Fix logical order in bidirectional lines (#6796)

crates/typst-layout/src/inline/line.rs

@@ -271,7 +271,7 @@ fn collect_range<'a>(
     items: &mut Items<'a>,
     fallback: &mut Option<ItemEntry<'a>>,
 ) {
-    for (idx, (subrange, item)) in p.slice(range.clone()).enumerate() {
+    for (idx, (subrange, item)) in p.slice(range.clone()) {
         // All non-text items are just kept, they can't be split.
         let Item::Text(shaped) = item else {
             items.push(item, idx);

crates/typst-layout/src/inline/prepare.rs

@@ -34,8 +34,12 @@ impl<'a> Preparation<'a> {
         &self.items[idx]
     }
 
-    /// Iterate over the items that intersect the given `sliced` range.
+    /// Iterate over the items that intersect the given `sliced` range alongside
-    pub fn slice(&self, sliced: Range) -> impl Iterator<Item = &(Range, Item<'a>)> {
+    /// their indices in `self.items` and their ranges in the paragraph's text.
+    pub fn slice(
+        &self,
+        sliced: Range,
+    ) -> impl Iterator<Item = (usize, &(Range, Item<'a>))> {
         // Usually, we don't want empty-range items at the start of the line
         // (because they will be part of the previous line), but for the first
         // line, we need to keep them.
@@ -43,7 +47,11 @@ impl<'a> Preparation<'a> {
             0 => 0,
             n => self.indices.get(n).copied().unwrap_or(0),
         };
-        self.items[start..].iter().take_while(move |(range, _)| {
+        self.items
+            .iter()
+            .enumerate()
+            .skip(start)
+            .take_while(move |(_, (range, _))| {
                 range.start < sliced.end || range.end <= sliced.end
             })
     }

tests/suite/introspection/counter.typ

@@ -174,3 +174,12 @@ B
 #context test(c.get(), (100000000002,))
 #c.update(n => n + 2)
 #context test(c.get(), (100000000004,))
+
+--- counter-rtl ---
+#set page(width: auto)
+#let c = counter("c")
+#let s = context c.display() + c.step()
+#let tree = [درخت]
+#let line = [A #s B #tree #s #tree #s #tree C #s D #s]
+#line \
+#line