copies-rust: add methods to build and update CopySource

Having explicit method with clear semantic help to clarify the code and prepare
an update to the underlying documentation without too much disruption.

Differential Revision: https://phab.mercurial-scm.org/D9643

rust/hg-core/src/copy_tracing.rs

@@ -15,7 +15,7 b' pub type PathCopies = HashMap<HgPathBuf,'
 
 type PathToken = usize;
 
-#[derive(Clone, Debug, PartialEq, Copy)]
+#[derive(Clone, Debug, PartialEq)]
 struct CopySource {
     /// revision at which the copy information was added
     rev: Revision,
@@ -24,6 +24,42 b' struct CopySource {'
     path: Option<PathToken>,
 }
 
+impl CopySource {
+    /// create a new CopySource
+    ///
+    /// Use this when no previous copy source existed.
+    fn new(rev: Revision, path: Option<PathToken>) -> Self {
+        Self { rev, path }
+    }
+
+    /// create a new CopySource from merging two others
+    ///
+    /// Use this when merging two InternalPathCopies requires active merging of
+    /// some entries.
+    fn new_from_merge(rev: Revision, winner: &Self, loser: &Self) -> Self {
+        Self {
+            rev,
+            path: winner.path,
+        }
+    }
+
+    /// Update the value of a pre-existing CopySource
+    ///
+    /// Use this when recording copy information from  parent → child edges
+    fn overwrite(&mut self, rev: Revision, path: Option<PathToken>) {
+        self.rev = rev;
+        self.path = path;
+    }
+
+    /// Mark pre-existing copy information as "dropped" by a file deletion
+    ///
+    /// Use this when recording copy information from  parent → child edges
+    fn mark_delete(&mut self, rev: Revision) {
+        self.rev = rev;
+        self.path = None;
+    }
+}
+
 /// maps CopyDestination to Copy Source (+ a "timestamp" for the operation)
 type InternalPathCopies = OrdMap<PathToken, CopySource>;
 
@@ -521,17 +557,13 b' fn add_from_changes<A: Fn(Revision, Revi'
                 // information. See merge_copies_dict for details.
                 match copies.entry(dest) {
                     Entry::Vacant(slot) => {
-                        let ttpc = CopySource {
-                            rev: current_rev,
-                            path: entry,
-                        };
+                        let ttpc = CopySource::new(current_rev, entry);
                         slot.insert(ttpc);
                     }
                     Entry::Occupied(mut slot) => {
-                        let mut ttpc = slot.get_mut();
+                        let ttpc = slot.get_mut();
                         oracle.record_overwrite(ttpc.rev, current_rev);
-                        ttpc.rev = current_rev;
-                        ttpc.path = entry;
+                        ttpc.overwrite(current_rev, entry);
                     }
                 }
             }
@@ -544,8 +576,7 b' fn add_from_changes<A: Fn(Revision, Revi'
                 let deleted = path_map.tokenize(deleted_path);
                 copies.entry(deleted).and_modify(|old| {
                     oracle.record_overwrite(old.rev, current_rev);
-                    old.rev = current_rev;
-                    old.path = None;
+                    old.mark_delete(current_rev);
                 });
             }
         }
@@ -614,14 +645,22 b' fn merge_copies_dict<A: Fn(Revision, Rev'
                     if overwrite {
                         oracle.record_overwrite(src_minor.rev, current_merge);
                         oracle.record_overwrite(src_major.rev, current_merge);
-                        let path = match pick {
-                            MergePick::Major => src_major.path,
-                            MergePick::Minor => src_minor.path,
-                            MergePick::Any => src_major.path,
-                        };
-                        let src = CopySource {
-                            rev: current_merge,
-                            path,
+                        let src = match pick {
+                            MergePick::Major => CopySource::new_from_merge(
+                                current_merge,
+                                src_major,
+                                &src_minor,
+                            ),
+                            MergePick::Minor => CopySource::new_from_merge(
+                                current_merge,
+                                &src_minor,
+                                src_major,
+                            ),
+                            MergePick::Any => CopySource::new_from_merge(
+                                current_merge,
+                                src_major,
+                                &src_minor,
+                            ),
                         };
                         major.insert(dest, src);
                     } else {
@@ -649,14 +688,22 b' fn merge_copies_dict<A: Fn(Revision, Rev'
                     if overwrite {
                         oracle.record_overwrite(src_minor.rev, current_merge);
                         oracle.record_overwrite(src_major.rev, current_merge);
-                        let path = match pick {
-                            MergePick::Major => src_minor.path,
-                            MergePick::Minor => src_major.path,
-                            MergePick::Any => src_major.path,
-                        };
-                        let src = CopySource {
-                            rev: current_merge,
-                            path,
+                        let src = match pick {
+                            MergePick::Major => CopySource::new_from_merge(
+                                current_merge,
+                                &src_major,
+                                src_minor,
+                            ),
+                            MergePick::Minor => CopySource::new_from_merge(
+                                current_merge,
+                                src_minor,
+                                &src_major,
+                            ),
+                            MergePick::Any => CopySource::new_from_merge(
+                                current_merge,
+                                &src_major,
+                                src_minor,
+                            ),
                         };
                         minor.insert(dest, src);
                     } else {
@@ -706,16 +753,22 b' fn merge_copies_dict<A: Fn(Revision, Rev'
                     if overwrite {
                         oracle.record_overwrite(src_minor.rev, current_merge);
                         oracle.record_overwrite(src_major.rev, current_merge);
-                        let path = match pick {
-                            MergePick::Major => src_major.path,
-                            MergePick::Minor => src_minor.path,
-                            // If the two entry are identical, no need to do
-                            // anything (but diff should not have yield them)
-                            MergePick::Any => src_major.path,
-                        };
-                        let src = CopySource {
-                            rev: current_merge,
-                            path,
+                        let src = match pick {
+                            MergePick::Major => CopySource::new_from_merge(
+                                current_merge,
+                                src_major,
+                                src_minor,
+                            ),
+                            MergePick::Minor => CopySource::new_from_merge(
+                                current_merge,
+                                src_minor,
+                                src_major,
+                            ),
+                            MergePick::Any => CopySource::new_from_merge(
+                                current_merge,
+                                src_major,
+                                src_minor,
+                            ),
                         };
                         to_minor(dest, &src);
                         to_major(dest, &src);