conflicts: replace missing files by empty in materialized conflict

When we materialize modify/delete conflicts, we currently don't
include any context lines. That's because modify/delete conflicts have
only two sides, so there's no common base to compare to. Hunks that
are unchanged on the "modify" side are therefore not considered
conflicting, and since they they don't contribute new changes, they're
simply skipped (here:
3dfedf5814/lib/src/files.rs (L228-L230)).

It seems more useful to instead pretend that the missing side is an
empty file. That way we'll get a conflict in the entire file.

We can still decide later to make e.g. `jj resolve` prompt the user on
modify/delete conflicts just like `hg resolve` does (or maybe it
actually happens earlier there, I don't remember).

Closes #1244.

CHANGELOG.md

@@ -13,6 +13,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed bugs
 
+* Modify/delete conflicts now include context lines
+  [#1244](https://github.com/martinvonz/jj/issues/1244).
+
 ## [0.7.0] - 2023-02-16
 
 ### Breaking changes

lib/src/conflicts.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::cmp::max;
 use std::io::{Cursor, Write};
 
 use itertools::Itertools;
@@ -154,14 +155,19 @@ pub fn extract_file_conflict_as_single_hunk(
     if file_adds.len() != conflict.adds.len() || file_removes.len() != conflict.removes.len() {
         return None;
     }
-    let added_content = file_adds
+    let mut added_content = file_adds
         .iter()
         .map(|part| get_file_contents(store, path, part))
         .collect_vec();
-    let removed_content = file_removes
+    let mut removed_content = file_removes
         .iter()
         .map(|part| get_file_contents(store, path, part))
         .collect_vec();
+    // If the conflict had removed the file on one side, we pretend that the file
+    // was empty there.
+    let l = max(added_content.len(), removed_content.len() + 1);
+    added_content.resize(l, vec![]);
+    removed_content.resize(l - 1, vec![]);
 
     Some(ConflictHunk {
         removes: removed_content,

lib/tests/test_conflicts.rs

@@ -197,12 +197,16 @@ line 5
         removes: vec![file_conflict_part(&base_id)],
         adds: vec![file_conflict_part(&modified_id)],
     };
-    // TODO: THis should have context around the conflict (#1244)
     insta::assert_snapshot!(&materialize_conflict_string(store, &path, &conflict), @r###"
     <<<<<<<
     %%%%%%%
+     line 1
+     line 2
     -line 3
     +modified
+     line 4
+     line 5
+    +++++++
     >>>>>>>
     "###
     );

tests/test_obslog_command.rs

@@ -49,8 +49,10 @@ fn test_obslog_with_or_without_diff() {
     │  Resolved conflict in file1:
     │     1    1: <<<<<<<resolved
     │     2     : %%%%%%%
-    │     3     : +bar
-    │     4     : >>>>>>>
+    │     3     :  foo
+    │     4     : +bar
+    │     5     : +++++++
+    │     6     : >>>>>>>
     o  rlvkpnrzqnoo test.user@example.com 2001-02-03 04:05:09.000 +07:00 af536e5af67e conflict
     │  my description
     o  rlvkpnrzqnoo test.user@example.com 2001-02-03 04:05:09.000 +07:00 6fbba7bcb590
@@ -86,10 +88,12 @@ fn test_obslog_with_or_without_diff() {
     index e155302a24...2ab19ae607 100644
     --- a/file1
     +++ b/file1
-    @@ -1,4 +1,1 @@
+    @@ -1,6 +1,1 @@
     -<<<<<<<
     -%%%%%%%
+    - foo
     -+bar
+    -+++++++
     ->>>>>>>
     +resolved
     rlvkpnrzqnoo test.user@example.com 2001-02-03 04:05:09.000 +07:00 af536e5af67e conflict

tests/test_resolve_command.rs

@@ -356,8 +356,8 @@ fn test_baseless_conflict_input_files() {
     std::fs::read_to_string(repo_path.join("file")).unwrap()
         , @r###"
     <<<<<<<
-    +++++++
-    a
+    %%%%%%%
+    +a
     +++++++
     b
     >>>>>>>
@@ -427,15 +427,14 @@ fn test_edit_delete_conflict_input_files() {
     <<<<<<<
     %%%%%%%
     -base
-    +a
+    +++++++
+    a
     >>>>>>>
     "###);
 
     check_resolve_produces_input_file(&mut test_env, &repo_path, "base", "base\n");
-    check_resolve_produces_input_file(&mut test_env, &repo_path, "left", "");
-    // Note that `a` ended up in "right" rather than "left". It's unclear if this
-    // can or should be fixed.
-    check_resolve_produces_input_file(&mut test_env, &repo_path, "right", "a\n");
+    check_resolve_produces_input_file(&mut test_env, &repo_path, "left", "a\n");
+    check_resolve_produces_input_file(&mut test_env, &repo_path, "right", "");
 }
 
 #[test]