officially make sibling branches 'planned' (#427)

git-repository/src/revision/spec/parse/delegate.rs

@@ -333,7 +333,10 @@ impl<'repo> delegate::Revision for Delegate<'repo> {
 
     fn sibling_branch(&mut self, _kind: SiblingBranch) -> Option<()> {
         self.unset_disambiguate_call();
-        todo!("once remote handling and ref-specs are fleshed out much more")
+        self.err.push(Error::Planned {
+            dependency: "remote handling and ref-specs are fleshed out more",
+        });
+        None
     }
 }
 

git-repository/src/revision/spec/parse/types.rs

@@ -61,6 +61,8 @@ pub struct Options {
 #[derive(Debug, thiserror::Error)]
 #[allow(missing_docs)]
 pub enum Error {
+    #[error("This feature will be implemented once {dependency}")]
+    Planned { dependency: &'static str },
     #[error("Reference {reference:?} does not have a reference log, cannot {action}")]
     MissingRefLog { reference: BString, action: &'static str },
     #[error("HEAD has {available} prior checkouts and checkout number {desired} is out of range")]

git-repository/tests/revision/spec/from_bytes/mod.rs

@@ -11,6 +11,25 @@ mod regex;
 mod reflog;
 mod traverse;
 
+mod sibling_branch {
+    use crate::revision::spec::from_bytes::{parse_spec_no_baseline, repo};
+    use git_repository::revision::spec::parse::Error;
+
+    #[test]
+    fn is_planned_and_delayed_until_remotes_are_sorted() {
+        let repo = repo("complex_graph").unwrap();
+        assert!(matches!(
+            parse_spec_no_baseline("main@{push}", &repo).unwrap_err(),
+            Error::Planned { .. }
+        ));
+
+        assert!(matches!(
+            parse_spec_no_baseline("main@{upstream}", &repo).unwrap_err(),
+            Error::Planned { .. }
+        ));
+    }
+}
+
 mod index {
     use crate::revision::spec::from_bytes::{parse_spec, repo};
     use git_repository::prelude::ObjectIdExt;