Move non-gfx things out of `gfx_traits` and create a `base` crate (#32296)

For a long time, `gfx_traits` has held a lot of things unrelated to graphics
and also unrelated to the `gfx` crate (which is mostly about fonts).
This is a cleanup which does a few things:

1. Move non `gfx` crate things out of `gfx_traits`. This is important in
   order to prevent dependency cycles with a different integration between
   layout, script, and fonts.
2. Rename the `msg` crate to `base`. It didn't really contain anything
   to do with messages and instead mostly holds ids, which are used
   across many different crates in Servo. This new crate will hold the
   *rare* data types that are widely used.

Details:

 - All BackgroundHangMonitor-related things from base to a new
   `background_hang_monitor_api` crate.
 - Moved `TraversalDirection` to `script_traits`
 - Moved `Epoch`-related things from `gfx_traits` to `base`.
 - Moved `PrintTree` to base. This should be widely useful in Servo.
 - Moved `WebrenderApi` from `base` to `webrender_traits` and renamed it
   to `WebRenderFontApi`.

17 files changed, 51 insertions, 24 deletions

diff --git a/components/layout/Cargo.toml b/components/layout/Cargo.toml
index 6df0d11c36f..f63002b5fe3 100644
--- a/components/layout/Cargo.toml
+++ b/components/layout/Cargo.toml
@@ -15,6 +15,7 @@ doctest = false
 [dependencies]
 app_units = { workspace = true }
 atomic_refcell = { workspace = true }
+base = { workspace = true }
 bitflags = { workspace = true }
 canvas_traits = { workspace = true }
 embedder_traits = { workspace = true }
@@ -27,7 +28,7 @@ ipc-channel = { workspace = true }
 lazy_static = { workspace = true }
 log = { workspace = true }
 malloc_size_of = { workspace = true }
-msg = { workspace = true }
+malloc_size_of_derive = { workspace = true }
 net_traits = { workspace = true }
 parking_lot = { workspace = true }
 profile_traits = { workspace = true }
diff --git a/components/layout/block.rs b/components/layout/block.rs
index 1aa32668e89..4ecdc44d2d1 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -30,9 +30,9 @@ use std::fmt;
 use std::sync::Arc;
 
 use app_units::{Au, MAX_AU};
+use base::print_tree::PrintTree;
 use bitflags::bitflags;
 use euclid::default::{Point2D, Rect, SideOffsets2D, Size2D};
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use serde::{Serialize, Serializer};
 use servo_geometry::MaxRect;
diff --git a/components/layout/context.rs b/components/layout/context.rs
index 5cedf5d38aa..5767717e371 100644
--- a/components/layout/context.rs
+++ b/components/layout/context.rs
@@ -9,10 +9,10 @@ use std::hash::BuildHasherDefault;
 use std::sync::{Arc, Mutex};
 use std::thread;
 
+use base::id::PipelineId;
 use fnv::FnvHasher;
 use gfx::font_cache_thread::FontCacheThread;
 use gfx::font_context::FontContext;
-use msg::constellation_msg::PipelineId;
 use net_traits::image_cache::{
     ImageCache, ImageCacheResult, ImageOrMetadataAvailable, UsePlaceholder,
 };
diff --git a/components/layout/display_list/builder.rs b/components/layout/display_list/builder.rs
index 335a5a66823..0fba6875d2f 100644
--- a/components/layout/display_list/builder.rs
+++ b/components/layout/display_list/builder.rs
@@ -13,6 +13,7 @@ use std::sync::Arc;
 use std::{f32, mem};
 
 use app_units::{Au, AU_PER_PX};
+use base::id::{BrowsingContextId, PipelineId};
 use bitflags::bitflags;
 use canvas_traits::canvas::{CanvasMsg, FromLayoutMsg};
 use embedder_traits::Cursor;
@@ -21,12 +22,11 @@ use euclid::{rect, SideOffsets2D};
 use fnv::FnvHashMap;
 use gfx::text::glyph::ByteIndex;
 use gfx::text::TextRun;
-use gfx_traits::{combine_id_with_fragment_type, FragmentType, StackingContextId};
 use ipc_channel::ipc;
 use log::{debug, warn};
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image_cache::UsePlaceholder;
 use range::Range;
+use script_layout_interface::{combine_id_with_fragment_type, FragmentType};
 use script_traits::compositor::ScrollSensitivity;
 use servo_config::opts;
 use servo_geometry::{self, MaxRect};
@@ -53,6 +53,7 @@ use webrender_api::{
     NinePatchBorderSource, NormalBorder, PropertyBinding, StickyOffsetBounds,
 };
 
+use super::StackingContextId;
 use crate::block::BlockFlow;
 use crate::context::LayoutContext;
 use crate::display_list::background::{self, get_cyclic};
diff --git a/components/layout/display_list/items.rs b/components/layout/display_list/items.rs
index 193a9f6c461..6fe14f03143 100644
--- a/components/layout/display_list/items.rs
+++ b/components/layout/display_list/items.rs
@@ -16,11 +16,10 @@ use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::{f32, fmt};
 
