1 files changed, 21 insertions, 0 deletions
diff --git a/third_party/rust/smart-default/src/util.rs b/third_party/rust/smart-default/src/util.rs
new file mode 100644
index 0000000000..0d4b247bbf
--- /dev/null
+++ b/third_party/rust/smart-default/src/util.rs
@@ -0,0 +1,21 @@
+use syn::parse::Error;
+use syn::spanned::Spanned;
+
+/// Return the value that fulfills the predicate if there is one in the slice. Panic if there is
+/// more than one.
+pub fn find_only<T, F>(iter: impl Iterator<Item = T>, pred: F) -> Result<Option<T>, Error>
+where
+    T: Spanned,
+    F: Fn(&T) -> Result<bool, Error>,
+{
+    let mut result = None;
+    for item in iter {
+        if pred(&item)? {
+            if result.is_some() {
+                return Err(Error::new(item.span(), "Multiple defaults"));
+            }
+            result = Some(item);
+        }
+    }
+    Ok(result)
+}