contracts: Add host function tracing

frame/contracts/README.md

@@ -135,6 +135,18 @@ to `error` in order to prevent them from spamming the console.
 `--dev`: Use a dev chain spec
 `--tmp`: Use temporary storage for chain data (the chain state is deleted on exit)
 
+## Host function tracing
+
+For contract authors, it can be a helpful debugging tool to see which host functions are called, with which arguments, and what the result was. 
+
+In order to see these messages on the node console, the log level for the `runtime::contracts::strace` target needs to be raised to the `trace` level. 
+
+Example: 
+
+```bash
+cargo run --release -- --dev -lerror,runtime::contracts::strace=trace,runtime::contracts=debug
+```
+
 ## Unstable Interfaces
 
 Driven by the desire to have an iterative approach in developing new contract interfaces

frame/contracts/proc-macro/src/lib.rs

@@ -596,6 +596,7 @@ fn expand_functions(def: &EnvDef, expand_blocks: bool, host_state: TokenStream2)
 	let impls = def.host_funcs.iter().map(|f| {
 		// skip the context and memory argument
 		let params = f.item.sig.inputs.iter().skip(2);
+
 		let (module, name, body, wasm_output, output) = (
 			f.module(),
 			&f.name,
@@ -606,6 +607,39 @@ fn expand_functions(def: &EnvDef, expand_blocks: bool, host_state: TokenStream2)
 		let is_stable = f.is_stable;
 		let not_deprecated = f.not_deprecated;
 
+		// wrapped host function body call with host function traces
+		// see https://github.com/paritytech/substrate/tree/master/frame/contracts#host-function-tracing
+		let wrapped_body_with_trace = {
+			let trace_fmt_args = params.clone().filter_map(|arg| match arg {
+				syn::FnArg::Receiver(_) => None,
+				syn::FnArg::Typed(p) => {
+					match *p.pat.clone() {
+						syn::Pat::Ident(ref pat_ident) => Some(pat_ident.ident.clone()),
+						_ => None,
+					}
+				},
+			});
+
+			let params_fmt_str = trace_fmt_args.clone().map(|s| format!("{s}: {{:?}}")).collect::<Vec<_>>().join(", ");
+			let trace_fmt_str = format!("{}::{}({}) = {{:?}}\n", module, name, params_fmt_str);
+
+			quote! {
+				if ::log::log_enabled!(target: "runtime::contracts::strace", ::log::Level::Trace) {
+					let result = #body;
+					{
+						use sp_std::fmt::Write;
+						let mut w = sp_std::Writer::default();
+						let _ = core::write!(&mut w, #trace_fmt_str, #( #trace_fmt_args, )* result);
+						let msg = core::str::from_utf8(&w.inner()).unwrap_or_default();
+						ctx.ext().append_debug_buffer(msg);
+					}
+					result
+				} else {
+					#body
+				}
+			}
+		};
+
 		// If we don't expand blocks (implementing for `()`) we change a few things:
 		// - We replace any code by unreachable!
 		// - Allow unused variables as the code that uses is not expanded
@@ -617,7 +651,7 @@ fn expand_functions(def: &EnvDef, expand_blocks: bool, host_state: TokenStream2)
 					.memory()
 					.expect("Memory must be set when setting up host data; qed")
 					.data_and_store_mut(&mut __caller__);
-				#body
+				#wrapped_body_with_trace
 			} }
 		} else {
 			quote! { || -> #wasm_output {

frame/contracts/src/wasm/runtime.rs

@@ -84,6 +84,7 @@ enum KeyType {
 /// will not be changed or removed. This means that any contract **must not** exhaustively
 /// match return codes. Instead, contracts should prepare for unknown variants and deal with
 /// those errors gracefully in order to be forward compatible.
+#[derive(Debug)]
 #[repr(u32)]
 pub enum ReturnCode {
 	/// API call successful.