1 files changed, 44 insertions, 0 deletions
diff --git a/third_party/rust/derive_more-impl/src/index_mut.rs b/third_party/rust/derive_more-impl/src/index_mut.rs
new file mode 100644
index 0000000000..0e030cf6f5
--- /dev/null
+++ b/third_party/rust/derive_more-impl/src/index_mut.rs
@@ -0,0 +1,44 @@
+use crate::utils::{add_where_clauses_for_new_ident, SingleFieldData, State};
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use syn::{parse::Result, DeriveInput};
+
+/// Provides the hook to expand `#[derive(IndexMut)]` into an implementation of `IndexMut`
+pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStream> {
+    let index_type = format_ident!("__IdxT");
+    let mut state = State::with_field_ignore(input, trait_name, "index_mut".into())?;
+    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! {
+        #[automatically_derived]
+        impl #impl_generics #trait_path_with_params for #input_type #ty_generics #where_clause {
+            #[inline]
+            fn index_mut(&mut self, idx: #index_type) -> &mut Self::Output {
+                #casted_trait::index_mut(&mut #member, idx)
+            }
+        }
+    })
+}