Support unit structs as contract events (#1917)

### What

The contractevent attribute macro now accepts fieldless unit structs
such as `pub struct MyEvent;`, treating them as events that carry no
topic or data fields and emit only the default prefix topic derived from
the struct name.

### Why

Issue #1571 reports that the macro only worked on structs with named
fields and rejected unit structs with a hard compile error, which
prevented defining simple event markers that carry no additional data.
Empty braced structs (`pub struct MyEvent {}`) already worked, so the
gap was specifically the unit-struct form.

### Known limitations

When a contract's spec is regenerated via contractimport!, a fieldless
event is rendered as an empty braced struct (`pub struct MyEvent {}`)
rather than a unit struct, because the on-wasm spec records only that
the event has no parameters and does not preserve the unit-versus-braced
distinction. The generated type produces an identical on-chain event so
that difference is of little consequence.

Close #1571

---
_Generated by [Claude
Code](https://claude.ai/code/session_011mKQ41rU7Ze9hxQAUEdfmk)_

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: leighmcculloch

soroban-sdk-macros/src/derive_event.rs

@@ -114,10 +114,7 @@ fn derive_impls(args: &ContractEventArgs, input: &DeriveInput) -> Result<TokenSt
                     "structs with unnamed fields are not supported as contract events",
                 )
                 .with_span(&struct_.fields.span()))?,
-                Fields::Unit => Err(Error::custom(
-                    "structs with no fields are not supported as contract events",
-                )
-                .with_span(&struct_.fields.span()))?,
+                Fields::Unit => Vec::new(),
             },
             Data::Enum(_) => Err(Error::custom("enums are not supported as contract events")
                 .with_span(&input.span()))?,

soroban-sdk/src/tests/contract_event.rs

@@ -213,6 +213,36 @@ fn test_no_topics_no_data() {
     assert_eq!(event.to_xdr(&env, &id), expected_event);
 }
 
+#[test]
+fn test_unit_struct() {
+    let env = Env::default();
+
+    #[contract]
+    pub struct Contract;
+    let id = env.register(Contract, ());
+
+    #[contractevent]
+    pub struct MyEvent;
+
+    let event = MyEvent;
+    env.as_contract(&id, || {
+        event.publish(&env);
+    });
+
+    let data: Val = Map::<Symbol, Val>::new(&env).into_val(&env);
+    let expected_event = xdr::ContractEvent {
+        ext: xdr::ExtensionPoint::V0,
+        type_: xdr::ContractEventType::Contract,
+        contract_id: Some(id.contract_id()),
+        body: xdr::ContractEventBody::V0(xdr::ContractEventV0 {
+            topics: vec![&env, symbol_short!("my_event")].into(),
+            data: xdr::ScVal::try_from_val(&env, &data).unwrap(),
+        }),
+    };
+    assert_eq!(env.events().all(), std::vec![expected_event.clone()]);
+    assert_eq!(event.to_xdr(&env, &id), expected_event);
+}
+
 #[test]
 fn test_data_single_value() {
     let env = Env::default();

soroban-sdk/test_snapshots/tests/contract_event/test_unit_struct.1.json

@@ -0,0 +1,82 @@
+{
+  "generators": {
+    "address": 1,
+    "nonce": 0,
+    "mux_id": 0
+  },
+  "auth": [
+    [],
+    []
+  ],
+  "ledger": {
+    "protocol_version": 27,
+    "sequence_number": 0,
+    "timestamp": 0,
+    "network_id": "0000000000000000000000000000000000000000000000000000000000000000",
+    "base_reserve": 0,
+    "min_persistent_entry_ttl": 4096,
+    "min_temp_entry_ttl": 16,
+    "max_entry_ttl": 6312000,
+    "ledger_entries": [
+      {
+        "entry": {
+          "last_modified_ledger_seq": 0,
+          "data": {
+            "contract_data": {
+              "ext": "v0",
+              "contract": "CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD2KM",
+              "key": "ledger_key_contract_instance",
+              "durability": "persistent",
+              "val": {
+                "contract_instance": {
+                  "executable": {
+                    "wasm": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
+                  },
+                  "storage": null
+                }
+              }
+            }
+          },
+          "ext": "v0"
+        },
+        "live_until": 4095
+      },
+      {
+        "entry": {
+          "last_modified_ledger_seq": 0,
+          "data": {
+            "contract_code": {
+              "ext": "v0",
+              "hash": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+              "code": ""
+            }
+          },
+          "ext": "v0"
+        },
+        "live_until": 4095
+      }
+    ]
+  },
+  "events": [
+    {
+      "event": {
+        "ext": "v0",
+        "contract_id": "CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD2KM",
+        "type_": "contract",
+        "body": {
+          "v0": {
+            "topics": [
+              {
+                "symbol": "my_event"
+              }
+            ],
+            "data": {
+              "map": []
+            }
+          }
+        }
+      },
+      "failed_call": false
+    }
+  ]
+}

