copies-rust: rewrite ChangedFiles binary parsing

by using the new from-bytes-safe crate and a custom struct
that encodes the expected data structure.

Differential Revision: https://phab.mercurial-scm.org/D10068

rust/hg-core/src/copy_tracing.rs

@@ -1,761 +1,673 b''
 use crate::utils::hg_path::HgPath;
 use crate::utils::hg_path::HgPathBuf;
 use crate::Revision;
 use crate::NULL_REVISION;
 
+use bytes_cast::{unaligned, BytesCast};
 use im_rc::ordmap::Entry;
 use im_rc::ordmap::OrdMap;
 use im_rc::OrdSet;
 
 use std::cmp::Ordering;
 use std::collections::HashMap;
-use std::convert::TryInto;
 
 pub type PathCopies = HashMap<HgPathBuf, HgPathBuf>;
 
 type PathToken = usize;
 
 #[derive(Clone, Debug)]
 struct CopySource {
     /// revision at which the copy information was added
     rev: Revision,
     /// the copy source, (Set to None in case of deletion of the associated
     /// key)
     path: Option<PathToken>,
     /// a set of previous `CopySource.rev` value directly or indirectly
     /// overwritten by this one.
     overwritten: OrdSet<Revision>,
 }
 
 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,
             overwritten: OrdSet::new(),
         }
     }
 
     /// 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 {
         let mut overwritten = OrdSet::new();
         overwritten.extend(winner.overwritten.iter().copied());
         overwritten.extend(loser.overwritten.iter().copied());
         overwritten.insert(winner.rev);
         overwritten.insert(loser.rev);
         Self {
             rev,
             path: winner.path,
             overwritten: overwritten,
         }
     }
 
     /// 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.overwritten.insert(self.rev);
         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.overwritten.insert(self.rev);
         self.rev = rev;
         self.path = None;
     }
 
     /// Mark pre-existing copy information as "dropped" by a file deletion
     ///
     /// Use this when recording copy information from  parent → child edges
     fn mark_delete_with_pair(&mut self, rev: Revision, other: &Self) {
         self.overwritten.insert(self.rev);
         if other.rev != rev {
             self.overwritten.insert(other.rev);
         }
         self.overwritten.extend(other.overwritten.iter().copied());
         self.rev = rev;
         self.path = None;
     }
 
     fn is_overwritten_by(&self, other: &Self) -> bool {
         other.overwritten.contains(&self.rev)
     }
 }
 
 // For the same "dest", content generated for a given revision will always be
 // the same.
 impl PartialEq for CopySource {
     fn eq(&self, other: &Self) -> bool {
         #[cfg(debug_assertions)]
         {
             if self.rev == other.rev {
                 debug_assert!(self.path == other.path);
                 debug_assert!(self.overwritten == other.overwritten);
             }
         }
         self.rev == other.rev
     }
 }
 
 /// maps CopyDestination to Copy Source (+ a "timestamp" for the operation)
 type InternalPathCopies = OrdMap<PathToken, CopySource>;
 
-/// represent the files affected by a changesets
-///
-/// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
-/// all the data categories tracked by it.
-/// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
-/// all the data categories tracked by it.
-pub struct ChangedFiles<'a> {
-    nb_items: u32,
-    index: &'a [u8],
-    data: &'a [u8],
-}
-
 /// Represent active changes that affect the copy tracing.
 enum Action<'a> {
     /// The parent ? children edge is removing a file
     ///
     /// (actually, this could be the edge from the other parent, but it does
     /// not matters)
     Removed(&'a HgPath),
     /// The parent ? children edge introduce copy information between (dest,
     /// source)
     CopiedFromP1(&'a HgPath, &'a HgPath),
     CopiedFromP2(&'a HgPath, &'a HgPath),
 }
 
 /// This express the possible "special" case we can get in a merge
 ///
 /// See mercurial/metadata.py for details on these values.
 #[derive(PartialEq)]
 enum MergeCase {
     /// Merged: file had history on both side that needed to be merged
     Merged,
     /// Salvaged: file was candidate for deletion, but survived the merge
     Salvaged,
     /// Normal: Not one of the two cases above
     Normal,
 }
 
-type FileChange<'a> = (u8, &'a HgPath, &'a HgPath);
-
-const EMPTY: &[u8] = b"";
 const COPY_MASK: u8 = 3;
 const P1_COPY: u8 = 2;
 const P2_COPY: u8 = 3;
 const ACTION_MASK: u8 = 28;
 const REMOVED: u8 = 12;
 const MERGED: u8 = 8;
 const SALVAGED: u8 = 16;
 
