Do layout short-circuit in flow instead of realization (#4231)

crates/typst/src/layout/flow.rs

@@ -46,7 +46,23 @@ impl LayoutMultiple for Packed<FlowElem> {
             bail!(self.span(), "cannot expand into infinite height");
         }
 
-        let mut layouter = FlowLayouter::new(regions, styles);
+        // Check whether we have just a single multiple-layoutable element. In
+        // that case, we do not set `expand.y` to `false`, but rather keep it at
+        // its original value (since that element can take the full space).
+        //
+        // Consider the following code: `block(height: 5cm, pad(10pt, align(bottom, ..)))`
+        // Thanks to the code below, the expansion will be passed all the way
+        // through the block & pad and reach the innermost flow, so that things
+        // are properly bottom-aligned.
+        let mut alone = false;
+        if let [child] = self.children().as_slice() {
+            alone = child
+                .to_packed::<StyledElem>()
+                .map_or(child, |styled| &styled.child)
+                .can::<dyn LayoutMultiple>();
+        }
+
+        let mut layouter = FlowLayouter::new(regions, styles, alone);
         for mut child in self.children().iter() {
             let outer = styles;
             let mut styles = styles;
@@ -70,8 +86,8 @@ impl LayoutMultiple for Packed<FlowElem> {
                 layouter.layout_par(engine, elem, styles)?;
             } else if let Some(layoutable) = child.with::<dyn LayoutSingle>() {
                 layouter.layout_single(engine, layoutable, styles)?;
-            } else if child.can::<dyn LayoutMultiple>() {
-                layouter.layout_multiple(engine, child, styles)?;
+            } else if let Some(layoutable) = child.with::<dyn LayoutMultiple>() {
+                layouter.layout_multiple(engine, child, layoutable, styles)?;
             } else {
                 bail!(child.span(), "unexpected flow child");
             }
@@ -179,11 +195,16 @@ impl FlowItem {
 
 impl<'a> FlowLayouter<'a> {
     /// Create a new flow layouter.
-    fn new(mut regions: Regions<'a>, styles: StyleChain<'a>) -> Self {
+    fn new(mut regions: Regions<'a>, styles: StyleChain<'a>, alone: bool) -> Self {
         let expand = regions.expand;
 
-        // Disable vertical expansion & root for children.
-        regions.expand.y = false;
+        // Disable vertical expansion when there are multiple or not directly
+        // layoutable children.
+        if !alone {
+            regions.expand.y = false;
+        }
+
+        // Disable root.
         let root = std::mem::replace(&mut regions.root, false);
 
         Self {
@@ -340,6 +361,7 @@ impl<'a> FlowLayouter<'a> {
         &mut self,
         engine: &mut Engine,
         child: &Content,
+        layoutable: &dyn LayoutMultiple,
         styles: StyleChain,
     ) -> SourceResult<()> {
         // Temporarily delegerate rootness to the columns.
@@ -368,7 +390,7 @@ impl<'a> FlowLayouter<'a> {
 
         // Layout the block itself.
         let sticky = BlockElem::sticky_in(styles);
-        let fragment = child.layout(engine, styles, self.regions)?;
+        let fragment = layoutable.layout(engine, styles, self.regions)?;
 
         for (i, mut frame) in fragment.into_iter().enumerate() {
             // Find footnotes in the frame.

crates/typst/src/layout/mod.rs

@@ -77,7 +77,7 @@ use crate::eval::Tracer;
 use crate::foundations::{category, Category, Content, Scope, StyleChain};
 use crate::introspection::{Introspector, Locator};
 use crate::model::Document;
-use crate::realize::{realize_block, realize_root, Arenas};
+use crate::realize::{realize_doc, realize_flow, Arenas};
 use crate::World;
 
 /// Arranging elements on the page in different ways.
@@ -207,7 +207,7 @@ impl LayoutRoot for Content {
                 tracer,
             };
             let arenas = Arenas::default();
-            let (document, styles) = realize_root(&mut engine, &arenas, content, styles)?;
+            let (document, styles) = realize_doc(&mut engine, &arenas, content, styles)?;
             document.layout_root(&mut engine, styles)
         }
 
