Merge subsettings when extending configurations (#4431)

crates/ruff_macros/src/combine_options.rs

@@ -0,0 +1,62 @@
+use quote::{quote, quote_spanned};
+use syn::{Data, DataStruct, DeriveInput, Field, Fields, Path, PathSegment, Type, TypePath};
+
+pub(crate) fn derive_impl(input: DeriveInput) -> syn::Result<proc_macro2::TokenStream> {
+    let DeriveInput { ident, data, .. } = input;
+
+    match data {
+        Data::Struct(DataStruct {
+            fields: Fields::Named(fields),
+            ..
+        }) => {
+            let output = fields
+                .named
+                .iter()
+                .map(handle_field)
+                .collect::<Result<Vec<_>, _>>()?;
+
+            Ok(quote! {
+                use crate::settings::configuration::CombinePluginOptions;
+
+                impl CombinePluginOptions for #ident {
+                    fn combine(self, other: Self) -> Self {
+                        Self {
+                        #(
+                            #output
+                        ),*
+                        }
+                    }
+                }
+            })
+        }
+        _ => Err(syn::Error::new(
+            ident.span(),
+            "Can only derive CombineOptions from structs with named fields.",
+        )),
+    }
+}
+
+fn handle_field(field: &Field) -> syn::Result<proc_macro2::TokenStream> {
+    let ident = field
+        .ident
+        .as_ref()
+        .expect("Expected to handle named fields");
+
+    match &field.ty {
+        Type::Path(TypePath {
+            path: Path { segments, .. },
+            ..
+        }) => match segments.first() {
+            Some(PathSegment {
+                ident: type_ident, ..
+            }) if type_ident == "Option" => Ok(quote_spanned!(
+                ident.span() => #ident: self.#ident.or(other.#ident)
+            )),
+            _ => Err(syn::Error::new(
+                ident.span(),
+                "Expected `Option<_>` or `Vec<_>` as type.",
+            )),
+        },
+        _ => Err(syn::Error::new(ident.span(), "Expected type.")),
+    }
+}