Make footnotes referenceable (#1546)

library/src/layout/flow.rs

@@ -488,6 +488,11 @@ impl FlowLayouter<'_> {
         // Process footnotes one at a time.
         let mut k = 0;
         while k < notes.len() {
+            if notes[k].is_ref() {
+                k += 1;
+                continue;
+            }
+
             if !self.has_footnotes {
                 self.layout_footnote_separator(vt)?;
             }

library/src/meta/bibliography.rs

@@ -545,7 +545,7 @@ fn create(
             }
 
             if style == CitationStyle::ChicagoNotes {
-                content = FootnoteElem::new(content).pack();
+                content = FootnoteElem::with_content(content).pack();
             }
 
             (location, Some(content))

library/src/meta/footnote.rs

@@ -1,15 +1,35 @@
+use comemo::Prehashed;
 use std::str::FromStr;
 
 use super::{Counter, Numbering, NumberingPattern};
 use crate::layout::{HElem, ParElem};
+use crate::meta::{Count, CounterUpdate};
 use crate::prelude::*;
 use crate::text::{SuperElem, TextElem, TextSize};
 use crate::visualize::LineElem;
 
+/// The body of a footnote can be either some content or a label referencing
+/// another footnote.
+#[derive(Debug)]
+pub enum FootnoteBody {
+    Content(Content),
+    Reference(Label),
+}
+
+cast! {
+    FootnoteBody,
+    self => match self {
+        Self::Content(v) => v.into_value(),
+        Self::Reference(v) => v.into_value(),
+    },
+    v: Content => Self::Content(v),
+    v: Label => Self::Reference(v),
+}
+
 /// A footnote.
 ///
-/// Include additional remarks and references on the same page with footnotes. A
-/// footnote will insert a superscript number that links to the note at the
+/// Includes additional remarks and references on the same page with footnotes.
+/// A footnote will insert a superscript number that links to the note at the
 /// bottom of the page. Notes are numbered sequentially throughout your document
 /// and can break across multiple pages.
 ///
@@ -28,6 +48,15 @@ use crate::visualize::LineElem;
 /// there is a space before it in the markup. To force space, you can use the
 /// string `[#" "]` or explicit [horizontal spacing]($func/h).
 ///
+/// By giving a label to a footnote, you can have multiple references to it.
+///
+/// ```example
+/// You can edit Typst documents online.
+/// #footnote[https://typst.app/app] <fn>
+/// Checkout Typst's website. @fn
+/// And the online app. #footnote(<fn>)
+/// ```
+///
 /// _Note:_ Set and show rules in the scope where `footnote` is called may not
 /// apply to the footnote's content. See [here][issue] more information.
 ///
@@ -35,7 +64,7 @@ use crate::visualize::LineElem;
 ///
 /// Display: Footnote
 /// Category: meta
-#[element(Locatable, Synthesize, Show)]
+#[element(Locatable, Synthesize, Show, Count)]
 #[scope(
     scope.define("entry", FootnoteEntry::func());
     scope
@@ -58,9 +87,49 @@ pub struct FootnoteElem {
     #[default(Numbering::Pattern(NumberingPattern::from_str("1").unwrap()))]
     pub numbering: Numbering,
 
-    /// The content to put into the footnote.
+    /// The content to put into the footnote. Can also be the label of another
+    /// footnote this one should point to.
     #[required]
-    pub body: Content,
+    pub body: FootnoteBody,
+}
+
+impl FootnoteElem {
+    /// Creates a new footnote that the passed content as its body.
+    pub fn with_content(content: Content) -> Self {
+        Self::new(FootnoteBody::Content(content))
+    }
+
+    /// Creates a new footnote referencing the footnote with the specified label.
+    pub fn with_label(label: Label) -> Self {
+        Self::new(FootnoteBody::Reference(label))
+    }
+
+    /// Tests if this footnote is a reference to another footnote.
+    pub fn is_ref(&self) -> bool {
+        matches!(self.body(), FootnoteBody::Reference(_))
+    }
+
+    /// Returns the content of the body of this footnote if it is not a ref.
+    pub fn body_content(&self) -> Option<Content> {
+        match self.body() {
+            FootnoteBody::Content(content) => Some(content),
+            _ => None,
+        }
+    }
+
+    /// Returns the location of the definition of this footnote.
+    pub fn declaration_location(&self, vt: &Vt) -> StrResult<Location> {
+        match self.body() {
+            FootnoteBody::Reference(label) => {
+                let element: Prehashed<Content> = vt.introspector.query_label(&label)?;
+                let footnote = element
+                    .to::<FootnoteElem>()
+                    .ok_or("referenced element should be a footnote")?;
+                footnote.declaration_location(vt)
+            }
+            _ => Ok(self.0.location().unwrap()),
+        }
+    }
 }
 
