fix(typst): fix paragraph break spans (#2080)

* refactor(core): rename `EMPTY` to `ZERO` in `Span`

Using `EMPTY` to represent a `Span` of `0..0` is confusing. A span that
starts and ends at 0 is always empty, but not all empty spans start and
end at 0.

* feat(core): add constructor `Span::empty()`

* fix(typst): fix paragraph break spans

Fix bug which caused paragraph break tokens to always be positioned at
the beginning of the document.

Fixes #2070

harper-core/src/span.rs

@@ -29,12 +29,8 @@ pub struct Span<T> {
 }
 
 impl<T> Span<T> {
-    /// An empty [`Span`].
-    pub const EMPTY: Self = Self {
-        start: 0,
-        end: 0,
-        span_type: PhantomData,
-    };
+    /// A [`Span`] with a start and end index of 0.
+    pub const ZERO: Self = Self::empty(0);
 
     /// Creates a new [`Span`] with the provided start and end indices.
     ///
@@ -61,6 +57,15 @@ impl<T> Span<T> {
         }
     }
 
+    /// Creates a new empty [`Span`] with the provided position.
+    pub const fn empty(pos: usize) -> Self {
+        Self {
+            start: pos,
+            end: pos,
+            span_type: PhantomData,
+        }
+    }
+
     /// The length of the [`Span`].
     pub fn len(&self) -> usize {
         self.end - self.start
@@ -179,7 +184,7 @@ impl Span<Token> {
     /// this span is required.
     pub fn to_char_span(&self, source_document_tokens: &[Token]) -> Span<char> {
         if self.is_empty() {
-            Span::EMPTY
+            Span::ZERO
         } else {
             let target_tokens = &source_document_tokens[self.start..self.end];
             Span::new(
@@ -263,7 +268,7 @@ mod tests {
         let doc = Document::new_plain_english_curated("Hello world!");
 
         // Empty span.
-        let token_span = Span::EMPTY;
+        let token_span = Span::ZERO;
         let converted = token_span.to_char_span(doc.get_tokens());
         assert!(converted.is_empty());
 

harper-typst/src/typst_translator.rs

@@ -152,15 +152,17 @@ impl<'a> TypstTranslator<'a> {
             };
         }
 
-        fn parbreak() -> Option<Vec<Token>> {
+        fn parbreak(pos: usize) -> Option<Vec<Token>> {
             Some(vec![Token {
-                span: harper_core::Span::EMPTY,
+                span: harper_core::Span::empty(pos),
                 kind: TokenKind::ParagraphBreak,
             }])
         }
 
         fn isolate(inner: Option<Vec<Token>>) -> Option<Vec<Token>> {
-            merge![parbreak(), inner, parbreak()]
+            let start = inner.as_ref()?.first().map_or(0, |token| token.span.start);
+            let end = inner.as_ref()?.last().map_or(0, |token| token.span.end);
+            merge![parbreak(start), inner, parbreak(end)]
         }
 
         // Recurse on each element of an iterator