-impl<'a> ChangedFiles<'a> {
-    const INDEX_START: usize = 4;
-    const ENTRY_SIZE: u32 = 9;
-    const FILENAME_START: u32 = 1;
-    const COPY_SOURCE_START: u32 = 5;
+#[derive(BytesCast)]
+#[repr(C)]
+struct ChangedFilesIndexEntry {
+    flags: u8,
 
-    pub fn new(data: &'a [u8]) -> Self {
-        assert!(
-            data.len() >= 4,
-            "data size ({}) is too small to contain the header (4)",
-            data.len()
-        );
-        let nb_items_raw: [u8; 4] = (&data[0..=3])
-            .try_into()
-            .expect("failed to turn 4 bytes into 4 bytes");
-        let nb_items = u32::from_be_bytes(nb_items_raw);
+    /// Only the end position is stored. The start is at the end of the
+    /// previous entry.
+    destination_path_end_position: unaligned::U32Be,
 
-        let index_size = (nb_items * Self::ENTRY_SIZE) as usize;
-        let index_end = Self::INDEX_START + index_size;
+    source_index_entry_position: unaligned::U32Be,
+}
+
+fn _static_assert_size_of() {
+    let _ = std::mem::transmute::<ChangedFilesIndexEntry, [u8; 9]>;
+}
 
-        assert!(
-            data.len() >= index_end,
-            "data size ({}) is too small to fit the index_data ({})",
-            data.len(),
-            index_end
-        );
+/// Represents the files affected by a changeset.
+///
+/// This holds a subset of `mercurial.metadata.ChangingFiles` as we do not need
+/// all the data categories tracked by it.
+pub struct ChangedFiles<'a> {
+    index: &'a [ChangedFilesIndexEntry],
+    paths: &'a [u8],
+}
 
-        let ret = ChangedFiles {
-            nb_items,
-            index: &data[Self::INDEX_START..index_end],
-            data: &data[index_end..],
-        };
-        let max_data = ret.filename_end(nb_items - 1) as usize;
-        assert!(
-            ret.data.len() >= max_data,
-            "data size ({}) is too small to fit all data ({})",
-            data.len(),
-            index_end + max_data
-        );
-        ret
+impl<'a> ChangedFiles<'a> {
+    pub fn new(data: &'a [u8]) -> Self {
+        let (header, rest) = unaligned::U32Be::from_bytes(data).unwrap();
+        let nb_index_entries = header.get() as usize;
+        let (index, paths) =
+            ChangedFilesIndexEntry::slice_from_bytes(rest, nb_index_entries)
+                .unwrap();
+        Self { index, paths }
     }
 
     pub fn new_empty() -> Self {
         ChangedFiles {
-            nb_items: 0,
-            index: EMPTY,
-            data: EMPTY,
+            index: &[],
+            paths: &[],
         }
     }
 
-    /// internal function to return an individual entry at a given index
-    fn entry(&'a self, idx: u32) -> FileChange<'a> {
-        if idx >= self.nb_items {
-            panic!(
-                "index for entry is higher that the number of file {} >= {}",
-                idx, self.nb_items
-            )
-        }
-        let flags = self.flags(idx);
-        let filename = self.filename(idx);
-        let copy_idx = self.copy_idx(idx);
-        let copy_source = self.filename(copy_idx);
-        (flags, filename, copy_source)
-    }
-
-    /// internal function to return the filename of the entry at a given index
-    fn filename(&self, idx: u32) -> &HgPath {
-        let filename_start;
-        if idx == 0 {
-            filename_start = 0;
+    /// Internal function to return the filename of the entry at a given index
+    fn path(&self, idx: usize) -> &HgPath {
+        let start = if idx == 0 {
+            0
         } else {
-            filename_start = self.filename_end(idx - 1)
-        }
-        let filename_end = self.filename_end(idx);
-        let filename_start = filename_start as usize;
-        let filename_end = filename_end as usize;
-        HgPath::new(&self.data[filename_start..filename_end])
-    }
-
-    /// internal function to return the flag field of the entry at a given
-    /// index
-    fn flags(&self, idx: u32) -> u8 {
-        let idx = idx as usize;
-        self.index[idx * (Self::ENTRY_SIZE as usize)]
-    }
-
-    /// internal function to return the end of a filename part at a given index
-    fn filename_end(&self, idx: u32) -> u32 {
-        let start = (idx * Self::ENTRY_SIZE) + Self::FILENAME_START;
-        let end = (idx * Self::ENTRY_SIZE) + Self::COPY_SOURCE_START;
-        let start = start as usize;
-        let end = end as usize;
-        let raw = (&self.index[start..end])
-            .try_into()
-            .expect("failed to turn 4 bytes into 4 bytes");
-        u32::from_be_bytes(raw)
-    }
-
-    /// internal function to return index of the copy source of the entry at a
-    /// given index
-    fn copy_idx(&self, idx: u32) -> u32 {
-        let start = (idx * Self::ENTRY_SIZE) + Self::COPY_SOURCE_START;
-        let end = (idx + 1) * Self::ENTRY_SIZE;
-        let start = start as usize;
-        let end = end as usize;
-        let raw = (&self.index[start..end])
-            .try_into()
-            .expect("failed to turn 4 bytes into 4 bytes");
-        u32::from_be_bytes(raw)
+            self.index[idx - 1].destination_path_end_position.get() as usize
+        };
+        let end = self.index[idx].destination_path_end_position.get() as usize;
+        HgPath::new(&self.paths[start..end])
     }
 