@@ -258,14 +258,16 @@ impl LayoutMultiple for Content {
                 );
             }
 
+            // If we are in a `PageElem`, this might already be a realized flow.
+            if let Some(flow) = content.to_packed::<FlowElem>() {
+                return flow.layout(&mut engine, styles, regions);
+            }
+
+            // Layout the content by first turning it into a `FlowElem` and then
+            // layouting that.
             let arenas = Arenas::default();
-            let (realized, styles) =
-                realize_block(&mut engine, &arenas, content, styles)?;
-            realized.with::<dyn LayoutMultiple>().unwrap().layout(
-                &mut engine,
-                styles,
-                regions,
-            )
+            let (flow, styles) = realize_flow(&mut engine, &arenas, content, styles)?;
+            flow.layout(&mut engine, styles, regions)
         }
 
         let fragment = cached(

crates/typst/src/realize/mod.rs

@@ -12,9 +12,7 @@ mod process;
 
 pub use self::arenas::Arenas;
 pub use self::behaviour::{Behave, BehavedBuilder, Behaviour};
-pub use self::process::{process, processable};
-
-use std::borrow::Cow;
+pub use self::process::process;
 
 use std::mem;
 
@@ -36,9 +34,10 @@ use crate::model::{
 use crate::syntax::Span;
 use crate::text::{LinebreakElem, SmartQuoteElem, SpaceElem, TextElem};
 
-/// Realize into an element that is capable of root-level layout.
-#[typst_macros::time(name = "realize root")]
-pub fn realize_root<'a>(
+/// Realize into a `DocumentElem`, an element that is capable of root-level
+/// layout.
+#[typst_macros::time(name = "realize doc")]
+pub fn realize_doc<'a>(
     engine: &mut Engine,
     arenas: &'a Arenas<'a>,
     content: &'a Content,
@@ -47,30 +46,21 @@ pub fn realize_root<'a>(
     let mut builder = Builder::new(engine, arenas, true);
     builder.accept(content, styles)?;
     builder.interrupt_page(Some(styles), true)?;
-    let (doc, trunk) = builder.doc.unwrap().finish();
-    Ok((doc, trunk))
+    Ok(builder.doc.unwrap().finish())
 }
 
-/// Realize into an element that is capable of block-level layout.
-#[typst_macros::time(name = "realize block")]
-pub fn realize_block<'a>(
+/// Realize into a `FlowElem`, an element that is capable of block-level layout.
+#[typst_macros::time(name = "realize flow")]
+pub fn realize_flow<'a>(
     engine: &mut Engine,
     arenas: &'a Arenas<'a>,
     content: &'a Content,
     styles: StyleChain<'a>,
-) -> SourceResult<(Cow<'a, Content>, StyleChain<'a>)> {
-    // These elements implement `Layout` but still require a flow for
-    // proper layout.
-    if content.can::<dyn LayoutMultiple>() && !processable(engine, content, styles) {
-        return Ok((Cow::Borrowed(content), styles));
-    }
-
+) -> SourceResult<(Packed<FlowElem>, StyleChain<'a>)> {
     let mut builder = Builder::new(engine, arenas, false);
     builder.accept(content, styles)?;
     builder.interrupt_par()?;
-
-    let (flow, trunk) = builder.flow.finish();
-    Ok((Cow::Owned(flow.pack()), trunk))
+    Ok(builder.flow.finish())
 }
 
 /// Builds a document or a flow element from content.

crates/typst/src/realize/process.rs

@@ -31,15 +31,6 @@ enum ShowStep<'a> {
     Builtin,
 }
 
-/// Returns whether the `target` element needs processing.
-pub fn processable<'a>(
-    engine: &mut Engine,
-    target: &'a Content,
-    styles: StyleChain<'a>,
-) -> bool {
-    verdict(engine, target, styles).is_some()
-}
-
 /// Processes the given `target` element when encountering it during realization.
 pub fn process(
     engine: &mut Engine,