lang: Optimize enums with all unit variants and empty arrays with Lazy (solana-foundation#4237)

Author: jamie-osec

lang/derive/serde/src/lazy.rs

@@ -1,5 +1,5 @@
 use proc_macro2::Literal;
-use quote::{format_ident, quote};
+use quote::{format_ident, quote, ToTokens};
 use syn::{spanned::Spanned, Fields, Item};
 
 pub fn gen_lazy(input: proc_macro::TokenStream) -> syn::Result<proc_macro2::TokenStream> {
@@ -24,7 +24,10 @@ pub fn gen_lazy(input: proc_macro::TokenStream) -> syn::Result<proc_macro2::Toke
                 .enumerate()
                 .map(|(i, size)| (Literal::usize_unsuffixed(i), size))
                 .map(|(i, size)| quote! { Some(#i) => { #size } });
-
+            let sized = enm
+                .variants
+                .iter()
+                .all(|variant| matches!(variant.fields, Fields::Unit));
             (
                 &enm.ident,
                 &enm.generics,
@@ -34,7 +37,7 @@ pub fn gen_lazy(input: proc_macro::TokenStream) -> syn::Result<proc_macro2::Toke
                         _ => unreachable!(),
                     }
                 },
-                quote!(false),
+                sized.to_token_stream(),
             )
         }
         Item::Union(_) => return Err(syn::Error::new(item.span(), "Unions are not supported")),

lang/src/lazy.rs

@@ -64,7 +64,7 @@ impl_sized!(f64);
 impl_sized!(Pubkey);
 
 impl<T: Lazy, const N: usize> Lazy for [T; N] {
-    const SIZED: bool = T::SIZED;
+    const SIZED: bool = N == 0 || T::SIZED;
 
     #[inline(always)]
     fn size_of(buf: &[u8]) -> usize {
@@ -196,6 +196,15 @@ mod tests {
             len!(MyEnum::Unnamed(1, 2))
         );
         assert!(!MyEnum::SIZED);
+
+        #[derive(AnchorSerialize, AnchorDeserialize)]
+        enum UnitEnum {
+            A,
+            B,
+        }
+        assert_eq!(UnitEnum::size_of(&[0]), len!(UnitEnum::A));
+        assert_eq!(UnitEnum::size_of(&[1]), len!(UnitEnum::B));
+        assert!(UnitEnum::SIZED);
     }
 
     #[test]