     /// Return an iterator over all the `Action` in this instance.
-    fn iter_actions(&self) -> ActionsIterator {
-        ActionsIterator {
-            changes: &self,
-            current: 0,
-        }
+    fn iter_actions(&self) -> impl Iterator<Item = Action> {
+        self.index.iter().enumerate().flat_map(move |(idx, entry)| {
+            let path = self.path(idx);
+            if (entry.flags & ACTION_MASK) == REMOVED {
+                Some(Action::Removed(path))
+            } else if (entry.flags & COPY_MASK) == P1_COPY {
+                let source_idx =
+                    entry.source_index_entry_position.get() as usize;
+                Some(Action::CopiedFromP1(path, self.path(source_idx)))
+            } else if (entry.flags & COPY_MASK) == P2_COPY {
+                let source_idx =
+                    entry.source_index_entry_position.get() as usize;
+                Some(Action::CopiedFromP2(path, self.path(source_idx)))
+            } else {
+                None
+            }
+        })
     }
 
     /// return the MergeCase value associated with a filename
     fn get_merge_case(&self, path: &HgPath) -> MergeCase {
-        if self.nb_items == 0 {
+        if self.index.is_empty() {
             return MergeCase::Normal;
         }
         let mut low_part = 0;
-        let mut high_part = self.nb_items;
+        let mut high_part = self.index.len();
 
         while low_part < high_part {
             let cursor = (low_part + high_part - 1) / 2;
-            let (flags, filename, _source) = self.entry(cursor);
-            match path.cmp(filename) {
+            match path.cmp(self.path(cursor)) {
                 Ordering::Less => low_part = cursor + 1,
                 Ordering::Greater => high_part = cursor,
                 Ordering::Equal => {
-                    return match flags & ACTION_MASK {
+                    return match self.index[cursor].flags & ACTION_MASK {
                         MERGED => MergeCase::Merged,
                         SALVAGED => MergeCase::Salvaged,
                         _ => MergeCase::Normal,
                     };
                 }
             }
         }
         MergeCase::Normal
     }
 }
 
-struct ActionsIterator<'a> {
-    changes: &'a ChangedFiles<'a>,
-    current: u32,
-}
-
-impl<'a> Iterator for ActionsIterator<'a> {
-    type Item = Action<'a>;
-
-    fn next(&mut self) -> Option<Action<'a>> {
-        while self.current < self.changes.nb_items {
-            let (flags, file, source) = self.changes.entry(self.current);
-            self.current += 1;
-            if (flags & ACTION_MASK) == REMOVED {
-                return Some(Action::Removed(file));
-            }
-            let copy = flags & COPY_MASK;
-            if copy == P1_COPY {
-                return Some(Action::CopiedFromP1(file, source));
-            } else if copy == P2_COPY {
-                return Some(Action::CopiedFromP2(file, source));
-            }
-        }
-        return None;
-    }
-}
-
 /// A small "tokenizer" responsible of turning full HgPath into lighter
 /// PathToken
 ///
 /// Dealing with small object, like integer is much faster, so HgPath input are
 /// turned into integer "PathToken" and converted back in the end.
 #[derive(Clone, Debug, Default)]
 struct TwoWayPathMap {
     token: HashMap<HgPathBuf, PathToken>,
     path: Vec<HgPathBuf>,
 }
 
 impl TwoWayPathMap {
     fn tokenize(&mut self, path: &HgPath) -> PathToken {
         match self.token.get(path) {
             Some(a) => *a,
             None => {
                 let a = self.token.len();
                 let buf = path.to_owned();
                 self.path.push(buf.clone());
                 self.token.insert(buf, a);
                 a
             }
         }
     }
 
     fn untokenize(&self, token: PathToken) -> &HgPathBuf {
         assert!(token < self.path.len(), "Unknown token: {}", token);
         &self.path[token]
     }
 }
 
