Improve justification of Chinese/Japanese text (#542)

library/src/layout/container.rs

@@ -84,6 +84,7 @@ pub struct BoxElem {
     ///   outset: (y: 3pt),
     ///   radius: 2pt,
     /// )[rectangle].
+    /// ```
     #[resolve]
     #[fold]
     pub outset: Sides<Option<Rel<Length>>>,

library/src/text/shaping.rs

@@ -4,6 +4,7 @@ use std::str::FromStr;
 use rustybuzz::{Feature, Tag, UnicodeBuffer};
 use typst::font::{Font, FontVariant};
 use typst::util::SliceExt;
+use unicode_script::{Script, UnicodeScript};
 
 use super::*;
 use crate::layout::SpanMapper;
@@ -69,11 +70,24 @@ impl ShapedGlyph {
     }
 
     /// Whether the glyph is justifiable.
+    ///
+    /// Typst's basic justification strategy is to stretch all the spaces
+    /// in a line until the line fills the available width. However, some
+    /// scripts (notably Chinese and Japanese) don't use spaces.
+    ///
+    /// In Japanese typography, the convention is to insert space evenly
+    /// between all glyphs. I assume it's the same in Chinese.
     pub fn is_justifiable(&self) -> bool {
-        self.is_space() || matches!(self.c, '，' | '。' | '、')
+        self.is_space() || is_spaceless(self.c.script())
     }
 }
 
+/// Does this script separate its words using spaces?
+fn is_spaceless(script: Script) -> bool {
+    use Script::*;
+    matches!(script, Hiragana | Katakana | Han)
+}
+
 /// A side you can go toward.
 enum Side {
     /// To the left-hand side.