Dead end with macros

src/utils/replace_first.rs

@@ -0,0 +1,14 @@
+use std::iter;
+
+pub fn replace_first<'a, T, F>(slice: &'a [T], mut f: F) -> Option<impl Iterator<Item = T> + 'a>
+where T: Clone, F: FnMut(&T) -> Option<T> {
+  for i in 0..slice.len() {
+    if let Some(new) = f(&slice[i]) {
+      let subbed_iter = slice[0..i].iter().cloned()
+        .chain(iter::once(new))
+        .chain(slice[i+1..].iter().cloned());
+      return Some(subbed_iter)
+    }
+  }
+  None
+}