Simplify highlighting infra

This also fixes the killer whale bug

crates/ide/src/syntax_highlighting/highlights.rs

@@ -0,0 +1,109 @@
+//! Collects a tree of highlighted ranges and flattens it.
+use std::{cmp::Ordering, iter};
+
+use stdx::equal_range_by;
+use syntax::TextRange;
+
+use crate::{HighlightTag, HighlightedRange};
+
+pub(super) struct Highlights {
+    root: Node,
+}
+
+struct Node {
+    highlighted_range: HighlightedRange,
+    nested: Vec<Node>,
+}
+
+impl Highlights {
+    pub(super) fn new(range: TextRange) -> Highlights {
+        Highlights {
+            root: Node::new(HighlightedRange {
+                range,
+                highlight: HighlightTag::Dummy.into(),
+                binding_hash: None,
+            }),
+        }
+    }
+
+    pub(super) fn add(&mut self, highlighted_range: HighlightedRange) {
+        self.root.add(highlighted_range);
+    }
+
+    pub(super) fn to_vec(self) -> Vec<HighlightedRange> {
+        let mut res = Vec::new();
+        self.root.flatten(&mut res);
+        res
+    }
+}
+
+impl Node {
+    fn new(highlighted_range: HighlightedRange) -> Node {
+        Node { highlighted_range, nested: Vec::new() }
+    }
+
+    fn add(&mut self, highlighted_range: HighlightedRange) {
+        assert!(self.highlighted_range.range.contains_range(highlighted_range.range));
+
+        // Fast path
+        if let Some(last) = self.nested.last_mut() {
+            if last.highlighted_range.range.contains_range(highlighted_range.range) {
+                return last.add(highlighted_range);
+            }
+            if last.highlighted_range.range.end() <= highlighted_range.range.start() {
+                return self.nested.push(Node::new(highlighted_range));
+            }
+        }
+
+        let (start, len) = equal_range_by(&self.nested, |n| {
+            ordering(n.highlighted_range.range, highlighted_range.range)
+        });
+
+        if len == 1
+            && self.nested[start].highlighted_range.range.contains_range(highlighted_range.range)
+        {
+            return self.nested[start].add(highlighted_range);
+        }
+
+        let nested = self
+            .nested
+            .splice(start..start + len, iter::once(Node::new(highlighted_range)))
+            .collect::<Vec<_>>();
+        self.nested[start].nested = nested;
+    }
+
+    fn flatten(&self, acc: &mut Vec<HighlightedRange>) {
+        let mut start = self.highlighted_range.range.start();
+        let mut nested = self.nested.iter();
+        loop {
+            let next = nested.next();
+            let end = next.map_or(self.highlighted_range.range.end(), |it| {
+                it.highlighted_range.range.start()
+            });
+            if start < end {
+                acc.push(HighlightedRange {
+                    range: TextRange::new(start, end),
+                    highlight: self.highlighted_range.highlight,
+                    binding_hash: self.highlighted_range.binding_hash,
+                });
+            }
+            start = match next {
+                Some(child) => {
+                    child.flatten(acc);
+                    child.highlighted_range.range.end()
+                }
+                None => break,
+            }
+        }
+    }
+}
+
+pub(super) fn ordering(r1: TextRange, r2: TextRange) -> Ordering {
+    if r1.end() <= r2.start() {
+        Ordering::Less
+    } else if r2.end() <= r1.start() {
+        Ordering::Greater
+    } else {
+        Ordering::Equal
+    }
+}