1 files changed, 50 insertions, 0 deletions
diff --git a/third_party/rust/derive_more/src/index.rs b/third_party/rust/derive_more/src/index.rs
new file mode 100644
index 0000000000..5aea597693
--- /dev/null
+++ b/third_party/rust/derive_more/src/index.rs
@@ -0,0 +1,50 @@
+use crate::utils::{add_where_clauses_for_new_ident, SingleFieldData, State};
+use proc_macro2::{Span, TokenStream};
+use quote::quote;
+use syn::{parse::Result, DeriveInput, Ident};
+
+/// Provides the hook to expand `#[derive(Index)]` into an implementation of `Index`
+pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStream> {
+    let index_type = &Ident::new("__IdxT", Span::call_site());
+    let mut state = State::with_field_ignore(
+        input,
+        trait_name,
+        quote!(::core::ops),
+        trait_name.to_lowercase(),
+    )?;
+    state.add_trait_path_type_param(quote!(#index_type));
+    let SingleFieldData {
+        field,
+        field_type,
+        input_type,
+        trait_path_with_params,
+        casted_trait,
+        member,
+        ..
+    } = state.assert_single_enabled_field();
+
+    let type_where_clauses = quote! {
+        where #field_type: #trait_path_with_params
+    };
+
+    let new_generics = add_where_clauses_for_new_ident(
+        &input.generics,
+        &[field],
+        index_type,
+        type_where_clauses,
+        true,
+    );
+
+    let (impl_generics, _, where_clause) = new_generics.split_for_impl();
+    let (_, ty_generics, _) = input.generics.split_for_impl();
+    Ok(quote! {
+        impl#impl_generics #trait_path_with_params for #input_type#ty_generics #where_clause
+        {
+            type Output = #casted_trait::Output;
+            #[inline]
+            fn index(&self, idx: #index_type) -> &Self::Output {
+                #casted_trait::index(&#member, idx)
+            }
+        }
+    })
+}