tests-expanded/test_spec_import_tests.rs

@@ -5,7 +5,7 @@ extern crate core;
 #[prelude_import]
 use core::prelude::rust_2021::*;
 use soroban_sdk::{contract, contractimpl, Address, Env, String};
-use test_spec_lib::{EnumA, EnumIntA, ErrorA, EventA, StructA, StructTupleA};
+use test_spec_lib::{EnumA, EnumIntA, ErrorA, EventA, EventD, StructA, StructTupleA};
 pub struct Contract;
 ///ContractArgs is a type for building arg lists for functions defined in "Contract".
 pub struct ContractArgs;
@@ -48,6 +48,9 @@ impl Contract {
     pub fn fn_event_a(env: Env, f1: Address, f2: String) {
         EventA { f1, f2 }.publish(&env);
     }
+    pub fn fn_event_d(env: Env) {
+        EventD.publish(&env);
+    }
 }
 #[doc(hidden)]
 #[allow(non_snake_case)]
@@ -135,6 +138,20 @@ impl Contract {
         *b"\0\0\0\0\0\0\0\0\0\0\0\nfn_event_a\0\0\0\0\0\x02\0\0\0\0\0\0\0\x02f1\0\0\0\0\0\x13\0\0\0\0\0\0\0\x02f2\0\0\0\0\0\x10\0\0\0\0"
     }
 }
+#[doc(hidden)]
+#[allow(non_snake_case)]
+pub mod __Contract__fn_event_d__spec {
+    #[doc(hidden)]
+    #[allow(non_snake_case)]
+    #[allow(non_upper_case_globals)]
+    pub static __SPEC_XDR_FN_FN_EVENT_D: [u8; 32usize] = super::Contract::spec_xdr_fn_event_d();
+}
+impl Contract {
+    #[allow(non_snake_case)]
+    pub const fn spec_xdr_fn_event_d() -> [u8; 32usize] {
+        *b"\0\0\0\0\0\0\0\0\0\0\0\nfn_event_d\0\0\0\0\0\0\0\0\0\0"
+    }
+}
 impl<'a> ContractClient<'a> {
     pub fn fn_struct_a(&self, f1: &u32, f2: &bool) -> StructA {
         use core::ops::Not;
@@ -325,6 +342,30 @@ impl<'a> ContractClient<'a> {
         );
         res
     }
+    pub fn fn_event_d(&self) -> () {
+        use core::ops::Not;
+        use soroban_sdk::{FromVal, IntoVal};
+        let res = self.env.invoke_contract(
+            &self.address,
+            &{ soroban_sdk::Symbol::new(&self.env, "fn_event_d") },
+            ::soroban_sdk::Vec::new(&self.env),
+        );
+        res
+    }
+    pub fn try_fn_event_d(
+        &self,
+    ) -> Result<
+        Result<(), <() as soroban_sdk::TryFromVal<soroban_sdk::Env, soroban_sdk::Val>>::Error>,
+        Result<soroban_sdk::Error, soroban_sdk::InvokeError>,
+    > {
+        use soroban_sdk::{FromVal, IntoVal};
+        let res = self.env.try_invoke_contract(
+            &self.address,
+            &{ soroban_sdk::Symbol::new(&self.env, "fn_event_d") },
+            ::soroban_sdk::Vec::new(&self.env),
+        );
+        res
+    }
 }
 impl ContractArgs {
     #[inline(always)]
@@ -357,6 +398,11 @@ impl ContractArgs {
     pub fn fn_event_a<'i>(f1: &'i Address, f2: &'i String) -> (&'i Address, &'i String) {
         (f1, f2)
     }
+    #[inline(always)]
+    #[allow(clippy::unused_unit)]
+    pub fn fn_event_d<'i>() -> () {
+        ()
+    }
 }
 #[doc(hidden)]
 #[allow(non_snake_case)]
@@ -527,6 +573,23 @@ pub extern "C" fn __Contract__fn_event_a__invoke_raw_extern(
     #[allow(deprecated)]
     __Contract__fn_event_a__invoke_raw(soroban_sdk::Env::default(), arg_0, arg_1)
 }
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[deprecated(note = "use `ContractClient::new(&env, &contract_id).fn_event_d` instead")]
+#[allow(deprecated)]
+pub fn __Contract__fn_event_d__invoke_raw(env: soroban_sdk::Env) -> soroban_sdk::Val {
+    soroban_sdk::IntoValForContractFn::into_val_for_contract_fn(
+        <Contract>::fn_event_d(env.clone()),
+        &env,
+    )
+}
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[deprecated(note = "use `ContractClient::new(&env, &contract_id).fn_event_d` instead")]
+pub extern "C" fn __Contract__fn_event_d__invoke_raw_extern() -> soroban_sdk::Val {
+    #[allow(deprecated)]
+    __Contract__fn_event_d__invoke_raw(soroban_sdk::Env::default())
+}
 #[rustc_main]
 #[coverage(off)]
 #[doc(hidden)]