+use base::id::PipelineId;
+use base::print_tree::PrintTree;
 use embedder_traits::Cursor;
 use euclid::{SideOffsets2D, Vector2D};
-use gfx_traits::print_tree::PrintTree;
-use gfx_traits::{self, StackingContextId};
-use msg::constellation_msg::PipelineId;
 use net_traits::image::base::Image;
 use script_traits::compositor::{ScrollSensitivity, ScrollTreeNodeId};
 use serde::Serialize;
@@ -35,6 +34,8 @@ use webrender_api::{
     SpatialId, StickyOffsetBounds, TransformStyle,
 };
 
+use super::StackingContextId;
+
 /// The factor that we multiply the blur radius by in order to inflate the boundaries of display
 /// items that involve a blur. This ensures that the display item boundaries include all the ink.
 pub static BLUR_INFLATION_FACTOR: i32 = 3;
diff --git a/components/layout/display_list/mod.rs b/components/layout/display_list/mod.rs
index aa6d3d2f500..ccd7763144f 100644
--- a/components/layout/display_list/mod.rs
+++ b/components/layout/display_list/mod.rs
@@ -2,6 +2,9 @@
  * 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/. */
 
+use malloc_size_of_derive::MallocSizeOf;
+use serde::{Deserialize, Serialize};
+
 pub use self::builder::{
     BorderPaintingMode, DisplayListBuildState, IndexableText, StackingContextCollectionFlags,
     StackingContextCollectionState,
@@ -15,3 +18,24 @@ pub(crate) mod conversions;
 mod gradient;
 pub mod items;
 mod webrender_helpers;
+
+/// A unique ID for every stacking context.
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]
+pub struct StackingContextId(
+    /// The identifier for this StackingContext, derived from the Flow's memory address
+    /// and fragment type.  As a space optimization, these are combined into a single word.
+    pub u64,
+);
+
+impl StackingContextId {
+    /// Returns the stacking context ID for the outer document/layout root.
+    #[inline]
+    pub fn root() -> StackingContextId {
+        StackingContextId(0)
+    }
+
+    pub fn next(&self) -> StackingContextId {
+        let StackingContextId(id) = *self;
+        StackingContextId(id + 1)
+    }
+}
diff --git a/components/layout/display_list/webrender_helpers.rs b/components/layout/display_list/webrender_helpers.rs
index 85c3694b98d..0ac79b4d8ca 100644
--- a/components/layout/display_list/webrender_helpers.rs
+++ b/components/layout/display_list/webrender_helpers.rs
@@ -7,9 +7,9 @@
 //           This might be achieved by sharing types between WR and Servo display lists, or
 //           completely converting layout to directly generate WebRender display lists, for example.
 
-use gfx_traits::WebRenderEpochToU16;
+use base::id::PipelineId;
+use base::WebRenderEpochToU16;
 use log::trace;
-use msg::constellation_msg::PipelineId;
 use script_traits::compositor::{
     CompositorDisplayListInfo, ScrollSensitivity, ScrollTreeNodeId, ScrollableNodeInfo,
 };
diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index 29152507c12..8cdaa6d521d 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -31,10 +31,9 @@ use std::sync::atomic::Ordering;
 use std::sync::Arc;
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use bitflags::bitflags;
 use euclid::default::{Point2D, Rect, Size2D, Vector2D};
-use gfx_traits::print_tree::PrintTree;
-use gfx_traits::StackingContextId;
 use log::debug;
 use serde::ser::{SerializeStruct, Serializer};
 use serde::Serialize;
@@ -54,7 +53,9 @@ use webrender_api::units::LayoutTransform;
 use crate::block::{BlockFlow, FormattingContextType};
 use crate::context::LayoutContext;
 use crate::display_list::items::ClippingAndScrolling;
-use crate::display_list::{DisplayListBuildState, StackingContextCollectionState};
+use crate::display_list::{
+    DisplayListBuildState, StackingContextCollectionState, StackingContextId,
+};
 use crate::flex::FlexFlow;
 use crate::floats::{Floats, SpeculatedFloatPlacement};
 use crate::flow_list::{FlowList, FlowListIterator, MutFlowListIterator};
diff --git a/components/layout/fragment.rs b/components/layout/fragment.rs
index 45843627493..171c529babc 100644
--- a/components/layout/fragment.rs
+++ b/components/layout/fragment.rs
@@ -11,16 +11,15 @@ use std::sync::{Arc, Mutex};
 use std::{f32, fmt};
 