 /// Same as mercurial.copies._combine_changeset_copies, but in Rust.
 pub struct CombineChangesetCopies {
     all_copies: HashMap<Revision, InternalPathCopies>,
     path_map: TwoWayPathMap,
     children_count: HashMap<Revision, usize>,
 }
 
 impl CombineChangesetCopies {
     pub fn new(children_count: HashMap<Revision, usize>) -> Self {
         Self {
             all_copies: HashMap::new(),
             path_map: TwoWayPathMap::default(),
             children_count,
         }
     }
 
     /// Combined the given `changes` data specific to `rev` with the data
     /// previously given for its parents (and transitively, its ancestors).
     pub fn add_revision(
         &mut self,
         rev: Revision,
         p1: Revision,
         p2: Revision,
         changes: ChangedFiles<'_>,
     ) {
         self.add_revision_inner(rev, p1, p2, changes.iter_actions(), |path| {
             changes.get_merge_case(path)
         })
     }
 
     /// Separated out from `add_revsion` so that unit tests can call this
     /// without synthetizing a `ChangedFiles` in binary format.
     fn add_revision_inner<'a>(
         &mut self,
         rev: Revision,
         p1: Revision,
         p2: Revision,
         copy_actions: impl Iterator<Item = Action<'a>>,
         get_merge_case: impl Fn(&HgPath) -> MergeCase + Copy,
     ) {
         // Retrieve data computed in a previous iteration
         let p1_copies = match p1 {
             NULL_REVISION => None,
             _ => get_and_clean_parent_copies(
                 &mut self.all_copies,
                 &mut self.children_count,
                 p1,
             ), // will be None if the vertex is not to be traversed
         };
         let p2_copies = match p2 {
             NULL_REVISION => None,
             _ => get_and_clean_parent_copies(
                 &mut self.all_copies,
                 &mut self.children_count,
                 p2,
             ), // will be None if the vertex is not to be traversed
         };
         // combine it with data for that revision
         let (p1_copies, p2_copies) = chain_changes(
             &mut self.path_map,
             p1_copies,
             p2_copies,
             copy_actions,
             rev,
         );
         let copies = match (p1_copies, p2_copies) {
             (None, None) => None,
             (c, None) => c,
             (None, c) => c,
             (Some(p1_copies), Some(p2_copies)) => Some(merge_copies_dict(
                 &self.path_map,
                 rev,
                 p2_copies,
                 p1_copies,
                 get_merge_case,
             )),
         };
         if let Some(c) = copies {
             self.all_copies.insert(rev, c);
         }
     }
 
     /// Drop intermediate data (such as which revision a copy was from) and
     /// return the final mapping.
     pub fn finish(mut self, target_rev: Revision) -> PathCopies {
         let tt_result = self
             .all_copies
             .remove(&target_rev)
             .expect("target revision was not processed");
         let mut result = PathCopies::default();
         for (dest, tt_source) in tt_result {
             if let Some(path) = tt_source.path {
                 let path_dest = self.path_map.untokenize(dest).to_owned();
                 let path_path = self.path_map.untokenize(path).to_owned();
                 result.insert(path_dest, path_path);
             }
         }
         result
     }
 }
 
 /// fetch previous computed information
 ///
 /// If no other children are expected to need this information, we drop it from
 /// the cache.
 ///
 /// If parent is not part of the set we are expected to walk, return None.
 fn get_and_clean_parent_copies(
     all_copies: &mut HashMap<Revision, InternalPathCopies>,
     children_count: &mut HashMap<Revision, usize>,
     parent_rev: Revision,
 ) -> Option<InternalPathCopies> {
     let count = children_count.get_mut(&parent_rev)?;
     *count -= 1;
     if *count == 0 {
         match all_copies.remove(&parent_rev) {
             Some(c) => Some(c),
             None => Some(InternalPathCopies::default()),
         }
     } else {
         match all_copies.get(&parent_rev) {
             Some(c) => Some(c.clone()),
             None => Some(InternalPathCopies::default()),
         }
     }
 }
 
