1 files changed, 53 insertions, 0 deletions
diff --git a/third_party/rust/derive_more-impl/src/sum_like.rs b/third_party/rust/derive_more-impl/src/sum_like.rs
new file mode 100644
index 0000000000..97d73fce4b
--- /dev/null
+++ b/third_party/rust/derive_more-impl/src/sum_like.rs
@@ -0,0 +1,53 @@
+use crate::utils::{
+    add_extra_ty_param_bound, add_extra_where_clauses, MultiFieldData, State,
+};
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use syn::{DeriveInput, Result};
+
+pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStream> {
+    let state = State::new(input, trait_name, trait_name.to_lowercase())?;
+    let multi_field_data = state.enabled_fields_data();
+    let MultiFieldData {
+        input_type,
+        field_types,
+        trait_path,
+        method_ident,
+        ..
+    } = multi_field_data.clone();
+
+    let op_trait_name = if trait_name == "Sum" { "Add" } else { "Mul" };
+    let op_trait_ident = format_ident!("{op_trait_name}");
+    let op_path = quote! { ::core::ops::#op_trait_ident };
+    let op_method_ident = format_ident!("{}", op_trait_name.to_lowercase());
+    let has_type_params = input.generics.type_params().next().is_none();
+    let generics = if has_type_params {
+        input.generics.clone()
+    } else {
+        let (_, ty_generics, _) = input.generics.split_for_impl();
+        let generics = add_extra_ty_param_bound(&input.generics, trait_path);
+        let operator_where_clause = quote! {
+            where #input_type #ty_generics: #op_path<Output=#input_type #ty_generics>
+        };
+        add_extra_where_clauses(&generics, operator_where_clause)
+    };
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    let initializers: Vec<_> = field_types
+        .iter()
+        .map(|field_type| {
+            quote! { #trait_path::#method_ident(::core::iter::empty::<#field_type>()) }
+        })
+        .collect();
+    let identity = multi_field_data.initializer(&initializers);
+
+    Ok(quote! {
+        #[automatically_derived]
+        impl #impl_generics #trait_path for #input_type #ty_generics #where_clause {
+            #[inline]
+            fn #method_ident<I: ::core::iter::Iterator<Item = Self>>(iter: I) -> Self {
+                iter.fold(#identity, #op_path::#op_method_ident)
+            }
+        }
+    })
+}