 use app_units::Au;
+use base::id::{BrowsingContextId, PipelineId};
 use bitflags::bitflags;
 use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use euclid::default::{Point2D, Rect, Size2D, Vector2D};
 use gfx::text::glyph::ByteIndex;
 use gfx::text::text_run::{TextRun, TextRunSlice};
-use gfx_traits::StackingContextId;
 use html5ever::{local_name, namespace_url, ns};
 use ipc_channel::ipc::IpcSender;
 use log::debug;
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image::base::{Image, ImageMetadata};
 use net_traits::image_cache::{ImageOrMetadataAvailable, UsePlaceholder};
 use range::*;
@@ -59,7 +58,7 @@ use webrender_api::{self, ImageKey};
 
 use crate::context::LayoutContext;
 use crate::display_list::items::{ClipScrollNodeIndex, OpaqueNode, BLUR_INFLATION_FACTOR};
-use crate::display_list::ToLayout;
+use crate::display_list::{StackingContextId, ToLayout};
 use crate::floats::ClearType;
 use crate::flow::{GetBaseFlow, ImmutableFlowUtils};
 use crate::flow_ref::FlowRef;
diff --git a/components/layout/inline.rs b/components/layout/inline.rs
index 5926e4b2e03..fa56f83979f 100644
--- a/components/layout/inline.rs
+++ b/components/layout/inline.rs
@@ -8,10 +8,10 @@ use std::sync::Arc;
 use std::{fmt, i32, isize, mem};
 
 use app_units::{Au, MIN_AU};
+use base::print_tree::PrintTree;
 use bitflags::bitflags;
 use euclid::default::{Point2D, Rect, Size2D};
 use gfx::font::FontMetrics;
-use gfx_traits::print_tree::PrintTree;
 use log::debug;
 use range::{int_range_index, Range, RangeIndex};
 use script_layout_interface::wrapper_traits::PseudoElementType;
diff --git a/components/layout/multicol.rs b/components/layout/multicol.rs
index 9e12a991772..ed82b41fe1c 100644
--- a/components/layout/multicol.rs
+++ b/components/layout/multicol.rs
@@ -9,8 +9,8 @@ use std::fmt;
 use std::sync::Arc;
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::{Point2D, Vector2D};
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use style::logical_geometry::LogicalSize;
 use style::properties::ComputedValues;
diff --git a/components/layout/table.rs b/components/layout/table.rs
index 1821769bc79..55f19c05867 100644
--- a/components/layout/table.rs
+++ b/components/layout/table.rs
@@ -7,8 +7,8 @@
 use std::{cmp, fmt};
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::Point2D;
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use serde::Serialize;
 use style::computed_values::{border_collapse, border_spacing, table_layout};
diff --git a/components/layout/table_caption.rs b/components/layout/table_caption.rs
index 398a43a7dbf..719d9a2309d 100644
--- a/components/layout/table_caption.rs
+++ b/components/layout/table_caption.rs
@@ -7,8 +7,8 @@
 use std::fmt;
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::Point2D;
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use style::logical_geometry::LogicalSize;
 use style::properties::ComputedValues;
diff --git a/components/layout/table_cell.rs b/components/layout/table_cell.rs
index 013702e9401..a70ea183931 100644
--- a/components/layout/table_cell.rs
+++ b/components/layout/table_cell.rs
@@ -7,8 +7,8 @@
 use std::fmt;
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::{Point2D, Rect, SideOffsets2D, Size2D};
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use script_layout_interface::wrapper_traits::ThreadSafeLayoutNode;
 use serde::Serialize;
diff --git a/components/layout/table_row.rs b/components/layout/table_row.rs
index 764386d9033..21aee72bba5 100644
--- a/components/layout/table_row.rs
+++ b/components/layout/table_row.rs
@@ -9,8 +9,8 @@ use std::fmt;
 use std::iter::{Enumerate, Peekable};
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::Point2D;
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, warn};
 use serde::{Serialize, Serializer};
 use style::computed_values::border_collapse::T as BorderCollapse;
diff --git a/components/layout/table_rowgroup.rs b/components/layout/table_rowgroup.rs
index 345bba4a588..4a21856b9f8 100644
--- a/components/layout/table_rowgroup.rs
+++ b/components/layout/table_rowgroup.rs
@@ -8,8 +8,8 @@ use std::fmt;
 use std::iter::{Iterator, Peekable};
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::Point2D;
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use serde::{Serialize, Serializer};
 use style::computed_values::{border_collapse, border_spacing};
diff --git a/components/layout/table_wrapper.rs b/components/layout/table_wrapper.rs
index 25832386566..3d5bc557561 100644
--- a/components/layout/table_wrapper.rs
+++ b/components/layout/table_wrapper.rs
@@ -16,8 +16,8 @@ use std::fmt;
 use std::ops::Add;
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::Point2D;
-use gfx_traits::print_tree::PrintTree;
 use log::{debug, trace};
 use serde::Serialize;
 use style::computed_values::{position, table_layout};