Fix up the VM trace.

ethcore/src/executive.rs

@@ -99,11 +99,11 @@ impl<'a> Executive<'a> {
 		let check = options.check_nonce;
 		match options.tracing {
 			true => match options.vm_tracing {
-				true => self.transact_with_tracer(t, check, ExecutiveTracer::default(), ExecutiveVMTracer::default()),
+				true => self.transact_with_tracer(t, check, ExecutiveTracer::default(), ExecutiveVMTracer::toplevel()),
 				false => self.transact_with_tracer(t, check, ExecutiveTracer::default(), NoopVMTracer),
 			},
 			false => match options.vm_tracing {
-				true => self.transact_with_tracer(t, check, NoopTracer, ExecutiveVMTracer::default()),
+				true => self.transact_with_tracer(t, check, NoopTracer, ExecutiveVMTracer::toplevel()),
 				false => self.transact_with_tracer(t, check, NoopTracer, NoopVMTracer),
 			},
 		}

ethcore/src/trace/executive_tracer.rs

@@ -160,11 +160,24 @@ impl Tracer for ExecutiveTracer {
 }
 
 /// Simple VM tracer. Traces all operations.
-#[derive(Default)]
 pub struct ExecutiveVMTracer {
 	data: VMTrace,
 }
 
+impl ExecutiveVMTracer {
+	/// Create a new top-level instance.
+	pub fn toplevel() -> Self {
+		ExecutiveVMTracer {
+			data: VMTrace {
+				parent_step: 0,
+				code: vec![],
+				operations: vec![Default::default()],	// prefill with a single entry so that prepare_subtrace can get the parent_step
+				subs: vec![],
+			}
+		}
+	}
+}
+
 impl VMTracer for ExecutiveVMTracer {
 	fn trace_prepare_execute(&mut self, pc: usize, instruction: u8, gas_cost: &U256) -> bool {
 		self.data.operations.push(VMOperation {
@@ -188,7 +201,7 @@ impl VMTracer for ExecutiveVMTracer {
 
 	fn prepare_subtrace(&self, code: &[u8]) -> Self {
 		ExecutiveVMTracer { data: VMTrace {
-			parent_step: self.data.operations.len(),
+			parent_step: self.data.operations.len() - 1,	// won't overflow since we must already have pushed an operation in trace_prepare_execute.
 			code: code.to_vec(),
 			operations: vec![],
 			subs: vec![],

ethcore/src/types/trace_types/trace.rs

@@ -473,7 +473,7 @@ impl Decodable for VMExecutedOperation {
 	}
 }
 
-#[derive(Debug, Clone, PartialEq, Binary)]
+#[derive(Debug, Clone, PartialEq, Binary, Default)]
 /// A record of the execution of a single VM operation.
 pub struct VMOperation {
 	/// The program counter.