Disable use of rayon with --layout-threads 1 instead of panicking

components/layout_2020/context.rs

@@ -11,6 +11,7 @@ use style::context::SharedStyleContext;
 
 pub struct LayoutContext<'a> {
     pub id: PipelineId,
+    pub use_rayon: bool,
     pub style_context: SharedStyleContext<'a>,
     pub font_cache_thread: Mutex<FontCacheThread>,
 }

components/layout_2020/flow/construct.rs

@@ -168,12 +168,11 @@ impl BlockContainer {
             builder.end_ongoing_inline_formatting_context();
         }
 
-        type Intermediate<Node> = IntermediateBlockLevelBox<Node>;
-        struct Target {
+        struct Accumulator {
             contains_floats: ContainsFloats,
             outer_content_sizes_of_children: ContentSizes,
         }
-        impl Default for Target {
+        impl Default for Accumulator {
             fn default() -> Self {
                 Self {
                     contains_floats: ContainsFloats::No,
@@ -181,37 +180,46 @@ impl BlockContainer {
                 }
             }
         }
-        let mut target = Target {
+        let mut acc = Accumulator {
             contains_floats: builder.contains_floats,
             outer_content_sizes_of_children: ContentSizes::zero(),
         };
-        let iter = builder.block_level_boxes.into_par_iter();
-        let iter = iter.mapfold_reduce_into(
-            &mut target,
-            |target, (intermediate, box_slot): (Intermediate<_>, BoxSlot<'_>)| {
+        let mapfold =
+            |acc: &mut Accumulator,
+             (intermediate, box_slot): (IntermediateBlockLevelBox<_>, BoxSlot<'_>)| {
                 let (block_level_box, box_contains_floats) = intermediate.finish(
                     context,
-                    content_sizes
-                        .if_requests_inline(|| &mut target.outer_content_sizes_of_children),
+                    content_sizes.if_requests_inline(|| &mut acc.outer_content_sizes_of_children),
                 );
-                target.contains_floats |= box_contains_floats;
+                acc.contains_floats |= box_contains_floats;
                 box_slot.set(LayoutBox::BlockLevel(block_level_box.clone()));
                 block_level_box
-            },
-            |left, right| {
+            };
+        let block_level_boxes = if context.use_rayon {
+            builder
+                .block_level_boxes
+                .into_par_iter()
+                .mapfold_reduce_into(&mut acc, mapfold, |left, right| {
                     left.contains_floats |= right.contains_floats;
                     if content_sizes.requests_inline() {
                         left.outer_content_sizes_of_children
                             .max_assign(&right.outer_content_sizes_of_children)
                     }
-            },
-        );
-        let container = BlockContainer::BlockLevelBoxes(iter.collect());
+                })
+                .collect()
+        } else {
+            builder
+                .block_level_boxes
+                .into_iter()
+                .map(|x| mapfold(&mut acc, x))
+                .collect()
+        };
+        let container = BlockContainer::BlockLevelBoxes(block_level_boxes);
 
-        let Target {
+        let Accumulator {
             contains_floats,
             outer_content_sizes_of_children,
-        } = target;
+        } = acc;
         let content_sizes = content_sizes.compute(|| outer_content_sizes_of_children);
         (container, contains_floats, content_sizes)
     }

components/layout_2020/flow/mod.rs

@@ -135,7 +135,7 @@ fn layout_block_level_children<'a>(
     containing_block: &ContainingBlock,
     tree_rank: usize,
     absolutely_positioned_fragments: &mut Vec<AbsolutelyPositionedFragment<'a>>,
-    float_context: Option<&mut FloatContext>,
+    mut float_context: Option<&mut FloatContext>,
     collapsible_with_parent_start_margin: CollapsibleWithParentStartMargin,
 ) -> FlowLayout {
     fn place_block_level_fragment(fragment: &mut Fragment, placement_state: &mut PlacementState) {
@@ -203,7 +203,7 @@ fn layout_block_level_children<'a>(
         current_block_direction_position: Length::zero(),
     };
     let mut fragments: Vec<_>;
-    if let Some(float_context) = float_context {
+    if float_context.is_some() || !layout_context.use_rayon {
         // Because floats are involved, we do layout for this block formatting context
         // in tree order without parallelism. This enables mutable access
         // to a `FloatContext` that tracks every float encountered so far (again in tree order).
@@ -216,7 +216,7 @@ fn layout_block_level_children<'a>(
                     containing_block,
                     tree_rank,
                     absolutely_positioned_fragments,
-                    Some(float_context),
+                    float_context.as_mut().map(|c| &mut **c),
                 );
                 place_block_level_fragment(&mut fragment, &mut placement_state);
                 fragment

components/layout_2020/flow/root.rs

@@ -12,7 +12,7 @@ use crate::formatting_contexts::IndependentFormattingContext;
 use crate::fragments::Fragment;
 use crate::geom;
 use crate::geom::flow_relative::Vec2;
-use crate::positioned::AbsolutelyPositionedBox;
+use crate::positioned::{AbsolutelyPositionedBox, AbsolutelyPositionedFragment};
 use crate::replaced::ReplacedContent;
 use crate::sizing::ContentSizesRequest;
 use crate::style_ext::{Display, DisplayGeneratingBox, DisplayInside};
@@ -118,11 +118,18 @@ impl BoxTreeRoot {
             &mut absolutely_positioned_fragments,
         );
 
-        independent_layout.fragments.par_extend(
-            absolutely_positioned_fragments
-                .par_iter()
-                .map(|a| a.layout(layout_context, &initial_containing_block)),
-        );
+        let map =
+            |a: &AbsolutelyPositionedFragment| a.layout(layout_context, &initial_containing_block);
+        if layout_context.use_rayon {
+            independent_layout
+                .fragments
+                .par_extend(absolutely_positioned_fragments.par_iter().map(map))
+        } else {
+            independent_layout
+                .fragments
+                .extend(absolutely_positioned_fragments.iter().map(map))
+        }
+
         FragmentTreeRoot(independent_layout.fragments)
     }
 }

components/layout_2020/positioned.rs

@@ -139,11 +139,14 @@ impl<'a> AbsolutelyPositionedFragment<'a> {
             size: padding_rect.size.clone(),
             style,
         };
+        let map = |a: &AbsolutelyPositionedFragment| a.layout(layout_context, &containing_block);
+        let children = if layout_context.use_rayon {
+            absolute.par_iter().map(map).collect()
+        } else {
+            absolute.iter().map(map).collect()
+        };
         fragments.push(Fragment::Anonymous(AnonymousFragment {
-            children: absolute
-                .par_iter()
-                .map(|a| a.layout(layout_context, &containing_block))
-                .collect(),
+            children,
             rect: padding_rect,
             mode: style.writing_mode,
         }))

components/layout_thread_2020/lib.rs

@@ -568,6 +568,7 @@ impl LayoutThread {
                 snapshot_map: snapshot_map,
             },
             font_cache_thread: Mutex::new(self.font_cache_thread.clone()),
+            use_rayon: STYLE_THREAD_POOL.pool().is_some(),
         }
     }