 /// Combine ChangedFiles with some existing PathCopies information and return
 /// the result
 fn chain_changes<'a>(
     path_map: &mut TwoWayPathMap,
     base_p1_copies: Option<InternalPathCopies>,
     base_p2_copies: Option<InternalPathCopies>,
     copy_actions: impl Iterator<Item = Action<'a>>,
     current_rev: Revision,
 ) -> (Option<InternalPathCopies>, Option<InternalPathCopies>) {
     // Fast path the "nothing to do" case.
     if let (None, None) = (&base_p1_copies, &base_p2_copies) {
         return (None, None);
     }
 
     let mut p1_copies = base_p1_copies.clone();
     let mut p2_copies = base_p2_copies.clone();
     for action in copy_actions {
         match action {
             Action::CopiedFromP1(path_dest, path_source) => {
                 match &mut p1_copies {
                     None => (), // This is not a vertex we should proceed.
                     Some(copies) => add_one_copy(
                         current_rev,
                         path_map,
                         copies,
                         base_p1_copies.as_ref().unwrap(),
                         path_dest,
                         path_source,
                     ),
                 }
             }
             Action::CopiedFromP2(path_dest, path_source) => {
                 match &mut p2_copies {
                     None => (), // This is not a vertex we should proceed.
                     Some(copies) => add_one_copy(
                         current_rev,
                         path_map,
                         copies,
                         base_p2_copies.as_ref().unwrap(),
                         path_dest,
                         path_source,
                     ),
                 }
             }
             Action::Removed(deleted_path) => {
                 // We must drop copy information for removed file.
                 //
                 // We need to explicitly record them as dropped to
                 // propagate this information when merging two
                 // InternalPathCopies object.
                 let deleted = path_map.tokenize(deleted_path);
 
                 let p1_entry = match &mut p1_copies {
                     None => None,
                     Some(copies) => match copies.entry(deleted) {
                         Entry::Occupied(e) => Some(e),
                         Entry::Vacant(_) => None,
                     },
                 };
                 let p2_entry = match &mut p2_copies {
                     None => None,
                     Some(copies) => match copies.entry(deleted) {
                         Entry::Occupied(e) => Some(e),
                         Entry::Vacant(_) => None,
                     },
                 };
 
                 match (p1_entry, p2_entry) {
                     (None, None) => (),
                     (Some(mut e), None) => {
                         e.get_mut().mark_delete(current_rev)
                     }
                     (None, Some(mut e)) => {
                         e.get_mut().mark_delete(current_rev)
                     }
                     (Some(mut e1), Some(mut e2)) => {
                         let cs1 = e1.get_mut();
                         let cs2 = e2.get();
                         if cs1 == cs2 {
                             cs1.mark_delete(current_rev);
                         } else {
                             cs1.mark_delete_with_pair(current_rev, &cs2);
                         }
                         e2.insert(cs1.clone());
                     }
                 }
             }
         }
     }
     (p1_copies, p2_copies)
 }
 
 // insert one new copy information in an InternalPathCopies
 //
 // This deal with chaining and overwrite.
 fn add_one_copy(
     current_rev: Revision,
     path_map: &mut TwoWayPathMap,
     copies: &mut InternalPathCopies,
     base_copies: &InternalPathCopies,
     path_dest: &HgPath,
     path_source: &HgPath,
 ) {
     let dest = path_map.tokenize(path_dest);
     let source = path_map.tokenize(path_source);
     let entry;
     if let Some(v) = base_copies.get(&source) {
         entry = match &v.path {
             Some(path) => Some((*(path)).to_owned()),
             None => Some(source.to_owned()),
         }
     } else {
         entry = Some(source.to_owned());
     }
     // Each new entry is introduced by the children, we
     // record this information as we will need it to take
     // the right decision when merging conflicting copy
     // information. See merge_copies_dict for details.
     match copies.entry(dest) {
         Entry::Vacant(slot) => {
             let ttpc = CopySource::new(current_rev, entry);
             slot.insert(ttpc);
         }
         Entry::Occupied(mut slot) => {
             let ttpc = slot.get_mut();
             ttpc.overwrite(current_rev, entry);
         }
     }
 }
 
