Conrad Irwin
b09764c54a
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
133 lines
4.7 KiB
Rust
133 lines
4.7 KiB
Rust
use proc_macro::TokenStream;
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use quote::quote;
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use syn::{Data, DeriveInput, Fields, Type, parse_macro_input};
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/// Derives the `MergeFrom` trait for a struct.
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///
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/// This macro automatically implements `MergeFrom` by calling `merge_from`
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/// on all fields in the struct. For `Option<T>` fields, it merges by taking
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/// the `other` value when `self` is `None`. For other types, it recursively
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/// calls `merge_from` on the field.
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///
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/// # Example
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///
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/// ```ignore
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/// #[derive(Clone, MergeFrom)]
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/// struct MySettings {
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/// field1: Option<String>,
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/// field2: SomeOtherSettings,
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/// }
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/// ```
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#[proc_macro_derive(MergeFrom)]
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pub fn derive_merge_from(input: TokenStream) -> TokenStream {
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let input = parse_macro_input!(input as DeriveInput);
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let name = &input.ident;
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let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
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let merge_body = match &input.data {
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Data::Struct(data_struct) => match &data_struct.fields {
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Fields::Named(fields) => {
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let field_merges = fields.named.iter().map(|field| {
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let field_name = &field.ident;
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let field_type = &field.ty;
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if is_option_type(field_type) {
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// For Option<T> fields, merge by taking the other value if self is None
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quote! {
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if let Some(other_value) = other.#field_name.as_ref() {
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if self.#field_name.is_none() {
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self.#field_name = Some(other_value.clone());
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} else if let Some(self_value) = self.#field_name.as_mut() {
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self_value.merge_from(Some(other_value));
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}
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}
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}
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} else {
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// For non-Option fields, recursively call merge_from
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quote! {
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self.#field_name.merge_from(Some(&other.#field_name));
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}
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}
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});
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quote! {
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if let Some(other) = other {
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#(#field_merges)*
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}
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}
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}
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Fields::Unnamed(fields) => {
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let field_merges = fields.unnamed.iter().enumerate().map(|(i, field)| {
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let field_index = syn::Index::from(i);
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let field_type = &field.ty;
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if is_option_type(field_type) {
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// For Option<T> fields, merge by taking the other value if self is None
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quote! {
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if let Some(other_value) = other.#field_index.as_ref() {
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if self.#field_index.is_none() {
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self.#field_index = Some(other_value.clone());
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} else if let Some(self_value) = self.#field_index.as_mut() {
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self_value.merge_from(Some(other_value));
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}
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}
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}
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} else {
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// For non-Option fields, recursively call merge_from
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quote! {
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self.#field_index.merge_from(Some(&other.#field_index));
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}
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}
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});
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quote! {
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if let Some(other) = other {
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#(#field_merges)*
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}
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}
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}
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Fields::Unit => {
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quote! {
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// No fields to merge for unit structs
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}
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}
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},
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Data::Enum(_) => {
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quote! {
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if let Some(other) = other {
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*self = other.clone();
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}
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}
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}
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Data::Union(_) => {
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panic!("MergeFrom cannot be derived for unions");
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}
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};
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let expanded = quote! {
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impl #impl_generics crate::merge_from::MergeFrom for #name #ty_generics #where_clause {
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fn merge_from(&mut self, other: ::core::option::Option<&Self>) {
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use crate::merge_from::MergeFrom as _;
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#merge_body
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}
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}
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};
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TokenStream::from(expanded)
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}
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/// Check if a type is `Option<T>`
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fn is_option_type(ty: &Type) -> bool {
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match ty {
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Type::Path(type_path) => {
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if let Some(segment) = type_path.path.segments.last() {
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segment.ident == "Option"
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} else {
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false
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}
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}
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_ => false,
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}
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}
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