Some more thought about whitespace and empty input (#427)

Also partially done with experimentation using `git rev-parse`.

git-revision/src/spec.rs

@@ -32,7 +32,9 @@ pub mod parse {
     /// - **Navigation** - where to go once from the initial revision.
     /// - **range** - to learn if the specification is for a single or multiple references.
     pub trait Delegate {
-        fn resolve_ref(&mut self, input: &BStr) -> Option<()>;
+        /// Resolve `name` as reference which might not be a valid reference name. The name may be partial like `main` or full like
+        /// `refs/heads/main` solely depending on the users input.
+        fn resolve_ref(&mut self, name: &BStr) -> Option<()>;
         fn find_by_prefix(&mut self, input: &BStr) -> Option<()>;
 
         fn nth_ancestor(&mut self, n: usize) -> Option<()>;

git-revision/tests/spec/mod.rs

@@ -6,6 +6,7 @@ mod parse {
     struct Recorder {
         resolve_ref_input: Option<BString>,
         kind: Option<spec::Kind>,
+        calls: usize,
     }
     impl spec::parse::Delegate for Recorder {
         fn resolve_ref(&mut self, input: &BStr) -> Option<()> {
@@ -15,6 +16,7 @@ mod parse {
                 input
             );
             self.resolve_ref_input = input.to_owned().into();
+            self.calls += 1;
             Some(())
         }
 
@@ -31,6 +33,7 @@ mod parse {
         }
 
         fn kind(&mut self, kind: spec::Kind) {
+            self.calls += 1;
             self.kind = Some(kind);
         }
     }
@@ -41,6 +44,26 @@ mod parse {
         rec
     }
 
+    #[test]
+    #[ignore]
+    fn empty_specs_are_valid() {
+        // they should of course be invalid for the delegate. CLIs may pre-process the input as well if they wish
+        // but git itself doesn't do that.
+        for spec in ["", " ", "\n\t"] {
+            let rec = parse(spec);
+            assert_eq!(rec.calls, 0);
+        }
+    }
+
+    #[test]
+    #[ignore]
+    fn all_characters_are_taken_verbatim_which_includes_whitespace() {
+        let spec = "  HEAD \n";
+        let rec = parse(spec);
+        assert!(rec.kind.is_none());
+        assert_eq!(rec.resolve_ref_input.unwrap(), spec);
+    }
+
     #[test]
     #[ignore]
     fn leading_caret_is_range_kind() {