layout: Combine layout_2020 and layout_thread_2020 into a crate called layout (#36613)

Now that legacy layout has been removed, the name `layout_2020` doesn't
make much sense any longer, also it's 2025 now for better or worse. The
split between the "layout thread" and "layout" also doesn't make as much
sense since layout doesn't run on it's own thread. There's a possibility
that it will in the future, but that should be something that the user
of the crate controls rather than layout iself.

This is part of the larger layout interface cleanup and optimization
that
@Looriool and I are doing.

Testing: Covered by existing tests as this is just code movement.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/taffy/mod.rs

@@ -0,0 +1,184 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+mod layout;
+mod stylo_taffy;
+use std::fmt;
+
+use app_units::Au;
+use malloc_size_of_derive::MallocSizeOf;
+use servo_arc::Arc;
+use style::properties::ComputedValues;
+use stylo_taffy::TaffyStyloStyle;
+
+use crate::PropagatedBoxTreeData;
+use crate::cell::ArcRefCell;
+use crate::construct_modern::{ModernContainerBuilder, ModernItemKind};
+use crate::context::LayoutContext;
+use crate::dom::{LayoutBox, NodeExt};
+use crate::dom_traversal::{NodeAndStyleInfo, NonReplacedContents};
+use crate::formatting_contexts::IndependentFormattingContext;
+use crate::fragment_tree::Fragment;
+use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
+
+#[derive(Debug, MallocSizeOf)]
+pub(crate) struct TaffyContainer {
+    children: Vec<ArcRefCell<TaffyItemBox>>,
+    #[conditional_malloc_size_of]
+    style: Arc<ComputedValues>,
+}
+
+impl TaffyContainer {
+    pub fn construct<'dom>(
+        context: &LayoutContext,
+        info: &NodeAndStyleInfo<impl NodeExt<'dom>>,
+        contents: NonReplacedContents,
+        propagated_data: PropagatedBoxTreeData,
+    ) -> Self {
+        let mut builder =
+            ModernContainerBuilder::new(context, info, propagated_data.union(&info.style));
+        contents.traverse(context, info, &mut builder);
+        let items = builder.finish();
+
+        let children = items
+            .into_iter()
+            .map(|item| {
+                let box_ = match item.kind {
+                    ModernItemKind::InFlow => ArcRefCell::new(TaffyItemBox::new(
+                        TaffyItemBoxInner::InFlowBox(item.formatting_context),
+                    )),
+                    ModernItemKind::OutOfFlow => {
+                        let abs_pos_box =
+                            ArcRefCell::new(AbsolutelyPositionedBox::new(item.formatting_context));
+                        ArcRefCell::new(TaffyItemBox::new(
+                            TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(abs_pos_box),
+                        ))
+                    },
+                };
+
+                if let Some(box_slot) = item.box_slot {
+                    box_slot.set(LayoutBox::TaffyItemBox(box_.clone()));
+                }
+
+                box_
+            })
+            .collect();
+
+        Self {
+            children,
+            style: info.style.clone(),
+        }
+    }
+}
+
+#[derive(MallocSizeOf)]
+pub(crate) struct TaffyItemBox {
+    pub(crate) taffy_layout: taffy::Layout,
+    pub(crate) child_fragments: Vec<Fragment>,
+    pub(crate) positioning_context: PositioningContext,
+    #[conditional_malloc_size_of]
+    pub(crate) style: Arc<ComputedValues>,
+    pub(crate) taffy_level_box: TaffyItemBoxInner,
+}
+
+#[derive(Debug, MallocSizeOf)]
+pub(crate) enum TaffyItemBoxInner {
+    InFlowBox(IndependentFormattingContext),
+    OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
+}
+
+impl fmt::Debug for TaffyItemBox {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TaffyItemBox")
+            .field("taffy_layout", &self.taffy_layout)
+            .field("child_fragments", &self.child_fragments.len())
+            .field("style", &self.style)
+            .field("taffy_level_box", &self.taffy_level_box)
+            .finish()
+    }
+}
+
+impl TaffyItemBox {
+    fn new(inner: TaffyItemBoxInner) -> Self {
+        let style: Arc<ComputedValues> = match &inner {
+            TaffyItemBoxInner::InFlowBox(item) => item.style().clone(),
+            TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(absbox) => {
+                (*absbox).borrow().context.style().clone()
+            },
+        };
+
+        Self {
+            taffy_layout: Default::default(),
+            child_fragments: Vec::new(),
+            positioning_context: PositioningContext::new_for_containing_block_for_all_descendants(),
+            style,
+            taffy_level_box: inner,
+        }
+    }
+
+    pub(crate) fn invalidate_cached_fragment(&mut self) {
+        self.taffy_layout = Default::default();
+        self.positioning_context =
+            PositioningContext::new_for_containing_block_for_all_descendants();
+        match self.taffy_level_box {
+            TaffyItemBoxInner::InFlowBox(ref independent_formatting_context) => {
+                independent_formatting_context
+                    .base
+                    .invalidate_cached_fragment()
+            },
+            TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(ref positioned_box) => {
+                positioned_box
+                    .borrow()
+                    .context
+                    .base
+                    .invalidate_cached_fragment()
+            },
+        }
+    }
+
+    pub(crate) fn fragments(&self) -> Vec<Fragment> {
+        match self.taffy_level_box {
+            TaffyItemBoxInner::InFlowBox(ref independent_formatting_context) => {
+                independent_formatting_context.base.fragments()
+            },
+            TaffyItemBoxInner::OutOfFlowAbsolutelyPositionedBox(ref positioned_box) => {
+                positioned_box.borrow().context.base.fragments()
+            },
+        }
+    }
+}
+
+/// Details from Taffy grid layout that will be stored
+#[derive(Clone, Debug, MallocSizeOf)]
+pub(crate) struct SpecificTaffyGridInfo {
+    pub rows: SpecificTaffyGridTrackInfo,
+    pub columns: SpecificTaffyGridTrackInfo,
+}
+
+impl SpecificTaffyGridInfo {
+    fn from_detailed_grid_layout(grid_info: taffy::DetailedGridInfo) -> Self {
+        Self {
+            rows: SpecificTaffyGridTrackInfo {
+                sizes: grid_info
+                    .rows
+                    .sizes
+                    .iter()
+                    .map(|size| Au::from_f32_px(*size))
+                    .collect(),
+            },
+            columns: SpecificTaffyGridTrackInfo {
+                sizes: grid_info
+                    .columns
+                    .sizes
+                    .iter()
+                    .map(|size| Au::from_f32_px(*size))
+                    .collect(),
+            },
+        }
+    }
+}
+
+#[derive(Clone, Debug, MallocSizeOf)]
+pub(crate) struct SpecificTaffyGridTrackInfo {
+    pub sizes: Box<[Au]>,
+}