Add basic multi-char symbol layout

Not perfect, but should handle most cases

Co-authored-by: Max <max@mkor.je>

crates/typst-layout/src/math/fragment.rs

@@ -300,6 +300,7 @@ impl GlyphFragment {
         );
 
         let buffer = rustybuzz::shape_with_plan(font.rusty(), &plan, buffer);
+        // TODO: deal with multiple glyphs.
         if buffer.len() != 1 {
             bail!(span, "did not get a single glyph after shaping {}", text);
         }

crates/typst-layout/src/math/text.rs

@@ -129,33 +129,29 @@ pub fn layout_symbol(
     ctx: &mut MathContext,
     styles: StyleChain,
 ) -> SourceResult<()> {
-    assert!(
-        elem.text.len() <= 4 && elem.text.chars().count() == 1,
-        "TODO: layout multi-char symbol"
-    );
-    let elem_char = elem
-        .text
-        .chars()
-        .next()
-        .expect("TODO: should an empty symbol value forbidden?");
-
-    // Switch dotless char to normal when we have the dtls OpenType feature.
-    // This should happen before the main styling pass.
-    let dtls = style_dtls();
-    let (unstyled_c, symbol_styles) = match try_dotless(elem_char) {
-        Some(c) if has_dtls_feat(ctx.font) => (c, styles.chain(&dtls)),
-        _ => (elem_char, styles),
-    };
-
     let variant = styles.get(EquationElem::variant);
     let bold = styles.get(EquationElem::bold);
     let italic = styles.get(EquationElem::italic);
-
+    let dtls = style_dtls();
-    let style = MathStyle::select(unstyled_c, variant, bold, italic);
+    let has_dtls_feat = has_dtls_feat(ctx.font);
-    let text: EcoString = to_style(unstyled_c, style).collect();
+    for cluster in elem.text.graphemes(true) {
+        // Switch dotless char to normal when we have the dtls OpenType feature.
+        // This should happen before the main styling pass.
+        let mut enable_dtls = false;
+        let text: EcoString = cluster
+            .chars()
+            .flat_map(|mut c| {
+                if has_dtls_feat && let Some(d) = try_dotless(c) {
+                    enable_dtls = true;
+                    c = d;
+                }
+                to_style(c, MathStyle::select(c, variant, bold, italic))
+            })
+            .collect();
+        let styles = if enable_dtls { styles.chain(&dtls) } else { styles };
 
         let fragment: MathFragment =
-        match GlyphFragment::new(ctx.font, symbol_styles, &text, elem.span()) {
+            match GlyphFragment::new(ctx.font, styles, &text, elem.span()) {
                 Ok(mut glyph) => {
                     adjust_glyph_layout(&mut glyph, ctx, styles);
                     glyph.into()
@@ -167,6 +163,7 @@ pub fn layout_symbol(
                 }
             };
         ctx.push(fragment);
+    }
     Ok(())
 }