lang: derive arbitrary traits for the instruction data

CHANGELOG.md

@@ -12,6 +12,7 @@ The minor version will be incremented upon a breaking change and the patch versi
 
 ### Features
 
+- lang: Implement `Arbitrary` trait for the anchor-generated instruction module when the program is compiled with the arbitrary feature (requires the arbitrary crate) ([2518](https://github.com/coral-xyz/anchor/pull/2518)).
 - idl: Allow overriding the idl build toolchain with the `RUSTUP_TOOLCHAIN` environment variable ([#2941](https://github.com/coral-xyz/anchor/pull/2941]))
 - avm: Support customizing the installation location using `AVM_HOME` environment variable ([#2917](https://github.com/coral-xyz/anchor/pull/2917))
 - idl, ts: Add accounts resolution for associated token accounts ([#2927](https://github.com/coral-xyz/anchor/pull/2927))

lang/syn/src/codegen/program/instruction.rs

@@ -44,6 +44,7 @@ pub fn generate(program: &Program) -> proc_macro2::TokenStream {
                 quote! {
                     /// Instruction.
                     #[derive(AnchorSerialize, AnchorDeserialize)]
+                    #[cfg_attr(feature = "arbitrary", derive(Debug, Clone, arbitrary::Arbitrary))]
                     pub struct #ix_name_camel;
 
                     #ix_data_trait
@@ -52,6 +53,7 @@ pub fn generate(program: &Program) -> proc_macro2::TokenStream {
                 quote! {
                     /// Instruction.
                     #[derive(AnchorSerialize, AnchorDeserialize)]
+                    #[cfg_attr(feature = "arbitrary", derive(Debug, Clone, arbitrary::Arbitrary))]
                     pub struct #ix_name_camel {
                         #(#raw_args),*
                     }