tests-expanded/test_spec_import_wasm32v1-none.rs

@@ -5,7 +5,7 @@ extern crate core;
 #[prelude_import]
 use core::prelude::rust_2021::*;
 use soroban_sdk::{contract, contractimpl, Address, Env, String};
-use test_spec_lib::{EnumA, EnumIntA, ErrorA, EventA, StructA, StructTupleA};
+use test_spec_lib::{EnumA, EnumIntA, ErrorA, EventA, EventD, StructA, StructTupleA};
 pub struct Contract;
 ///ContractArgs is a type for building arg lists for functions defined in "Contract".
 pub struct ContractArgs;
@@ -48,6 +48,9 @@ impl Contract {
     pub fn fn_event_a(env: Env, f1: Address, f2: String) {
         EventA { f1, f2 }.publish(&env);
     }
+    pub fn fn_event_d(env: Env) {
+        EventD.publish(&env);
+    }
 }
 #[doc(hidden)]
 #[allow(non_snake_case)]
@@ -141,6 +144,21 @@ impl Contract {
         *b"\0\0\0\0\0\0\0\0\0\0\0\nfn_event_a\0\0\0\0\0\x02\0\0\0\0\0\0\0\x02f1\0\0\0\0\0\x13\0\0\0\0\0\0\0\x02f2\0\0\0\0\0\x10\0\0\0\0"
     }
 }
+#[doc(hidden)]
+#[allow(non_snake_case)]
+pub mod __Contract__fn_event_d__spec {
+    #[doc(hidden)]
+    #[allow(non_snake_case)]
+    #[allow(non_upper_case_globals)]
+    #[link_section = "contractspecv0"]
+    pub static __SPEC_XDR_FN_FN_EVENT_D: [u8; 32usize] = super::Contract::spec_xdr_fn_event_d();
+}
+impl Contract {
+    #[allow(non_snake_case)]
+    pub const fn spec_xdr_fn_event_d() -> [u8; 32usize] {
+        *b"\0\0\0\0\0\0\0\0\0\0\0\nfn_event_d\0\0\0\0\0\0\0\0\0\0"
+    }
+}
 impl<'a> ContractClient<'a> {
     pub fn fn_struct_a(&self, f1: &u32, f2: &bool) -> StructA {
         use core::ops::Not;
@@ -331,6 +349,30 @@ impl<'a> ContractClient<'a> {
         );
         res
     }
+    pub fn fn_event_d(&self) -> () {
+        use core::ops::Not;
+        use soroban_sdk::{FromVal, IntoVal};
+        let res = self.env.invoke_contract(
+            &self.address,
+            &{ soroban_sdk::Symbol::new(&self.env, "fn_event_d") },
+            ::soroban_sdk::Vec::new(&self.env),
+        );
+        res
+    }
+    pub fn try_fn_event_d(
+        &self,
+    ) -> Result<
+        Result<(), <() as soroban_sdk::TryFromVal<soroban_sdk::Env, soroban_sdk::Val>>::Error>,
+        Result<soroban_sdk::Error, soroban_sdk::InvokeError>,
+    > {
+        use soroban_sdk::{FromVal, IntoVal};
+        let res = self.env.try_invoke_contract(
+            &self.address,
+            &{ soroban_sdk::Symbol::new(&self.env, "fn_event_d") },
+            ::soroban_sdk::Vec::new(&self.env),
+        );
+        res
+    }
 }
 impl ContractArgs {
     #[inline(always)]
@@ -363,6 +405,11 @@ impl ContractArgs {
     pub fn fn_event_a<'i>(f1: &'i Address, f2: &'i String) -> (&'i Address, &'i String) {
         (f1, f2)
     }
+    #[inline(always)]
+    #[allow(clippy::unused_unit)]
+    pub fn fn_event_d<'i>() -> () {
+        ()
+    }
 }
 #[doc(hidden)]
 #[allow(non_snake_case)]
@@ -539,3 +586,21 @@ pub extern "C" fn __Contract__fn_event_a__invoke_raw_extern(
     #[allow(deprecated)]
     __Contract__fn_event_a__invoke_raw(soroban_sdk::Env::default(), arg_0, arg_1)
 }
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[deprecated(note = "use `ContractClient::new(&env, &contract_id).fn_event_d` instead")]
+#[allow(deprecated)]
+pub fn __Contract__fn_event_d__invoke_raw(env: soroban_sdk::Env) -> soroban_sdk::Val {
+    soroban_sdk::IntoValForContractFn::into_val_for_contract_fn(
+        <Contract>::fn_event_d(env.clone()),
+        &env,
+    )
+}
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[deprecated(note = "use `ContractClient::new(&env, &contract_id).fn_event_d` instead")]
+#[export_name = "fn_event_d"]
+pub extern "C" fn __Contract__fn_event_d__invoke_raw_extern() -> soroban_sdk::Val {
+    #[allow(deprecated)]
+    __Contract__fn_event_d__invoke_raw(soroban_sdk::Env::default())
+}