Custom outline indenting (#1157)

library/src/meta/outline.rs

@@ -5,7 +5,7 @@ use typst::util::option_eq;
 use super::{
     Counter, CounterKey, HeadingElem, LocalName, Numbering, NumberingPattern, Refable,
 };
-use crate::layout::{BoxElem, HElem, HideElem, ParbreakElem, RepeatElem};
+use crate::layout::{BoxElem, HElem, HideElem, ParbreakElem, RepeatElem, Spacing};
 use crate::prelude::*;
 use crate::text::{LinebreakElem, SpaceElem, TextElem};
 
@@ -110,13 +110,54 @@ pub struct OutlineElem {
     /// ```
     pub depth: Option<NonZeroUsize>,
 
-    /// Whether to indent the sub-elements to align the start of their numbering
-    /// with the title of their parents. This will only have an effect if a
-    /// [heading numbering]($func/heading.numbering) is set.
+    /// How to indent the outline's entry lines. This defaults to `{none}`,
+    /// which does not apply any indentation at all upon the outline's entries,
+    /// which will then all be placed at the start of each line.
+    ///
+    /// If this option is set to `{auto}`, each entry (usually headings) will
+    /// be indented enough to align with the last character of its parent's
+    /// numbering. For example, if the parent entry is "3. Top heading" and the
+    /// child entry is "3.1. Inner heading", the end result is that the child
+    /// entry's line will begin where the "3." from the parent ends (after the
+    /// last dot), but below. Consequently, if you specify `{auto}` indentation,
+    /// you will only see a visible effect if a
+    /// [heading numbering]($func/heading.numbering) is set for your headings
+    /// (if using headings), or, in general, if your entries have some form of
+    /// automatic numbering (generated by Typst) enabled.
+    ///
+    /// Note that specifying `{true}` (equivalent to `{auto}`) or `{false}`
+    /// (equivalent to `{none}`) for this option is deprecated and will be
+    /// removed in a future release. Please use `{none}` for no indentation
+    /// or `{auto}` for automatic indentation instead.
+    ///
+    /// Alternatively, you may specify a custom indentation method, which
+    /// doesn't depend on numbering. Setting this option to a length, such as
+    /// `{2em}`, will indent each nested level by that much length, multiplied
+    /// by the current nesting level (so a top-level heading, nested 0 times,
+    /// wouldn't be indented at all; a heading nested once would be `{2em}`
+    /// away from the start of the line, a heading nested twice would be
+    /// `{4em}` away, and so on).
+    ///
+    /// It is also possible to set this option to a function, allowing for a
+    /// more complete customization of the indentation. A function is expected
+    /// to take a single parameter indcating the current nesting level
+    /// (starting at `{0}` for top-level headings/elements), and return the
+    /// indentation option for that level (or `{none}`). Such a function could
+    /// be, for example, {n => n * 2em}` (indenting by `{2em}` times the
+    /// nesting level), or `{n => [*!*] * n * n}` (indenting by a bold
+    /// exclamation mark times the nesting level squared). Please note that the
+    /// function is also called for nesting level 0, so be careful to not
+    /// return a fixed value if you don't want to accidentally indent top-level
+    /// entries by it (if that's not your intention), which you can solve by
+    /// returning `{none}` when the received parameter is equal to `{0}`.
+    ///
     ///
     /// ```example
     /// #set heading(numbering: "1.a.")
-    /// #outline(indent: true)
+    ///
+    /// #outline(title: "Contents (Automatic indentation)", indent: auto)
+    /// #outline(title: "Contents (Length indentation)", indent: 2em)
+    /// #outline(title: "Contents (Function indentation)", indent: n => [*!*] * n * n)
     ///
     /// = About ACME Corp.
     ///
@@ -126,8 +167,8 @@ pub struct OutlineElem {
     /// == Products
     /// #lorem(10)
     /// ```
-    #[default(false)]
-    pub indent: bool,
+    #[default(None)]
+    pub indent: Option<Smart<OutlineIndent>>,
 
     /// Content to fill the space between the title and the page number. Can be
     /// set to `none` to disable filling.
@@ -190,27 +231,7 @@ impl Show for OutlineElem {
                 ancestors.pop();
             }
 
-            // Add hidden ancestors numberings to realize the indent.
-            if indent {
-                let mut hidden = Content::empty();
-                for ancestor in &ancestors {
-                    let ancestor_outlinable = ancestor.with::<dyn Outlinable>().unwrap();
-
-                    if let Some(numbering) = ancestor_outlinable.numbering() {
-                        let numbers = ancestor_outlinable
-                            .counter()
-                            .at(vt, ancestor.location().unwrap())?
-                            .display(vt, &numbering)?;
-
-                        hidden += numbers + SpaceElem::new().pack();
-                    };
-                }
-
-                if !ancestors.is_empty() {
-                    seq.push(HideElem::new(hidden).pack());
-                    seq.push(SpaceElem::new().pack());
-                }
-            }
+            OutlineIndent::apply(&indent, vt, &ancestors, &mut seq, self.span())?;
 
