conflicts: add "snapshot" conflict marker style

Adds a new "snapshot" conflict marker style which returns a series of
snapshots, similar to Git's "diff3" conflict style. The "snapshot"
option uses a subset of the conflict hunk headers as the "diff" option
(it just doesn't use "%%%%%%%"), meaning that the two options are
trivially compatible with each other (i.e. a file materialized with
"snapshot" can be parsed with "diff" and vice versa).

Example of "snapshot" conflict markers:

```
<<<<<<< Conflict 1 of 1
+++++++ Contents of side #1
fn example(word: String) {
    println!("word is {word}");
------- Contents of base
fn example(w: String) {
    println!("word is {w}");
+++++++ Contents of side #2
fn example(w: &str) {
    println!("word is {w}");
>>>>>>> Conflict 1 of 1 ends
}
```

CHANGELOG.md

@@ -66,6 +66,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * New `ui.conflict-marker-style` config option to change how conflicts are
   materialized in the working copy. The default option ("diff") renders
   conflicts as a snapshot with a list of diffs to apply to the snapshot.
+  The new "snapshot" option renders conflicts as a series of snapshots, showing
+  each side and base of the conflict.
 
 ### Fixed bugs
 

cli/src/config-schema.json

@@ -163,7 +163,8 @@
                     "type": "string",
                     "description": "Conflict marker style to use when materializing conflicts in the working copy",
                     "enum": [
-                        "diff"
+                        "diff",
+                        "snapshot"
                     ],
                     "default": "diff"
                 }

lib/src/conflicts.rs

@@ -240,6 +240,8 @@ pub enum ConflictMarkerStyle {
     /// Style which shows a snapshot and a series of diffs to apply.
     #[default]
     Diff,
+    /// Style which shows a snapshot for each base and side.
+    Snapshot,
 }
 
 pub fn materialize_merge_result<T: AsRef<[u8]>>(
@@ -274,7 +276,7 @@ pub fn materialize_merge_result_to_bytes<T: AsRef<[u8]>>(
 
 fn materialize_conflict_hunks(
     hunks: &[Merge<BString>],
-    _conflict_marker_style: ConflictMarkerStyle,
+    conflict_marker_style: ConflictMarkerStyle,
     output: &mut dyn Write,
 ) -> io::Result<()> {
     // Write a positive snapshot (side) of a conflict
@@ -338,6 +340,15 @@ fn materialize_conflict_hunks(
                     write_base(&base_str, left, output)?;
                     continue;
                 };
+
+                // For any style other than "diff", always emit sides and bases separately
+                if conflict_marker_style != ConflictMarkerStyle::Diff {
+                    write_side(add_index, right1, output)?;
+                    write_base(&base_str, left, output)?;
+                    add_index += 1;
+                    continue;
+                }
+
                 let diff1 = Diff::by_line([&left, &right1]).hunks().collect_vec();
                 // Check if the diff against the next positive term is better. Since
                 // we want to preserve the order of the terms, we don't match against