 /// merge two copies-mapping together, minor and major
 ///
 /// In case of conflict, value from "major" will be picked, unless in some
 /// cases. See inline documentation for details.
 fn merge_copies_dict(
     path_map: &TwoWayPathMap,
     current_merge: Revision,
     minor: InternalPathCopies,
     major: InternalPathCopies,
     get_merge_case: impl Fn(&HgPath) -> MergeCase + Copy,
 ) -> InternalPathCopies {
     use crate::utils::{ordmap_union_with_merge, MergeResult};
 
     ordmap_union_with_merge(minor, major, |&dest, src_minor, src_major| {
         let (pick, overwrite) = compare_value(
             current_merge,
             || get_merge_case(path_map.untokenize(dest)),
             src_minor,
             src_major,
         );
         if overwrite {
             let (winner, loser) = match pick {
                 MergePick::Major | MergePick::Any => (src_major, src_minor),
                 MergePick::Minor => (src_minor, src_major),
             };
             MergeResult::UseNewValue(CopySource::new_from_merge(
                 current_merge,
                 winner,
                 loser,
             ))
         } else {
             match pick {
                 MergePick::Any | MergePick::Major => {
                     MergeResult::UseRightValue
                 }
                 MergePick::Minor => MergeResult::UseLeftValue,
             }
         }
     })
 }
 
 /// represent the side that should prevail when merging two
 /// InternalPathCopies
 #[derive(Debug, PartialEq)]
 enum MergePick {
     /// The "major" (p1) side prevails
     Major,
     /// The "minor" (p2) side prevails
     Minor,
     /// Any side could be used (because they are the same)
     Any,
 }
 
 /// decide which side prevails in case of conflicting values
 #[allow(clippy::if_same_then_else)]
 fn compare_value(
     current_merge: Revision,
     merge_case_for_dest: impl Fn() -> MergeCase,
     src_minor: &CopySource,
     src_major: &CopySource,
 ) -> (MergePick, bool) {
     if src_major == src_minor {
         (MergePick::Any, false)
     } else if src_major.rev == current_merge {
         // minor is different according to per minor == major check earlier
         debug_assert!(src_minor.rev != current_merge);
 
         // The last value comes the current merge, this value -will- win
         // eventually.
         (MergePick::Major, true)
     } else if src_minor.rev == current_merge {
         // The last value comes the current merge, this value -will- win
         // eventually.
         (MergePick::Minor, true)
     } else if src_major.path == src_minor.path {
         debug_assert!(src_major.rev != src_major.rev);
         // we have the same value, but from other source;
         if src_major.is_overwritten_by(src_minor) {
             (MergePick::Minor, false)
         } else if src_minor.is_overwritten_by(src_major) {
             (MergePick::Major, false)
         } else {
             (MergePick::Any, true)
         }
     } else {
         debug_assert!(src_major.rev != src_major.rev);
         let action = merge_case_for_dest();
         if src_minor.path.is_some()
             && src_major.path.is_none()
             && action == MergeCase::Salvaged
         {
             // If the file is "deleted" in the major side but was
             // salvaged by the merge, we keep the minor side alive
             (MergePick::Minor, true)
         } else if src_major.path.is_some()
             && src_minor.path.is_none()
             && action == MergeCase::Salvaged
         {
             // If the file is "deleted" in the minor side but was
             // salvaged by the merge, unconditionnaly preserve the
             // major side.
             (MergePick::Major, true)
         } else if src_minor.is_overwritten_by(src_major) {
             // The information from the minor version are strictly older than
             // the major version
             if action == MergeCase::Merged {
                 // If the file was actively merged, its means some non-copy
                 // activity happened on the other branch. It
                 // mean the older copy information are still relevant.
                 //
                 // The major side wins such conflict.
                 (MergePick::Major, true)
             } else {
                 // No activity on the minor branch, pick the newer one.
                 (MergePick::Major, false)
             }
         } else if src_major.is_overwritten_by(src_minor) {
             if action == MergeCase::Merged {
                 // If the file was actively merged, its means some non-copy
                 // activity happened on the other branch. It
                 // mean the older copy information are still relevant.
                 //
                 // The major side wins such conflict.
                 (MergePick::Major, true)
             } else {
                 // No activity on the minor branch, pick the newer one.
                 (MergePick::Minor, false)
             }
         } else if src_minor.path.is_none() {
             // the minor side has no relevant information, pick the alive one
             (MergePick::Major, true)
         } else if src_major.path.is_none() {
             // the major side has no relevant information, pick the alive one
             (MergePick::Minor, true)
         } else {
             // by default the major side wins
             (MergePick::Major, true)
         }
     }
 }