             // Add the outline of the element.
             seq.push(outline.linked(Destination::Location(location)));
@@ -301,3 +322,91 @@ pub trait Outlinable: Refable {
         NonZeroUsize::ONE
     }
 }
+
+#[derive(Debug, Clone)]
+pub enum OutlineIndent {
+    Bool(bool),
+    Length(Spacing),
+    Function(Func),
+}
+
+impl OutlineIndent {
+    fn apply(
+        indent: &Option<Smart<Self>>,
+        vt: &mut Vt,
+        ancestors: &Vec<&Content>,
+        seq: &mut Vec<Content>,
+        span: Span,
+    ) -> SourceResult<()> {
+        match &indent {
+            // 'none' | 'false' => no indenting
+            None | Some(Smart::Custom(OutlineIndent::Bool(false))) => {}
+
+            // 'auto' | 'true' => use numbering alignment for indenting
+            Some(Smart::Auto | Smart::Custom(OutlineIndent::Bool(true))) => {
+                // Add hidden ancestors numberings to realize the indent.
+                let mut hidden = Content::empty();
+                for ancestor in ancestors {
+                    let ancestor_outlinable = ancestor.with::<dyn Outlinable>().unwrap();
+
+                    if let Some(numbering) = ancestor_outlinable.numbering() {
+                        let numbers = ancestor_outlinable
+                            .counter()
+                            .at(vt, ancestor.location().unwrap())?
+                            .display(vt, &numbering)?;
+
+                        hidden += numbers + SpaceElem::new().pack();
+                    };
+                }
+
+                if !ancestors.is_empty() {
+                    seq.push(HideElem::new(hidden).pack());
+                    seq.push(SpaceElem::new().pack());
+                }
+            }
+
+            // Length => indent with some fixed spacing per level
+            Some(Smart::Custom(OutlineIndent::Length(length))) => {
+                let Ok(depth): Result<i64, _> = ancestors.len().try_into() else {
+                    bail!(span, "outline element depth too large");
+                };
+
+                let hspace = HElem::new(*length).pack().repeat(depth).unwrap();
+                seq.push(hspace);
+            }
+
+            // Function => call function with the current depth and take
+            // the returned content
+            Some(Smart::Custom(OutlineIndent::Function(func))) => {
+                let depth = ancestors.len();
+                let returned = func.call_vt(vt, [depth.into()])?;
+                let Ok(returned) = returned.cast::<Content>() else {
+                    bail!(
+                        span,
+                        "indent function must return content"
+                    );
+                };
+                if !returned.is_empty() {
+                    seq.push(returned);
+                }
+            }
+        };
+
+        Ok(())
+    }
+}
+
+cast_from_value! {
+    OutlineIndent,
+    b: bool => OutlineIndent::Bool(b),
+    s: Spacing => OutlineIndent::Length(s),
+    f: Func => OutlineIndent::Function(f),
+}
+
+cast_to_value! {
+    v: OutlineIndent => match v {
+        OutlineIndent::Bool(b) => b.into(),
+        OutlineIndent::Length(s) => s.into(),
+        OutlineIndent::Function(f) => f.into()
+    }
+}

tests/typ/meta/outline-indent.typ

@@ -0,0 +1,60 @@
+// Tests outline 'indent' option.
+
+---
+// With heading numbering
+#set page(width: 200pt)
+#set heading(numbering: "1.a.")
+#outline()
+#outline(indent: false)
+#outline(indent: true)
+#outline(indent: none)
+#outline(indent: auto)
+#outline(indent: 2em)
+#outline(indent: n => ([-], [], [==], [====]).at(n))
+#outline(indent: n => "!" * calc.pow(2, n))
+
+= About ACME Corp.
+
+== History
+#lorem(10)
+
+== Products
+#lorem(10)
+
+=== Categories
+#lorem(10)
+
+==== General
+#lorem(10)
+
+---
+// Without heading numbering
+#set page(width: 200pt)
+#outline()
+#outline(indent: false)
+#outline(indent: true)
+#outline(indent: none)
+#outline(indent: auto)
+#outline(indent: 2em)
+#outline(indent: n => ([-], [], [==], [====]).at(n))
+#outline(indent: n => "!" * calc.pow(2, n))
+
+= About ACME Corp.
+
+== History
+#lorem(10)
+
+== Products
+#lorem(10)
+
+=== Categories
+#lorem(10)
+
+==== General
+#lorem(10)
+
+---
+// Error: 2-35 indent function must return content
+#outline(indent: n => (a: "dict"))
+
+= Heading