Various improvements to address the PR review (part 1).

rust-lang#11840 (review)

src/cargo/sources/git/source.rs

@@ -69,6 +69,7 @@ impl<'cfg> GitSource<'cfg> {
     }
 }
 
+/// Create an identifier from a URL, essentially turning `proto://host/path/repo` into `repo-<hash-of-url>`.
 fn ident(id: &SourceId) -> String {
     let ident = id
         .canonical_url()
@@ -82,6 +83,10 @@ fn ident(id: &SourceId) -> String {
     format!("{}-{}", ident, short_hash(id.canonical_url()))
 }
 
+/// Like `ident()`, but appends `-shallow` to it, turning `proto://host/path/repo` into `repo-<hash-of-url>-shallow`.
+///
+/// It's important to separate shallow from non-shallow clones for reasons of backwards compatibility - older
+/// cargo's aren't necessarily handling shallow clones correctly.
 fn ident_shallow(id: &SourceId, is_shallow: bool) -> String {
     let mut ident = ident(id);
     if is_shallow {

src/cargo/sources/git/utils.rs

@@ -111,7 +111,7 @@ impl GitRemote {
                 cargo_config,
                 history,
             )
-            .context(format!("failed to fetch into: {}", into.display()))?;
+            .with_context(|| format!("failed to fetch into: {}", into.display()))?;
             match locked_rev {
                 Some(rev) => {
                     if db.contains(rev) {
@@ -142,7 +142,7 @@ impl GitRemote {
             cargo_config,
             history,
         )
-        .context(format!("failed to clone into: {}", into.display()))?;
+        .with_context(|| format!("failed to clone into: {}", into.display()))?;
         let rev = match locked_rev {
             Some(rev) => rev,
             None => reference.resolve(&repo)?,
@@ -302,6 +302,10 @@ impl<'a> GitCheckout<'a> {
                 .with_checkout(checkout)
                 .fetch_options(fopts)
                 .clone(url.as_str(), into)?;
+            // `git2` doesn't seem to handle shallow repos correctly when doing a local clone.
+            // Fortunately all that's needed is the copy of the one file that defines the
+            // shallow boundary, the commits which have their parents omitted as part of the
+            // shallow clone.
             if database.repo.is_shallow() {
                 std::fs::copy(
                     database.repo.path().join("shallow"),

tests/testsuite/git.rs

@@ -2183,7 +2183,6 @@ fn gitoxide_clones_git_dependency_with_shallow_protocol_and_follow_up_fetch_main
     .map(|path| -> anyhow::Result<usize> {
         let path = path?;
         let dep_checkout = gix::open_opts(&path, gix::open::Options::isolated())?;
-        dbg!(dep_checkout.git_dir());
         assert_eq!(
             dep_checkout.is_shallow(),
             path.to_string_lossy().contains("-shallow"),