Use the log crate to output contract generated messages. (#8403)

substrate/frame/contracts/README.md

@@ -57,4 +57,19 @@ will make things a lot easier. One such language is [`ink`](https://github.com/p
 which is an [`eDSL`](https://wiki.haskell.org/Embedded_domain_specific_language) that enables
 writing WebAssembly based smart contracts in the Rust programming language.
 
+## Debugging
+
+Contracts can emit messages to the node console when run on a development chain through the
+`seal_println` API. This is exposed in ink! via
+[`ink_env::debug_println()`](https://docs.rs/ink_env/latest/ink_env/fn.debug_println.html).
+
+In order to see these messages the log level for the `runtime::contracts` target needs to be raised
+to at least the `info` level which is the default. However, those messages are easy to overlook
+because of the noise generated by block production. A good starting point for contract debugging
+could be:
+
+```bash
+cargo run --release -- --dev --tmp -lerror,runtime::contracts
+```
+
 License: Apache-2.0

substrate/frame/contracts/src/wasm/runtime.rs

@@ -1366,7 +1366,7 @@ define_env!(Env, <E: Ext>,
 	seal_println(ctx, str_ptr: u32, str_len: u32) => {
 		let data = ctx.read_sandbox_memory(str_ptr, str_len)?;
 		if let Ok(utf8) = core::str::from_utf8(&data) {
-			sp_runtime::print(utf8);
+			log::info!(target: "runtime::contracts", "seal_println: {}", utf8);
 		}
 		Ok(())
 	},