Re-resolve anchor before applying AI inline assist edits (#43103)

Closes #39088

Release Notes:

- Fixed AI assistant edits being scrambled when file was modified while
it was open

--

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/agent_ui/src/buffer_codegen.rs

@@ -491,6 +491,14 @@ impl CodegenAlternative {
         cx: &mut Context<Self>,
     ) {
         let start_time = Instant::now();
+
+        // Make a new snapshot and re-resolve anchor in case the document was modified.
+        // This can happen often if the editor loses focus and is saved + reformatted,
+        // as in https://github.com/zed-industries/zed/issues/39088
+        self.snapshot = self.buffer.read(cx).snapshot(cx);
+        self.range = self.snapshot.anchor_after(self.range.start)
+            ..self.snapshot.anchor_after(self.range.end);
+
         let snapshot = self.snapshot.clone();
         let selected_text = snapshot
             .text_for_range(self.range.start..self.range.end)

crates/language/src/buffer_tests.rs

@@ -3455,6 +3455,25 @@ fn test_contiguous_ranges() {
     );
 }
 
+#[gpui::test]
+fn test_insertion_after_deletion(cx: &mut gpui::App) {
+    let buffer = cx.new(|cx| Buffer::local("struct Foo {\n    \n}", cx));
+    buffer.update(cx, |buffer, cx| {
+        let mut anchor = buffer.anchor_after(17);
+        buffer.edit([(12..18, "")], None, cx);
+        let snapshot = buffer.snapshot();
+        assert_eq!(snapshot.text(), "struct Foo {}");
+        if !anchor.is_valid(&snapshot) {
+            anchor = snapshot.anchor_after(snapshot.offset_for_anchor(&anchor));
+        }
+        buffer.edit([(anchor..anchor, "\n")], None, cx);
+        buffer.edit([(anchor..anchor, "field1:")], None, cx);
+        buffer.edit([(anchor..anchor, " i32,")], None, cx);
+        let snapshot = buffer.snapshot();
+        assert_eq!(snapshot.text(), "struct Foo {\nfield1: i32,}");
+    })
+}
+
 #[gpui::test(iterations = 500)]
 fn test_trailing_whitespace_ranges(mut rng: StdRng) {
     // Generate a random multi-line string containing