layout: Simplify `Contents` a little (#32487)

Instead of duplicating some of `NonReplacedContents` in `Contents`,
divide it into either replaced and non-replaced content, since this is
how the layout system processes `Contents` always. In addition, stop
using `TryInto` to match replaced or non-replaced contents, as it is
quite confusing to handle an `Err` as a success case.

4 files changed, 32 insertions, 39 deletions

diff --git a/components/layout_2020/dom_traversal.rs b/components/layout_2020/dom_traversal.rs
index 10ae9ae3978..7d9be02634e 100644
--- a/components/layout_2020/dom_traversal.rs
+++ b/components/layout_2020/dom_traversal.rs
@@ -122,20 +122,19 @@ where
 
 #[derive(Debug)]
 pub(super) enum Contents {
-    /// Refers to a DOM subtree, plus `::before` and `::after` pseudo-elements.
-    OfElement,
-
+    /// Any kind of content that is not replaced, including the contents of pseudo-elements.
+    NonReplaced(NonReplacedContents),
     /// Example: an `<img src=…>` element.
     /// <https://drafts.csswg.org/css2/conform.html#replaced-element>
     Replaced(ReplacedContent),
-
-    /// Content of a `::before` or `::after` pseudo-element that is being generated.
-    /// <https://drafts.csswg.org/css2/generate.html#content>
-    OfPseudoElement(Vec<PseudoElementContentItem>),
 }
 
+#[derive(Debug)]
 pub(super) enum NonReplacedContents {
+    /// Refers to a DOM subtree, plus `::before` and `::after` pseudo-elements.
     OfElement,
+    /// Content of a `::before` or `::after` pseudo-element that is being generated.
+    /// <https://drafts.csswg.org/css2/generate.html#content>
     OfPseudoElement(Vec<PseudoElementContentItem>),
 }
 
@@ -221,7 +220,8 @@ fn traverse_element<'dom, Node>(
             }
         },
         Display::GeneratingBox(display) => {
-            let contents = replaced.map_or(Contents::OfElement, Contents::Replaced);
+            let contents =
+                replaced.map_or(NonReplacedContents::OfElement.into(), Contents::Replaced);
             let display = display.used_value_for_contents(&contents);
             let box_slot = element.element_box_slot();
             let info = NodeAndStyleInfo::new(element, style);
@@ -251,7 +251,7 @@ fn traverse_pseudo_element<'dom, Node>(
             Display::GeneratingBox(display) => {
                 let items = generate_pseudo_element_content(&info.style, element, context);
                 let box_slot = element.pseudo_element_box_slot(which);
-                let contents = Contents::OfPseudoElement(items);
+                let contents = NonReplacedContents::OfPseudoElement(items).into();
                 handler.handle_element(&info, display, contents, box_slot);
             },
         }
@@ -310,30 +310,25 @@ fn traverse_pseudo_element_contents<'dom, Node>(
 impl Contents {
     /// Returns true iff the `try_from` impl below would return `Err(_)`
     pub fn is_replaced(&self) -> bool {
-        match self {
-            Contents::OfElement | Contents::OfPseudoElement(_) => false,
-            Contents::Replaced(_) => true,
-        }
+        matches!(self, Contents::Replaced(_))
     }
 }
 
-impl std::convert::TryFrom<Contents> for NonReplacedContents {
-    type Error = ReplacedContent;
-
-    fn try_from(contents: Contents) -> Result<Self, Self::Error> {
-        match contents {
-            Contents::OfElement => Ok(NonReplacedContents::OfElement),
-            Contents::OfPseudoElement(items) => Ok(NonReplacedContents::OfPseudoElement(items)),
-            Contents::Replaced(replaced) => Err(replaced),
-        }
+impl From<NonReplacedContents> for Contents {
+    fn from(non_replaced_contents: NonReplacedContents) -> Self {
+        Contents::NonReplaced(non_replaced_contents)
     }
 }
 
-impl From<NonReplacedContents> for Contents {
-    fn from(contents: NonReplacedContents) -> Self {
+impl std::convert::TryFrom<Contents> for NonReplacedContents {
+    type Error = &'static str;
+
+    fn try_from(contents: Contents) -> Result<Self, Self::Error> {
         match contents {
-            NonReplacedContents::OfElement => Contents::OfElement,
-            NonReplacedContents::OfPseudoElement(items) => Contents::OfPseudoElement(items),
+            Contents::NonReplaced(non_replaced_contents) => Ok(non_replaced_contents),
+            Contents::Replaced(_) => {
+                Err("Tried to covnert a `Contents::Replaced` into `NonReplacedContent`")
+            },
         }
     }
 }
diff --git a/components/layout_2020/flow/construct.rs b/components/layout_2020/flow/construct.rs
index 5a53bbd12d9..986ae3e46d7 100644
--- a/components/layout_2020/flow/construct.rs
+++ b/components/layout_2020/flow/construct.rs
@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::borrow::Cow;
-use std::convert::{TryFrom, TryInto};
+use std::convert::TryFrom;
 
