Make wasm-builder print the rustc version (#7351)

* Make `wasm-builder` print the rustc version

This makes `wasm-builder` print the rustc version that is being used to
compile the project. This is rather useful, because people can check
faster if the used rustc version is maybe known for being broken with
Substrate.

* Apply suggestions from code review

* Add some comments

substrate/utils/wasm-builder/src/lib.rs

@@ -92,7 +92,7 @@
 //! as well. For example if installing the rust nightly from 20.02.2020 using `rustup install nightly-2020-02-20`,
 //! the wasm target needs to be installed as well `rustup target add wasm32-unknown-unknown --toolchain nightly-2020-02-20`.
 
-use std::{env, fs, path::PathBuf, process::{Command, self}, io::BufRead};
+use std::{env, fs, path::{PathBuf, Path}, process::{Command, self}, io::BufRead};
 
 mod prerequisites;
 mod wasm_project;
@@ -158,14 +158,18 @@ pub fn build_project_with_default_rustflags(
 		panic!("'{}' no valid path to a `Cargo.toml`!", cargo_manifest.display());
 	}
 
-	if let Some(err_msg) = prerequisites::check() {
-		eprintln!("{}", err_msg);
-		process::exit(1);
-	}
+	let cargo_cmd = match prerequisites::check() {
+		Ok(cmd) => cmd,
+		Err(err_msg) => {
+			eprintln!("{}", err_msg);
+			process::exit(1);
+		},
+	};
 
 	let (wasm_binary, bloaty) = wasm_project::create_and_compile(
 		&cargo_manifest,
 		default_rustflags,
+		cargo_cmd,
 	);
 
 	let (wasm_binary, wasm_binary_bloaty) = if let Some(wasm_binary) = wasm_binary {
@@ -181,7 +185,7 @@ pub fn build_project_with_default_rustflags(
 	};
 
 	write_file_if_changed(
-		file_name.into(),
+		file_name,
 		format!(
 			r#"
 				pub const WASM_BINARY: Option<&[u8]> = Some(include_bytes!("{wasm_binary}"));
@@ -199,9 +203,10 @@ fn check_skip_build() -> bool {
 }
 
 /// Write to the given `file` if the `content` is different.
-fn write_file_if_changed(file: PathBuf, content: String) {
-	if fs::read_to_string(&file).ok().as_ref() != Some(&content) {
-		fs::write(&file, content).unwrap_or_else(|_| panic!("Writing `{}` can not fail!", file.display()));
+fn write_file_if_changed(file: impl AsRef<Path>, content: impl AsRef<str>) {
+	if fs::read_to_string(file.as_ref()).ok().as_deref() != Some(content.as_ref()) {
+		fs::write(file.as_ref(), content.as_ref())
+			.unwrap_or_else(|_| panic!("Writing `{}` can not fail!", file.as_ref().display()));
 	}
 }
 
@@ -268,7 +273,7 @@ fn get_rustup_nightly(selected: Option<String>) -> Option<CargoCommand> {
 	Some(CargoCommand::new_with_args("rustup", &["run", &version, "cargo"]))
 }
 
-/// Builder for cargo commands
+/// Wraps a specific command which represents a cargo invocation.
 #[derive(Debug)]
 struct CargoCommand {
 	program: String,
@@ -310,6 +315,34 @@ impl CargoCommand {
 	}
 }
 
+/// Wraps a [`CargoCommand`] and the version of `rustc` the cargo command uses.
+struct CargoCommandVersioned {
+	command: CargoCommand,
+	version: String,
+}
+
+impl CargoCommandVersioned {
+	fn new(command: CargoCommand, version: String) -> Self {
+		Self {
+			command,
+			version,
+		}
+	}
+
+	/// Returns the `rustc` version.
+	fn rustc_version(&self) -> &str {
+		&self.version
+	}
+}
+
+impl std::ops::Deref for CargoCommandVersioned {
+	type Target = CargoCommand;
+
+	fn deref(&self) -> &CargoCommand {
+		&self.command
+	}
+}
+
 /// Returns `true` when color output is enabled.
 fn color_output_enabled() -> bool {
 	env::var(crate::WASM_BUILD_NO_COLOR).is_err()