feat: add Repository::index_or_load_from_head_or_empty().

It's useful to get a reasonable index in any case, even on unborn repositories.
It's for cases where the `HEAD` isn't setup at all, despite content being available,
and to avoid unnecessary restrictions on what works.

gix/src/repository/diff.rs

@@ -19,7 +19,7 @@ impl Repository {
         mode: gix_diff::blob::pipeline::Mode,
         worktree_roots: gix_diff::blob::pipeline::WorktreeRoots,
     ) -> Result<gix_diff::blob::Platform, diff_resource_cache::Error> {
-        let index = self.index_or_load_from_head()?;
+        let index = self.index_or_load_from_head_or_empty()?;
         Ok(crate::diff::resource_cache(
             self,
             mode,

gix/src/repository/index.rs

@@ -96,8 +96,10 @@ impl crate::Repository {
     ///
     /// ### Note
     ///
-    /// The locally stored index is not guaranteed to represent `HEAD^{tree}` if this repository is bare - bare repos
-    /// don't naturally have an index and if an index is present it must have been generated by hand.
+    /// * The locally stored index is not guaranteed to represent `HEAD^{tree}` if this repository is bare - bare repos
+    ///   don't naturally have an index and if an index is present it must have been generated by hand.
+    /// * This method will fail on unborn repositories as `HEAD` doesn't point to a reference yet, which is needed to resolve
+    ///   the revspec. If that is a concern, use [`Self::index_or_load_from_head_or_empty()`] instead.
     pub fn index_or_load_from_head(
         &self,
     ) -> Result<IndexPersistedOrInMemory, crate::repository::index_or_load_from_head::Error> {
@@ -110,6 +112,29 @@ impl crate::Repository {
         })
     }
 
+    /// Open the persisted worktree index or generate it from the current `HEAD^{tree}` to live in-memory only,
+    /// or resort to an empty index if `HEAD` is unborn.
+    ///
+    /// Use this method to get an index in any repository, even bare ones that don't have one naturally, or those
+    /// that are in a state where `HEAD` is invalid or points to an unborn reference.
+    pub fn index_or_load_from_head_or_empty(
+        &self,
+    ) -> Result<IndexPersistedOrInMemory, crate::repository::index_or_load_from_head_or_empty::Error> {
+        Ok(match self.try_index()? {
+            Some(index) => IndexPersistedOrInMemory::Persisted(index),
+            None => match self.head()?.id() {
+                Some(id) => {
+                    let head_tree_id = id.object()?.peel_to_commit()?.tree_id()?;
+                    IndexPersistedOrInMemory::InMemory(self.index_from_tree(&head_tree_id)?)
+                }
+                None => IndexPersistedOrInMemory::InMemory(gix_index::File::from_state(
+                    gix_index::State::new(self.object_hash()),
+                    self.index_path(),
+                )),
+            },
+        })
+    }
+
     /// Create new index-file, which would live at the correct location, in memory from the given `tree`.
     ///
     /// Note that this is an expensive operation as it requires recursively traversing the entire tree to unpack it into the index.

gix/src/repository/merge.rs

@@ -18,7 +18,7 @@ impl Repository {
         &self,
         worktree_roots: gix_merge::blob::pipeline::WorktreeRoots,
     ) -> Result<gix_merge::blob::Platform, merge_resource_cache::Error> {
-        let index = self.index_or_load_from_head()?;
+        let index = self.index_or_load_from_head_or_empty()?;
         let mode = {
             let renormalize = self
                 .config