 use rayon::iter::{IntoParallelIterator, ParallelIterator};
 use servo_arc::Arc;
@@ -401,7 +401,7 @@ where
             DisplayInside::Flow {
                 is_list_item: false,
             },
-            Contents::OfPseudoElement(contents),
+            NonReplacedContents::OfPseudoElement(contents).into(),
             BoxSlot::dummy(),
         );
     }
@@ -494,8 +494,8 @@ where
         }
 
         let propagated_text_decoration_line = self.text_decoration_line;
-        let kind = match contents.try_into() {
-            Ok(contents) => match display_inside {
+        let kind = match contents {
+            Contents::NonReplaced(contents) => match display_inside {
                 DisplayInside::Flow { is_list_item }
                     if !info.style.establishes_block_formatting_context() =>
                 {
@@ -513,7 +513,7 @@ where
                     propagated_text_decoration_line,
                 },
             },
-            Err(contents) => {
+            Contents::Replaced(contents) => {
                 let contents = Contents::Replaced(contents);
                 BlockLevelCreator::Independent {
                     display_inside,
diff --git a/components/layout_2020/flow/root.rs b/components/layout_2020/flow/root.rs
index cea4063592c..8a1865621cb 100644
--- a/components/layout_2020/flow/root.rs
+++ b/components/layout_2020/flow/root.rs
@@ -18,7 +18,7 @@ use webrender_traits::display_list::ScrollSensitivity;
 use crate::cell::ArcRefCell;
 use crate::context::LayoutContext;
 use crate::dom::{LayoutBox, NodeExt};
-use crate::dom_traversal::{iter_child_nodes, Contents, NodeAndStyleInfo};
+use crate::dom_traversal::{iter_child_nodes, Contents, NodeAndStyleInfo, NonReplacedContents};
 use crate::flexbox::FlexLevelBox;
 use crate::flow::float::FloatBox;
 use crate::flow::inline::InlineItem;
@@ -206,7 +206,7 @@ impl BoxTree {
         loop {
             if let Some((primary_style, display_inside, update_point)) = update_point(dirty_node) {
                 let contents = ReplacedContent::for_element(dirty_node, context)
-                    .map_or(Contents::OfElement, Contents::Replaced);
+                    .map_or_else(|| NonReplacedContents::OfElement.into(), Contents::Replaced);
                 let info = NodeAndStyleInfo::new(dirty_node, Arc::clone(&primary_style));
                 let out_of_flow_absolutely_positioned_box = ArcRefCell::new(
                     AbsolutelyPositionedBox::construct(context, &info, display_inside, contents),
@@ -264,7 +264,7 @@ fn construct_for_root_element<'dom>(
     };
 
     let contents = ReplacedContent::for_element(root_element, context)
-        .map_or(Contents::OfElement, Contents::Replaced);
+        .map_or_else(|| NonReplacedContents::OfElement.into(), Contents::Replaced);
     let root_box = if box_style.position.is_absolutely_positioned() {
         BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(ArcRefCell::new(
             AbsolutelyPositionedBox::construct(context, &info, display_inside, contents),
diff --git a/components/layout_2020/formatting_contexts.rs b/components/layout_2020/formatting_contexts.rs
index 1b15baa7eca..f3c51e9579b 100644
--- a/components/layout_2020/formatting_contexts.rs
+++ b/components/layout_2020/formatting_contexts.rs
@@ -2,8 +2,6 @@
  * 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 std::convert::TryInto;
-
 use app_units::Au;
 use serde::Serialize;
 use servo_arc::Arc;
@@ -92,8 +90,8 @@ impl IndependentFormattingContext {
         contents: Contents,
         propagated_text_decoration_line: TextDecorationLine,
     ) -> Self {
-        match contents.try_into() {
-            Ok(non_replaced_contents) => {
+        match contents {
+            Contents::NonReplaced(non_replaced_contents) => {
                 let contents = match display_inside {
                     DisplayInside::Flow { is_list_item } |
                     DisplayInside::FlowRoot { is_list_item } => {
@@ -131,7 +129,7 @@ impl IndependentFormattingContext {
                     contents,
                 })
             },
-            Err(contents) => {
+            Contents::Replaced(contents) => {
                 let mut base_fragment_info: BaseFragmentInfo = node_and_style_info.into();
                 base_fragment_info.flags.insert(FragmentFlags::IS_REPLACED);
                 Self::Replaced(ReplacedFormattingContext {