lib/tests/test_conflicts.rs

@@ -116,6 +116,145 @@ fn test_materialize_conflict_basic() {
     line 5
     "###
     );
+    // Test materializing "snapshot" conflict markers
+    let conflict = Merge::from_removes_adds(
+        vec![Some(base_id.clone())],
+        vec![Some(left_id.clone()), Some(right_id.clone())],
+    );
+    insta::assert_snapshot!(
+        &materialize_conflict_string(store, path, &conflict, ConflictMarkerStyle::Snapshot),
+        @r##"
+    line 1
+    line 2
+    <<<<<<< Conflict 1 of 1
+    +++++++ Contents of side #1
+    left 3.1
+    left 3.2
+    left 3.3
+    ------- Contents of base
+    line 3
+    +++++++ Contents of side #2
+    right 3.1
+    >>>>>>> Conflict 1 of 1 ends
+    line 4
+    line 5
+    "##
+    );
+}
+
+#[test]
+fn test_materialize_conflict_three_sides() {
+    let test_repo = TestRepo::init();
+    let store = test_repo.repo.store();
+
+    let path = RepoPath::from_internal_string("file");
+    let base_1_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 base
+            line 3 base
+            line 4 base
+            line 5
+        "},
+    );
+    let base_2_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 base
+            line 5
+        "},
+    );
+    let a_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 a.1
+            line 3 a.2
+            line 4 base
+            line 5
+        "},
+    );
+    let b_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 b.1
+            line 3 base
+            line 4 b.2
+            line 5
+        "},
+    );
+    let c_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 base
+            line 3 c.2
+            line 5
+        "},
+    );
+
+    let conflict = Merge::from_removes_adds(
+        vec![Some(base_1_id.clone()), Some(base_2_id.clone())],
+        vec![Some(a_id.clone()), Some(b_id.clone()), Some(c_id.clone())],
+    );
+    // Test materializing "diff" conflict markers
+    insta::assert_snapshot!(
+        &materialize_conflict_string(store, path, &conflict, ConflictMarkerStyle::Diff),
+        @r##"
+    line 1
+    <<<<<<< Conflict 1 of 1
+    %%%%%%% Changes from base #1 to side #1
+    -line 2 base
+    -line 3 base
+    +line 2 a.1
+    +line 3 a.2
+     line 4 base
+    +++++++ Contents of side #2
+    line 2 b.1
+    line 3 base
+    line 4 b.2
+    %%%%%%% Changes from base #2 to side #3
+     line 2 base
+    +line 3 c.2
+    >>>>>>> Conflict 1 of 1 ends
+    line 5
+    "##
+    );
+    // Test materializing "snapshot" conflict markers
+    insta::assert_snapshot!(
+        &materialize_conflict_string(store, path, &conflict, ConflictMarkerStyle::Snapshot),
+        @r##"
+    line 1
+    <<<<<<< Conflict 1 of 1
+    +++++++ Contents of side #1
+    line 2 a.1
+    line 3 a.2
+    line 4 base
+    ------- Contents of base #1
+    line 2 base
+    line 3 base
+    line 4 base
+    +++++++ Contents of side #2
+    line 2 b.1
+    line 3 base
+    line 4 b.2
+    ------- Contents of base #2
+    line 2 base
+    +++++++ Contents of side #3
+    line 2 base
+    line 3 c.2
+    >>>>>>> Conflict 1 of 1 ends
+    line 5
+    "##
+    );
 }
 
 #[test]
@@ -341,6 +480,72 @@ fn test_materialize_parse_roundtrip() {
     "###);
 }
 
