Support pointwise addition for arrays and tuples

Fixes #342

We want to support examples like these:

```rust
struct StructRecursive {
    a: i32,
    b: [i32; 2],
    c: [[i32; 2]; 3],
    d: (i32, i32),
    e: ((u8, [i32; 3]), i32),
    f: ((u8, i32), (u8, ((i32, u64, ((u8, u8), u16)), u8))),
    g: i32,
}

struct TupleRecursive((i32, u8), [(i32, u8); 10]);
```

Supporting arrays and tuples inside of enums would also be useful, but
that's not in this PR.

impl/src/add_helpers.rs

@@ -1,28 +1,69 @@
-use proc_macro2::TokenStream;
+use proc_macro2::{Span, TokenStream};
 use quote::quote;
-use syn::{Field, Ident, Index};
+use syn::{Field, Ident, Index, Type, TypeArray, TypeTuple};
 
 pub fn tuple_exprs(fields: &[&Field], method_ident: &Ident) -> Vec<TokenStream> {
-    let mut exprs = vec![];
-
-    for i in 0..fields.len() {
-        let i = Index::from(i);
-        // generates `self.0.add(rhs.0)`
-        let expr = quote! { self.#i.#method_ident(rhs.#i) };
-        exprs.push(expr);
-    }
-    exprs
+    let fields: Vec<&Type> = fields.iter().map(|field| &field.ty).collect::<Vec<_>>();
+    inner_tuple_exprs(0, &quote! {}, &fields, method_ident)
 }
 
 pub fn struct_exprs(fields: &[&Field], method_ident: &Ident) -> Vec<TokenStream> {
-    let mut exprs = vec![];
+    fields
+        .iter()
+        .map(|field| {
+            // It's safe to unwrap because struct fields always have an identifier
+            let field_path = field.ident.as_ref().unwrap();
+            elem_content(0, &quote! { .#field_path }, &field.ty, method_ident)
+        })
+        .collect()
+}
+
+pub fn inner_tuple_exprs(
+    // `depth` is needed for `index_var` generation for nested arrays
+    depth: usize,
+    field_path: &TokenStream,
+    fields: &[&Type],
+    method_ident: &Ident,
+) -> Vec<TokenStream> {
+    fields
+        .iter()
+        .enumerate()
+        .map(|(i, ty)| {
+            let i = Index::from(i);
+            elem_content(depth + 1, &quote! { #field_path.#i }, ty, method_ident)
+        })
+        .collect()
+}
+
+pub fn elem_content(
+    depth: usize,
+    field_path: &TokenStream,
+    ty: &Type,
+    method_ident: &Ident,
+) -> TokenStream {
+    match ty {
+        Type::Array(TypeArray { elem, .. }) => {
+            let index_var = Ident::new(&format!("i{}", depth), Span::call_site());
+            let fn_body = elem_content(
+                depth + 1,
+                &quote! { #field_path[#index_var] },
+                elem.as_ref(),
+                method_ident,
+            );
 
-    for field in fields {
-        // It's safe to unwrap because struct fields always have an identifier
-        let field_id = field.ident.as_ref().unwrap();
-        // generates `x: self.x.add(rhs.x)`
-        let expr = quote! { self.#field_id.#method_ident(rhs.#field_id) };
-        exprs.push(expr)
+            // generates `core::array::from_fn(|i0| self.x[i0].add(rhs.x[i0]))`
+            quote! { core::array::from_fn(|#index_var| #fn_body) }
+        }
+        Type::Tuple(TypeTuple { elems, .. }) => {
+            let exprs = inner_tuple_exprs(
+                depth + 1,
+                field_path,
+                &elems.iter().collect::<Vec<_>>(),
+                method_ident,
+            );
+            quote! { (#(#exprs),*) }
+        }
+        // generates `self.x.add(rhs.x)`
+        _ => quote! { self #field_path.#method_ident(rhs #field_path) },
     }
-    exprs
 }

impl/src/add_like.rs

@@ -18,6 +18,7 @@ pub fn expand(input: &DeriveInput, trait_name: &str) -> TokenStream {
     let generics = add_extra_type_param_bound_op_output(&input.generics, &trait_ident);
     let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
 
+    // TODO(Matthias): do we add support for arrays here?
     let (output_type, block) = match input.data {
         Data::Struct(ref data_struct) => match data_struct.fields {
             Fields::Unnamed(ref fields) => (
@@ -53,7 +54,7 @@ pub fn expand(input: &DeriveInput, trait_name: &str) -> TokenStream {
     }
 }
 
-fn tuple_content<T: ToTokens>(
+pub(crate) fn tuple_content<T: ToTokens>(
     input_type: &T,
     fields: &[&Field],
     method_ident: &Ident,

tests/add.rs

@@ -22,3 +22,28 @@ enum MixedInts {
     UnsignedTwo(u32),
     Unit,
 }
+
+#[derive(Add)]
+#[derive(Default)]
+struct StructRecursive {
+    a: i32,
+    b: [i32; 2],
+    c: [[i32; 2]; 3],
+    d: (i32, i32),
+    e: ((u8, [i32; 3]), i32),
+    f: ((u8, i32), (u8, ((i32, u64, ((u8, u8), u16)), u8))),
+    g: i32,
+}
+
+#[test]
+fn test_sanity() {
+    let mut a: StructRecursive = Default::default();
+    let mut b: StructRecursive = Default::default();
+    a.c[0][1] = 1;
+    b.c[0][1] = 2;
+    let c = a + b;
+    assert_eq!(c.c[0][1], 3);
+}
+
+#[derive(Add)]
+struct TupleRecursive((i32, u8), [(i32, u8); 10]);