allow(unsafe_code)]
-pub fn base<T: ?Sized + Flow>(this: &T) -> &BaseFlow {
+pub fn base<T: ?Sized + HasBaseFlow>(this: &T) -> &BaseFlow {
     let ptr: *const T = this;
     let ptr = ptr as *const BaseFlow;
     unsafe { &*ptr }
@@ -464,7 +472,7 @@ pub fn child_iter<'a>(flow: &'a Flow) -> FlowListIterator {
 
 #[inline(always)]
 #[allow(unsafe_code)]
-pub fn mut_base<T: ?Sized + Flow>(this: &mut T) -> &mut BaseFlow {
+pub fn mut_base<T: ?Sized + HasBaseFlow>(this: &mut T) -> &mut BaseFlow {
     let ptr: *mut T = this;
     let ptr = ptr as *mut BaseFlow;
     unsafe { &mut *ptr }

components/layout/inline.rs

@@ -861,8 +861,12 @@ impl InlineFragments {
     }
 }
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for InlineFlow {}
+
 /// Flows for inline layout.
 #[derive(Serialize)]
+#[repr(C)]
 pub struct InlineFlow {
     /// Data common to all flows.
     pub base: BaseFlow,

components/layout/list_item.rs

@@ -24,8 +24,12 @@ use style::logical_geometry::LogicalSize;
 use style::properties::ComputedValues;
 use style::servo::restyle_damage::RESOLVE_GENERATED_CONTENT;
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for ListItemFlow {}
+
 /// A block with the CSS `display` property equal to `list-item`.
 #[derive(Debug)]
+#[repr(C)]
 pub struct ListItemFlow {
     /// Data common to all block flows.
     pub block_flow: BlockFlow,

components/layout/multicol.rs

@@ -24,6 +24,10 @@ use style::properties::ComputedValues;
 use style::values::Either;
 use style::values::computed::{LengthOrPercentageOrAuto, LengthOrPercentageOrNone};
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for MulticolFlow {}
+
+#[repr(C)]
 pub struct MulticolFlow {
     pub block_flow: BlockFlow,
 
@@ -32,6 +36,10 @@ pub struct MulticolFlow {
     pub column_pitch: Au,
 }
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for MulticolColumnFlow {}
+
+#[repr(C)]
 pub struct MulticolColumnFlow {
     pub block_flow: BlockFlow,
 }

components/layout/table.rs

@@ -34,10 +34,14 @@ use table_row::{self, CellIntrinsicInlineSize, CollapsedBorder, CollapsedBorderP
 use table_row::TableRowFlow;
 use table_wrapper::TableLayout;
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableFlow {}
+
 /// A table flow corresponded to the table's internal table fragment under a table wrapper flow.
 /// The properties `position`, `float`, and `margin-*` are used on the table wrapper fragment,
 /// not table fragment per CSS 2.1 § 10.5.
 #[derive(Serialize)]
+#[repr(C)]
 pub struct TableFlow {
     pub block_flow: BlockFlow,
 

components/layout/table_caption.rs

@@ -19,7 +19,11 @@ use std::fmt;
 use style::logical_geometry::LogicalSize;
 use style::properties::ComputedValues;
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableCaptionFlow {}
+
 /// A table formatting context.
+#[repr(C)]
 pub struct TableCaptionFlow {
     pub block_flow: BlockFlow,
 }

components/layout/table_cell.rs

@@ -28,8 +28,12 @@ use style::values::generics::box_::VerticalAlign;
 use table::InternalTable;
 use table_row::{CollapsedBorder, CollapsedBorderProvenance};
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableCellFlow {}
+
 /// A table formatting context.
 #[derive(Serialize)]
+#[repr(C)]
 pub struct TableCellFlow {
     /// Data common to all block flows.
     pub block_flow: BlockFlow,

components/layout/table_colgroup.rs

@@ -19,7 +19,11 @@ use style::logical_geometry::LogicalSize;
 use style::properties::ComputedValues;
 use style::values::computed::LengthOrPercentageOrAuto;
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableColGroupFlow {}
+
 /// A table formatting context.
+#[repr(C)]
 pub struct TableColGroupFlow {
     /// Data common to all flows.
     pub base: BaseFlow,

components/layout/table_row.rs

@@ -31,7 +31,11 @@ use style::values::computed::{Color, LengthOrPercentageOrAuto};
 use table::{ColumnComputedInlineSize, ColumnIntrinsicInlineSize, InternalTable, VecExt};
 use table_cell::{CollapsedBordersForCell, TableCellFlow};
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableRowFlow {}
+
 /// A single row of a table.
+#[repr(C)]
 pub struct TableRowFlow {
     /// Fields common to all block flows.
     pub block_flow: BlockFlow,

components/layout/table_rowgroup.rs

@@ -24,7 +24,11 @@ use style::logical_geometry::LogicalSize;
 use style::properties::ComputedValues;
 use table::{ColumnIntrinsicInlineSize, InternalTable, TableLikeFlow};
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableRowGroupFlow {}
+
 /// A table formatting context.
+#[repr(C)]
 pub struct TableRowGroupFlow {
     /// Fields common to all block flows.
     pub block_flow: BlockFlow,

components/layout/table_wrapper.rs

@@ -43,8 +43,12 @@ pub enum TableLayout {
     Auto
 }
 
+#[allow(unsafe_code)]
+unsafe impl ::flow::HasBaseFlow for TableWrapperFlow {}
+
 /// A table wrapper flow based on a block formatting context.
 #[derive(Serialize)]
+#[repr(C)]
 pub struct TableWrapperFlow {
     pub block_flow: BlockFlow,