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/debugger_ui/src/debugger_panel.rs

@@ -14,13 +14,12 @@ use collections::IndexMap;
 use dap::adapters::DebugAdapterName;
 use dap::{DapRegistry, StartDebuggingRequestArguments};
 use dap::{client::SessionId, debugger_settings::DebuggerSettings};
-use editor::Editor;
+use editor::{Editor, MultiBufferOffset, ToPoint};
 use gpui::{
     Action, App, AsyncWindowContext, ClipboardItem, Context, DismissEvent, Entity, EntityId,
     EventEmitter, FocusHandle, Focusable, MouseButton, MouseDownEvent, Point, Subscription, Task,
     WeakEntity, anchored, deferred,
 };
-use text::ToPoint as _;
 
 use itertools::Itertools as _;
 use language::Buffer;
@@ -1216,11 +1215,11 @@ impl DebugPanel {
         let mut last_offset = None;
         while let Some(mat) = matches.next() {
             if let Some(pos) = mat.captures.first().map(|m| m.node.byte_range().end) {
-                last_offset = Some(pos)
+                last_offset = Some(MultiBufferOffset(pos))
             }
         }
         let mut edits = Vec::new();
-        let mut cursor_position = 0;
+        let mut cursor_position = MultiBufferOffset(0);
 
         if let Some(pos) = last_offset {
             edits.push((pos..pos, format!(",\n{new_scenario}")));
@@ -1234,24 +1233,25 @@ impl DebugPanel {
 
             if let Some(mat) = matches.next() {
                 if let Some(pos) = mat.captures.first().map(|m| m.node.byte_range().end - 1) {
-                    edits.push((pos..pos, format!("\n{new_scenario}\n")));
-                    cursor_position = pos + "\n  ".len();
+                    edits.push((
+                        MultiBufferOffset(pos)..MultiBufferOffset(pos),
+                        format!("\n{new_scenario}\n"),
+                    ));
+                    cursor_position = MultiBufferOffset(pos) + "\n  ".len();
                 }
             } else {
-                edits.push((0..0, format!("[\n{}\n]", new_scenario)));
-                cursor_position = "[\n  ".len();
+                edits.push((
+                    MultiBufferOffset(0)..MultiBufferOffset(0),
+                    format!("[\n{}\n]", new_scenario),
+                ));
+                cursor_position = MultiBufferOffset("[\n  ".len());
             }
         }
         editor.transact(window, cx, |editor, window, cx| {
             editor.edit(edits, cx);
-            let snapshot = editor
-                .buffer()
-                .read(cx)
-                .as_singleton()
-                .unwrap()
-                .read(cx)
-                .snapshot();
+            let snapshot = editor.buffer().read(cx).read(cx);
             let point = cursor_position.to_point(&snapshot);
+            drop(snapshot);
             editor.go_to_singleton_buffer_point(point, window, cx);
         });
         Ok(editor.save(SaveOptions::default(), project, window, cx))