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/settings_macros/src/settings_macros.rs

@@ -0,0 +1,132 @@
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{Data, DeriveInput, Fields, Type, parse_macro_input};
+
+/// Derives the `MergeFrom` trait for a struct.
+///
+/// This macro automatically implements `MergeFrom` by calling `merge_from`
+/// on all fields in the struct. For `Option<T>` fields, it merges by taking
+/// the `other` value when `self` is `None`. For other types, it recursively
+/// calls `merge_from` on the field.
+///
+/// # Example
+///
+/// ```ignore
+/// #[derive(Clone, MergeFrom)]
+/// struct MySettings {
+///     field1: Option<String>,
+///     field2: SomeOtherSettings,
+/// }
+/// ```
+#[proc_macro_derive(MergeFrom)]
+pub fn derive_merge_from(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    let merge_body = match &input.data {
+        Data::Struct(data_struct) => match &data_struct.fields {
+            Fields::Named(fields) => {
+                let field_merges = fields.named.iter().map(|field| {
+                    let field_name = &field.ident;
+                    let field_type = &field.ty;
+
+                    if is_option_type(field_type) {
+                        // For Option<T> fields, merge by taking the other value if self is None
+                        quote! {
+                            if let Some(other_value) = other.#field_name.as_ref() {
+                                if self.#field_name.is_none() {
+                                    self.#field_name = Some(other_value.clone());
+                                } else if let Some(self_value) = self.#field_name.as_mut() {
+                                    self_value.merge_from(Some(other_value));
+                                }
+                            }
+                        }
+                    } else {
+                        // For non-Option fields, recursively call merge_from
+                        quote! {
+                            self.#field_name.merge_from(Some(&other.#field_name));
+                        }
+                    }
+                });
+
+                quote! {
+                    if let Some(other) = other {
+                        #(#field_merges)*
+                    }
+                }
+            }
+            Fields::Unnamed(fields) => {
+                let field_merges = fields.unnamed.iter().enumerate().map(|(i, field)| {
+                    let field_index = syn::Index::from(i);
+                    let field_type = &field.ty;
+
+                    if is_option_type(field_type) {
+                        // For Option<T> fields, merge by taking the other value if self is None
+                        quote! {
+                            if let Some(other_value) = other.#field_index.as_ref() {
+                                if self.#field_index.is_none() {
+                                    self.#field_index = Some(other_value.clone());
+                                } else if let Some(self_value) = self.#field_index.as_mut() {
+                                    self_value.merge_from(Some(other_value));
+                                }
+                            }
+                        }
+                    } else {
+                        // For non-Option fields, recursively call merge_from
+                        quote! {
+                            self.#field_index.merge_from(Some(&other.#field_index));
+                        }
+                    }
+                });
+
+                quote! {
+                    if let Some(other) = other {
+                        #(#field_merges)*
+                    }
+                }
+            }
+            Fields::Unit => {
+                quote! {
+                    // No fields to merge for unit structs
+                }
+            }
+        },
+        Data::Enum(_) => {
+            quote! {
+               if let Some(other) = other {
+                   *self = other.clone();
+               }
+            }
+        }
+        Data::Union(_) => {
+            panic!("MergeFrom cannot be derived for unions");
+        }
+    };
+
+    let expanded = quote! {
+        impl #impl_generics crate::merge_from::MergeFrom for #name #ty_generics #where_clause {
+            fn merge_from(&mut self, other: ::core::option::Option<&Self>) {
+                use crate::merge_from::MergeFrom as _;
+                #merge_body
+            }
+        }
+    };
+
+    TokenStream::from(expanded)
+}
+
+/// Check if a type is `Option<T>`
+fn is_option_type(ty: &Type) -> bool {
+    match ty {
+        Type::Path(type_path) => {
+            if let Some(segment) = type_path.path.segments.last() {
+                segment.ident == "Option"
+            } else {
+                false
+            }
+        }
+        _ => false,
+    }
+}