Shaping: Arabic keeps contextual alternates

Contextual alternates are part of how many Arabic fonts join (connecting
strokes, tatweel and letter variants). InDesign keeps them in Arabic text
when Contextual Alternates is off, so `calt` off is now dropped for
Arabic runs.

crates/fonts/src/lib.rs

@@ -195,6 +195,20 @@ pub fn shape_with_context(
     out
 }
 
+/// `features` as a run in `script` takes them. Contextual alternates are part of how Arabic joins
+/// (connecting strokes, tatweel and letter variants): InDesign keeps them in Arabic text with
+/// Contextual Alternates off, so `calt` off is dropped there.
+fn script_features(features: &[Feature], script: unicode_script::Script) -> std::borrow::Cow<'_, [Feature]> {
+    use unicode_script::Script;
+    let calt = Tag::new(b"calt");
+    let off = |f: &Feature| f.tag == calt && f.value == 0;
+    if script == Script::Arabic && features.iter().any(off) {
+        features.iter().filter(|f| !off(f)).copied().collect::<Vec<_>>().into()
+    } else {
+        features.into()
+    }
+}
+
 fn shape_dir(
     face: &FontFace,
     text: &str,
@@ -205,6 +219,8 @@ fn shape_dir(
     context: ShapeContext<'_>,
 ) -> Vec<ShapedGlyph> {
     let mut out = Vec::with_capacity(text.len());
+    let features = script_features(features, script);
+    let features = &features[..];
     let shaped = face.hb().map(|hb| {
         let shaper = face.shaper.shaper(&hb).instance(face.instance.as_ref()).build();
         let mut buf = UnicodeBuffer::new();
@@ -332,6 +348,16 @@ pub fn first_glyph(face: &FontFace, chars: &[char]) -> u32 {
 mod tests {
     use super::*;
 
+    #[test]
+    fn arabic_keeps_contextual_alternates() {
+        use unicode_script::Script;
+        let off = feature("-calt").unwrap();
+        let liga = feature("-liga").unwrap();
+        let tags = |s: Script| script_features(&[off, liga], s).iter().map(|f| (f.tag, f.value)).collect::<Vec<_>>();
+        assert_eq!(tags(Script::Arabic), vec![(liga.tag, 0)]);
+        assert_eq!(tags(Script::Latin), vec![(off.tag, 0), (liga.tag, 0)]);
+    }
+
     #[test]
     fn arabic_script_runs_do_not_inherit_the_hebrew_shaper() {
         use unicode_script::Script;