settings: Use a derive macro for refine (#38451)

When we refactored settings to not pass JSON blobs around, we ended up
needing
to write *a lot* of code that just merged things (like json merge used
to do).

Use a derive macro to prevent typos in this logic.

Release Notes:

- N/A

crates/util/src/schemars.rs

@@ -18,9 +18,8 @@ pub fn replace_subschema<T: JsonSchema>(
     let schema_name = T::schema_name();
     let definitions = generator.definitions_mut();
     assert!(!definitions.contains_key(&format!("{schema_name}2")));
-    if definitions.contains_key(schema_name.as_ref()) {
-        definitions.insert(schema_name.to_string(), schema().to_value());
-    }
+    assert!(definitions.contains_key(schema_name.as_ref()));
+    definitions.insert(schema_name.to_string(), schema().to_value());
     schemars::Schema::new_ref(format!("{DEFS_PATH}{schema_name}"))
 }
 

crates/util/src/util.rs

@@ -1390,21 +1390,3 @@ Line 3"#
         assert_eq!(result[1], (10..15, "world")); // '🦀' is 4 bytes
     }
 }
-
-pub fn refine<T: Clone>(dest: &mut T, src: &Option<T>) {
-    if let Some(src) = src {
-        *dest = src.clone()
-    }
-}
-
-pub trait MergeFrom: Sized + Clone {
-    fn merge_from(&mut self, src: &Option<Self>);
-}
-
-impl<T: Clone> MergeFrom for T {
-    fn merge_from(&mut self, src: &Option<Self>) {
-        if let Some(src) = src {
-            *self = src.clone();
-        }
-    }
-}