+#[test]
+fn test_materialize_parse_roundtrip_different_markers() {
+    let test_repo = TestRepo::init();
+    let store = test_repo.repo.store();
+
+    let path = RepoPath::from_internal_string("file");
+    let base_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 base
+            line 3 base
+            line 4 base
+            line 5
+        "},
+    );
+    let a_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 a.1
+            line 3 a.2
+            line 4 base
+            line 5
+        "},
+    );
+    let b_id = testutils::write_file(
+        store,
+        path,
+        indoc! {"
+            line 1
+            line 2 b.1
+            line 3 base
+            line 4 b.2
+            line 5
+        "},
+    );
+
+    let conflict = Merge::from_removes_adds(
+        vec![Some(base_id.clone())],
+        vec![Some(a_id.clone()), Some(b_id.clone())],
+    );
+
+    let all_styles = [ConflictMarkerStyle::Diff, ConflictMarkerStyle::Snapshot];
+
+    // For every pair of conflict marker styles, materialize the conflict using the
+    // first style and parse it using the second. It should return the same result
+    // regardless of the conflict markers used for materialization and parsing.
+    for materialize_style in all_styles {
+        let materialized = materialize_conflict_string(store, path, &conflict, materialize_style);
+        for parse_style in all_styles {
+            let parsed =
+                update_from_content(&conflict, store, path, materialized.as_bytes(), parse_style)
+                    .block_on()
+                    .unwrap();
+
+            assert_eq!(
+                parsed, conflict,
+                "parse {materialize_style:?} conflict markers with {parse_style:?}"
+            );
+        }
+    }
+}
+
 #[test]
 fn test_materialize_conflict_no_newlines_at_eof() {
     let test_repo = TestRepo::init();
@@ -615,6 +820,84 @@ fn test_parse_conflict_simple() {
     )
     "###
     );
+    // Test "snapshot" style
+    insta::assert_debug_snapshot!(
+        parse_conflict(indoc! {b"
+            line 1
+            <<<<<<< Random text
+            +++++++ Random text
+            line 3.1
+            line 3.2
+            ------- Random text
+            line 3
+            line 4
+            +++++++ Random text
+            line 3
+            line 4.1
+            >>>>>>> Random text
+            line 5
+            "},
+            2
+        ),
+        @r#"
+    Some(
+        [
+            Resolved(
+                "line 1\n",
+            ),
+            Conflicted(
+                [
+                    "line 3.1\nline 3.2\n",
+                    "line 3\nline 4\n",
+                    "line 3\nline 4.1\n",
+                ],
+            ),
+            Resolved(
+                "line 5\n",
+            ),
+        ],
+    )
+    "#
+    );
+    // Test "snapshot" style with reordered sections
+    insta::assert_debug_snapshot!(
+        parse_conflict(indoc! {b"
+            line 1
+            <<<<<<< Random text
+            ------- Random text
+            line 3
+            line 4
+            +++++++ Random text
+            line 3.1
+            line 3.2
+            +++++++ Random text
+            line 3
+            line 4.1
+            >>>>>>> Random text
+            line 5
+            "},
+            2
+        ),
+        @r#"
+    Some(
+        [
+            Resolved(
+                "line 1\n",
+            ),
+            Conflicted(
+                [
+                    "line 3.1\nline 3.2\n",
+                    "line 3\nline 4\n",
+                    "line 3\nline 4.1\n",
+                ],
+            ),
+            Resolved(
+                "line 5\n",
+            ),
+        ],
+    )
+    "#
+    );
     // The conflict markers are too long and shouldn't parse (though we may
     // decide to change this in the future)
     insta::assert_debug_snapshot!(
@@ -726,6 +1009,52 @@ fn test_parse_conflict_multi_way() {
     )
     "###
     );
+    // Test "snapshot" style
+    insta::assert_debug_snapshot!(
+        parse_conflict(indoc! {b"
+            line 1
+            <<<<<<< Random text
+            +++++++ Random text
+            line 3.1
+            line 3.2
+            +++++++ Random text
+            line 3
+            line 4.1
+            ------- Random text
+            line 3
+            line 4
+            ------- Random text
+            line 3
+            +++++++ Random text
+            line 3
+            line 4
+            >>>>>>> Random text
+            line 5
+            "},
+            3
+        ),
+        @r#"
+    Some(
+        [
+            Resolved(
+                "line 1\n",
+            ),
+            Conflicted(
+                [
+                    "line 3.1\nline 3.2\n",
+                    "line 3\nline 4\n",
+                    "line 3\nline 4.1\n",
+                    "line 3\n",
+                    "line 3\nline 4\n",
+                ],
+            ),
+            Resolved(
+                "line 5\n",
+            ),
+        ],
+    )
+    "#
+    );
 }
 
 #[test]
@@ -906,6 +1235,28 @@ fn test_parse_conflict_malformed_diff() {
     );
 }
 
+#[test]
+fn test_parse_conflict_snapshot_missing_header() {
+    // The "+++++++" header is missing
+    assert_eq!(
+        parse_conflict(
+            indoc! {b"
+            line 1
+            <<<<<<<
+            left
+            -------
+            base
+            +++++++
+            right
+            >>>>>>>
+            line 5
+            "},
+            2
+        ),
+        None
+    );
+}
+
 #[test]
 fn test_update_conflict_from_content() {
     let test_repo = TestRepo::init();