gix/src/repository/mod.rs

@@ -100,7 +100,7 @@ pub mod merge_resource_cache {
         #[error(transparent)]
         PipelineOptions(#[from] crate::config::merge::pipeline_options::Error),
         #[error(transparent)]
-        Index(#[from] crate::repository::index_or_load_from_head::Error),
+        Index(#[from] crate::repository::index_or_load_from_head_or_empty::Error),
         #[error(transparent)]
         AttributeStack(#[from] crate::config::attribute_stack::Error),
         #[error(transparent)]
@@ -154,7 +154,7 @@ pub mod diff_resource_cache {
         #[error("Could not obtain resource cache for diffing")]
         ResourceCache(#[from] crate::diff::resource_cache::Error),
         #[error(transparent)]
-        Index(#[from] crate::repository::index_or_load_from_head::Error),
+        Index(#[from] crate::repository::index_or_load_from_head_or_empty::Error),
         #[error(transparent)]
         AttributeStack(#[from] crate::config::attribute_stack::Error),
     }
@@ -289,7 +289,7 @@ pub mod pathspec_defaults_ignore_case {
 ///
 #[cfg(feature = "index")]
 pub mod index_or_load_from_head {
-    /// The error returned by [`Repository::index_or_load_from_head()`][crate::Repository::index_or_load_from_head()].
+    /// The error returned by [`Repository::index_or_load_from_head()`](crate::Repository::index_or_load_from_head()).
     #[derive(thiserror::Error, Debug)]
     #[allow(missing_docs)]
     pub enum Error {
@@ -304,10 +304,32 @@ pub mod index_or_load_from_head {
     }
 }
 
+///
+#[cfg(feature = "index")]
+pub mod index_or_load_from_head_or_empty {
+    /// The error returned by [`Repository::index_or_load_from_head_or_empty()`](crate::Repository::index_or_load_from_head_or_empty()).
+    #[derive(thiserror::Error, Debug)]
+    #[allow(missing_docs)]
+    pub enum Error {
+        #[error(transparent)]
+        ReadHead(#[from] crate::reference::find::existing::Error),
+        #[error(transparent)]
+        FindCommit(#[from] crate::object::find::existing::Error),
+        #[error(transparent)]
+        PeelToTree(#[from] crate::object::peel::to_kind::Error),
+        #[error(transparent)]
+        TreeId(#[from] gix_object::decode::Error),
+        #[error(transparent)]
+        TraverseTree(#[from] crate::repository::index_from_tree::Error),
+        #[error(transparent)]
+        OpenIndex(#[from] crate::worktree::open_index::Error),
+    }
+}
+
 ///
 #[cfg(feature = "worktree-stream")]
 pub mod worktree_stream {
-    /// The error returned by [`Repository::worktree_stream()`][crate::Repository::worktree_stream()].
+    /// The error returned by [`Repository::worktree_stream()`](crate::Repository::worktree_stream()).
     #[derive(Debug, thiserror::Error)]
     #[allow(missing_docs)]
     pub enum Error {
@@ -332,6 +354,6 @@ pub mod worktree_stream {
 ///
 #[cfg(feature = "worktree-archive")]
 pub mod worktree_archive {
-    /// The error returned by [`Repository::worktree_archive()`][crate::Repository::worktree_archive()].
+    /// The error returned by [`Repository::worktree_archive()`](crate::Repository::worktree_archive()).
     pub type Error = gix_archive::Error;
 }

gix/tests/fixtures/make_basic_repo.sh

@@ -41,4 +41,6 @@ git init empty-core-excludes
 git clone --bare . non-bare-without-worktree
 (cd non-bare-without-worktree
   git config core.bare false
-)
+)
+
+git init unborn;

gix/tests/gix/repository/mod.rs

@@ -17,6 +17,23 @@ mod state;
 mod submodule;
 mod worktree;
 
+#[cfg(feature = "index")]
+mod index {
+    #[test]
+    fn basics() -> crate::Result {
+        let repo = crate::named_subrepo_opts("make_basic_repo.sh", "unborn", gix::open::Options::isolated())?;
+        assert!(
+            repo.index_or_load_from_head().is_err(),
+            "can't read index if `HEAD^{{tree}}` can't be resolved"
+        );
+        assert!(
+            repo.index_or_load_from_head_or_empty()?.entries().is_empty(),
+            "an empty index is created on the fly"
+        );
+        Ok(())
+    }
+}
+
 #[cfg(feature = "dirwalk")]
 mod dirwalk {
     use gix_dir::entry::Kind::*;
@@ -44,6 +61,7 @@ mod dirwalk {
             ("non-bare-repo-without-index".into(), Repository),
             ("non-bare-without-worktree".into(), Directory),
             ("some".into(), Directory),
+            ("unborn".into(), Repository),
         ];
         assert_eq!(
             collect