multi_buffer: Typed MultiBufferOffset (#42707)

This PR introduces a new `MultiBufferOffset` new type wrapping size. The
goal of this is to make it clear at the type level when we are
interacting with offsets of a multi buffer versus offsets of a language
/ text buffer. This improves readability of things quite a bit by making
it clear what kind of offsets one is working with while also reducing
accidental bugs by using the wrong kin of offset for the wrong API.

This PR also uncovered two minor bugs due to that.

Does not yet introduce the MultiBufferPoint equivalent, that is for a
follow up PR.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/multi_buffer/src/anchor.rs

@@ -1,8 +1,10 @@
+use crate::{MultiBufferDimension, MultiBufferOffset, MultiBufferOffsetUtf16};
+
 use super::{ExcerptId, MultiBufferSnapshot, ToOffset, ToPoint};
-use language::{OffsetUtf16, Point, TextDimension};
+use language::Point;
 use std::{
     cmp::Ordering,
-    ops::{Range, Sub},
+    ops::{AddAssign, Range, Sub},
 };
 use sum_tree::Bias;
 use text::BufferId;
@@ -162,7 +164,11 @@ impl Anchor {
 
     pub fn summary<D>(&self, snapshot: &MultiBufferSnapshot) -> D
     where
-        D: TextDimension + Ord + Sub<D, Output = D>,
+        D: MultiBufferDimension
+            + Ord
+            + Sub<Output = D::TextDimension>
+            + AddAssign<D::TextDimension>,
+        D::TextDimension: Sub<Output = D::TextDimension> + Ord,
     {
         snapshot.summary_for_anchor(self)
     }
@@ -182,10 +188,10 @@ impl Anchor {
 }
 
 impl ToOffset for Anchor {
-    fn to_offset(&self, snapshot: &MultiBufferSnapshot) -> usize {
+    fn to_offset(&self, snapshot: &MultiBufferSnapshot) -> MultiBufferOffset {
         self.summary(snapshot)
     }
-    fn to_offset_utf16(&self, snapshot: &MultiBufferSnapshot) -> OffsetUtf16 {
+    fn to_offset_utf16(&self, snapshot: &MultiBufferSnapshot) -> MultiBufferOffsetUtf16 {
         self.summary(snapshot)
     }
 }
@@ -203,7 +209,7 @@ pub trait AnchorRangeExt {
     fn cmp(&self, other: &Range<Anchor>, buffer: &MultiBufferSnapshot) -> Ordering;
     fn includes(&self, other: &Range<Anchor>, buffer: &MultiBufferSnapshot) -> bool;
     fn overlaps(&self, other: &Range<Anchor>, buffer: &MultiBufferSnapshot) -> bool;
-    fn to_offset(&self, content: &MultiBufferSnapshot) -> Range<usize>;
+    fn to_offset(&self, content: &MultiBufferSnapshot) -> Range<MultiBufferOffset>;
     fn to_point(&self, content: &MultiBufferSnapshot) -> Range<Point>;
 }
 
@@ -223,7 +229,7 @@ impl AnchorRangeExt for Range<Anchor> {
         self.end.cmp(&other.start, buffer).is_ge() && self.start.cmp(&other.end, buffer).is_le()
     }
 
-    fn to_offset(&self, content: &MultiBufferSnapshot) -> Range<usize> {
+    fn to_offset(&self, content: &MultiBufferSnapshot) -> Range<MultiBufferOffset> {
         self.start.to_offset(content)..self.end.to_offset(content)
     }
 
@@ -231,6 +237,3 @@ impl AnchorRangeExt for Range<Anchor> {
         self.start.to_point(content)..self.end.to_point(content)
     }
 }
-
-#[derive(Clone, Copy, Eq, PartialEq, Debug, Hash, Ord, PartialOrd)]
-pub struct Offset(pub usize);