Change diff line numbers to 1-based indexing (#799)

## Summary
- Changed snapshot diff line numbers from 0-based to 1-based indexing
- Line numbers now start at 1 instead of 0, matching text editor
conventions
- Added functional test to verify the 1-based indexing behavior

## Test plan
- [x] Added new functional test `test_line_numbers_1_based` that
verifies line numbers are displayed as 1-based
- [x] All existing tests pass
- [x] Manual verification shows diffs with line numbers starting at 1

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: max-sixty

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 All notable changes to insta and cargo-insta are documented here.
 
+## Unreleased
+
+- Changed diff line numbers to 1-based indexing.
+
 ## 1.43.2
 
 - Preserve snapshot names with `INSTA_GLOB_FILTER`. #786

cargo-insta/tests/functional/main.rs

@@ -43,7 +43,7 @@
 /// Note that the packages must have different names, or we'll see interference
 /// between the tests[^1].
 ///
-/// [1]: That seems to be because they all share the same `target` directory, which
+/// [^1]: That seems to be because they all share the same `target` directory, which
 ///      cargo will confuse for each other if they share the same name. I haven't
 ///      worked out why — this is the case even if the files are the same between
 ///      two tests but with different commands — and those files exist in different
@@ -804,3 +804,58 @@ src/
         stderr
     );
 }
+
+#[test]
+fn test_line_numbers_1_based() {
+    let test_project = TestFiles::new()
+        .add_cargo_toml("test_line_numbers")
+        .add_file(
+            "src/lib.rs",
+            r#"
+#[test]
+fn test_snapshot() {
+    insta::assert_snapshot!("line1\nline2\nline3\nline4\nline5", @"line1\nmodified_line2\nline3\nline4\nline5");
+}
+"#
+            .to_string(),
+        )
+        .create_project();
+
+    // Run test to trigger failure and capture diff output
+    // Use --no-ignore and -- --nocapture to get the full diff output
+    let output = test_project
+        .insta_cmd()
+        .args(["test", "--no-ignore", "--", "--nocapture"])
+        .stderr(Stdio::piped())
+        .stdout(Stdio::piped())
+        .output()
+        .unwrap();
+
+    // Check both stdout and stderr for the diff output
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    let combined_output = format!("{}\n{}", stdout, stderr);
+
+    // Check that line numbers in the diff start at 1, not 0
+    // The diff should show line numbers like "1     1 │" for the first line
+    assert!(
+        combined_output.contains("    1     1 │ line1"),
+        "Expected line numbers to start at 1, but got:\n{}",
+        combined_output
+    );
+
+    // Also check line 2 which has the modification
+    assert!(
+        combined_output.contains("    2       │-modified_line2")
+            || combined_output.contains("          2 │+line2"),
+        "Expected line 2 to be numbered as 2, but got:\n{}",
+        combined_output
+    );
+
+    // And verify line 5 is numbered as 5
+    assert!(
+        combined_output.contains("    5     5 │ line5"),
+        "Expected line 5 to be numbered as 5, but got:\n{}",
+        combined_output
+    );
+}

insta/src/output.rs

@@ -236,7 +236,7 @@ impl<'a> SnapshotPrinter<'a> {
                                 print!(
                                     "{:>5} {:>5} │{}",
                                     "",
-                                    style(change.new_index().unwrap()).cyan().dim().bold(),
+                                    style(change.new_index().unwrap() + 1).cyan().dim().bold(),
                                     style("+").green(),
                                 );
                                 for &(emphasized, change) in change.values() {
@@ -252,7 +252,7 @@ impl<'a> SnapshotPrinter<'a> {
                                 has_changes = true;
                                 print!(
                                     "{:>5} {:>5} │{}",
-                                    style(change.old_index().unwrap()).cyan().dim(),
+                                    style(change.old_index().unwrap() + 1).cyan().dim(),
                                     "",
                                     style("-").red(),
                                 );
@@ -268,8 +268,8 @@ impl<'a> SnapshotPrinter<'a> {
                             ChangeTag::Equal => {
                                 print!(
                                     "{:>5} {:>5} │ ",
-                                    style(change.old_index().unwrap()).cyan().dim(),
-                                    style(change.new_index().unwrap()).cyan().dim().bold(),
+                                    style(change.old_index().unwrap() + 1).cyan().dim(),
+                                    style(change.new_index().unwrap() + 1).cyan().dim().bold(),
                                 );
                                 for &(_, change) in change.values() {
                                     let change = render_invisible(change, newlines_matter);