Simplify highlighting infra

This also fixes the killer whale bug

crates/ide/src/syntax_highlighting/injector.rs

@@ -0,0 +1,83 @@
+//! Extracts a subsequence of a text document, remembering the mapping of ranges
+//! between original and extracted texts.
+use std::ops::{self, Sub};
+
+use stdx::equal_range_by;
+use syntax::{TextRange, TextSize};
+
+use super::highlights::ordering;
+
+#[derive(Default)]
+pub(super) struct Injector {
+    buf: String,
+    ranges: Vec<(TextRange, Option<Delta<TextSize>>)>,
+}
+
+impl Injector {
+    pub(super) fn add(&mut self, text: &str, source_range: TextRange) {
+        let len = TextSize::of(text);
+        assert_eq!(len, source_range.len());
+
+        let target_range = TextRange::at(TextSize::of(&self.buf), len);
+        self.ranges
+            .push((target_range, Some(Delta::new(target_range.start(), source_range.start()))));
+        self.buf.push_str(text);
+    }
+    pub(super) fn add_unmapped(&mut self, text: &str) {
+        let len = TextSize::of(text);
+
+        let target_range = TextRange::at(TextSize::of(&self.buf), len);
+        self.ranges.push((target_range, None));
+        self.buf.push_str(text);
+    }
+
+    pub(super) fn text(&self) -> &str {
+        &self.buf
+    }
+    pub(super) fn map_range_up(&self, range: TextRange) -> impl Iterator<Item = TextRange> + '_ {
+        let (start, len) = equal_range_by(&self.ranges, |&(r, _)| ordering(r, range));
+        (start..start + len).filter_map(move |i| {
+            let (target_range, delta) = self.ranges[i];
+            let intersection = target_range.intersect(range).unwrap();
+            Some(intersection + delta?)
+        })
+    }
+}
+
+#[derive(Clone, Copy)]
+enum Delta<T> {
+    Add(T),
+    Sub(T),
+}
+
+impl<T> Delta<T> {
+    fn new(from: T, to: T) -> Delta<T>
+    where
+        T: Ord + Sub<Output = T>,
+    {
+        if to >= from {
+            Delta::Add(to - from)
+        } else {
+            Delta::Sub(from - to)
+        }
+    }
+}
+
+impl ops::Add<Delta<TextSize>> for TextSize {
+    type Output = TextSize;
+
+    fn add(self, rhs: Delta<TextSize>) -> TextSize {
+        match rhs {
+            Delta::Add(it) => self + it,
+            Delta::Sub(it) => self - it,
+        }
+    }
+}
+
+impl ops::Add<Delta<TextSize>> for TextRange {
+    type Output = TextRange;
+
+    fn add(self, rhs: Delta<TextSize>) -> TextRange {
+        TextRange::at(self.start() + rhs, self.len())
+    }
+}