 impl Synthesize for FootnoteElem {
@@ -73,14 +142,22 @@ impl Synthesize for FootnoteElem {
 impl Show for FootnoteElem {
     #[tracing::instrument(name = "FootnoteElem::show", skip_all)]
     fn show(&self, vt: &mut Vt, styles: StyleChain) -> SourceResult<Content> {
-        let loc = self.0.location().unwrap();
-        let numbering = self.numbering(styles);
-        let counter = Counter::of(Self::func());
-        let num = counter.at(vt, loc)?.display(vt, &numbering)?;
-        let sup = SuperElem::new(num).pack();
-        let hole = HElem::new(Abs::zero().into()).with_weak(true).pack();
-        let loc = self.0.location().unwrap().variant(1);
-        Ok(hole + sup.linked(Destination::Location(loc)))
+        Ok(vt.delayed(|vt| {
+            let loc = self.declaration_location(vt).at(self.span())?;
+            let numbering = self.numbering(styles);
+            let counter = Counter::of(Self::func());
+            let num = counter.at(vt, loc)?.display(vt, &numbering)?;
+            let sup = SuperElem::new(num).pack();
+            let hole = HElem::new(Abs::zero().into()).with_weak(true).pack();
+            let loc = loc.variant(1);
+            Ok(hole + sup.linked(Destination::Location(loc)))
+        }))
+    }
+}
+
+impl Count for FootnoteElem {
+    fn update(&self) -> Option<CounterUpdate> {
+        (!self.is_ref()).then(|| CounterUpdate::Step(NonZeroUsize::ONE))
     }
 }
 
@@ -201,7 +278,7 @@ impl Show for FootnoteEntry {
             HElem::new(self.indent(styles).into()).pack(),
             sup,
             HElem::new(number_gap.into()).with_weak(true).pack(),
-            note.body(),
+            note.body_content().unwrap(),
         ]))
     }
 }
@@ -218,5 +295,5 @@ impl Finalize for FootnoteEntry {
 
 cast! {
     FootnoteElem,
-    v: Content => v.to::<Self>().cloned().unwrap_or_else(|| Self::new(v.clone())),
+    v: Content => v.to::<Self>().cloned().unwrap_or_else(|| Self::with_content(v.clone())),
 }

library/src/meta/reference.rs

@@ -1,4 +1,5 @@
 use super::{BibliographyElem, CiteElem, Counter, Figurable, Numbering};
+use crate::meta::FootnoteElem;
 use crate::prelude::*;
 use crate::text::TextElem;
 
@@ -11,11 +12,11 @@ use crate::text::TextElem;
 /// bibliography.
 ///
 /// Referenceable elements include [headings]($func/heading),
-/// [figures]($func/figure), and [equations]($func/math.equation). To create a
-/// custom referenceable element like a theorem, you can create a figure of a
-/// custom [`kind`]($func/figure.kind) and write a show rule for it. In the
-/// future, there might be a more direct way to define a custom referenceable
-/// element.
+/// [figures]($func/figure), [equations]($func/math.equation), and
+/// [footnotes]($func/footnote). To create a custom referenceable element like a
+/// theorem, you can create a figure of a custom [`kind`]($func/figure.kind) and
+/// write a show rule for it. In the future, there might be a more direct way to
+/// define a custom referenceable element.
 ///
 /// If you just want to link to a labelled element and not get an automatic
 /// textual reference, consider using the [`link`]($func/link) function instead.
@@ -160,6 +161,11 @@ impl Show for RefElem {
             }
 
             let elem = elem.at(span)?;
+
+            if elem.func() == FootnoteElem::func() {
+                return Ok(FootnoteElem::with_label(target).pack().spanned(span));
+            }
+
             let refable = elem
                 .with::<dyn Refable>()
                 .ok_or_else(|| {

tests/typ/meta/footnote-refs.typ

@@ -0,0 +1,40 @@
+// Test references to footnotes.
+
+---
+A footnote #footnote[Hi]<fn> \
+A reference to it @fn
+
+---
+// Multiple footnotes are refs
+First #footnote[A]<fn1> \
+Second #footnote[B]<fn2> \
+First ref @fn1 \
+Third #footnote[C] \
+Fourth #footnote[D]<fn4> \
+Fourth ref @fn4 \
+Second ref @fn2 \
+Second ref again @fn2
+
+---
+// Forward reference
+Usage @fn \
+Definition #footnote[Hi]<fn>
+
+---
+// Footnote ref in footnote
+#footnote[Reference to next @fn]
+#footnote[Reference to myself @fn]<fn>
+#footnote[Reference to previous @fn]
+
+---
+// Styling
+#show footnote: text.with(fill: red)
+Real #footnote[...]<fn> \
+Ref @fn
+
+---
+// Footnote call with label
+#footnote(<fn>)
+#footnote[Hi]<fn>
+#ref(<fn>)
+#footnote(<fn>)