Optimize :(optional) prefix stripping to avoid unnecessary allocations

Co-authored-by: Byron <[email protected]>

Author: Copilot

gix-config-value/src/path.rs

@@ -112,10 +112,16 @@ impl<'a> From<Cow<'a, BStr>> for Path<'a> {
 
         // Check if the value starts with ":(optional)" prefix
         if value.starts_with(OPTIONAL_PREFIX) {
-            // Strip the prefix and create a path marked as optional
-            let stripped = &value[OPTIONAL_PREFIX.len()..];
+            // Strip the prefix while preserving the Cow variant to avoid unnecessary allocations
+            let stripped = match value {
+                Cow::Borrowed(b) => Cow::Borrowed(&b[OPTIONAL_PREFIX.len()..]),
+                Cow::Owned(mut b) => {
+                    b.drain(..OPTIONAL_PREFIX.len());
+                    Cow::Owned(b)
+                }
+            };
             Path {
-                value: Cow::Owned(stripped.into()),
+                value: stripped,
                 is_optional: true,
             }
         } else {

gix-config-value/tests/value/path.rs

@@ -131,6 +131,7 @@ mod interpolate {
 mod optional_prefix {
     use std::borrow::Cow;
 
+    use bstr::ByteSlice;
     use crate::{b, cow_str};
 
     #[test]
@@ -215,4 +216,28 @@ mod optional_prefix {
 
         Ok(())
     }
+
+    #[test]
+    fn borrowed_path_stays_borrowed_after_prefix_stripping() {
+        // Verify that we don't unnecessarily allocate when stripping the prefix from borrowed data
+        let borrowed_input: &[u8] = b":(optional)/some/path";
+        let path = gix_config_value::Path::from(Cow::Borrowed(borrowed_input.as_bstr()));
+        
+        assert!(path.is_optional());
+        assert_eq!(path.value.as_ref(), b"/some/path");
+        // Verify it's still borrowed (no unnecessary allocation)
+        assert!(matches!(path.value, Cow::Borrowed(_)));
+    }
+
+    #[test]
+    fn owned_path_stays_owned_after_prefix_stripping() {
+        // Verify that owned data remains owned after prefix stripping (but efficiently)
+        let owned_input = bstr::BString::from(":(optional)/some/path");
+        let path = gix_config_value::Path::from(Cow::Owned(owned_input));
+        
+        assert!(path.is_optional());
+        assert_eq!(path.value.as_ref(), b"/some/path");
+        // Verify it's still owned
+        assert!(matches!(path.value, Cow::Owned(_)));
+    }
 }