Also build the standard library crates when building the runtimes

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

@@ -87,6 +87,9 @@
 //!   required as we walk up from the target directory until we find a `Cargo.toml`. If the target
 //!   directory is changed for the build, this environment variable can be used to point to the
 //!   actual workspace.
+//! - `WASM_BUILD_STD` - Sets whether the Rust's standard library crates will also be built. This is
+//!   necessary to make sure the standard library crates only use the exact WASM feature set that
+//!   our executor supports. Enabled by default.
 //! - `CARGO_NET_OFFLINE` - If `true`, `--offline` will be passed to all processes launched to
 //!   prevent network access. Useful in offline environments.
 //!
@@ -158,6 +161,9 @@ const FORCE_WASM_BUILD_ENV: &str = "FORCE_WASM_BUILD";
 /// Environment variable that hints the workspace we are building.
 const WASM_BUILD_WORKSPACE_HINT: &str = "WASM_BUILD_WORKSPACE_HINT";
 
+/// Environment variable to set whether we'll build `core`/`std`.
+const WASM_BUILD_STD: &str = "WASM_BUILD_STD";
+
 /// Write to the given `file` if the `content` is different.
 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()) {
@@ -282,6 +288,12 @@ impl CargoCommand {
 		self.version
 	}
 
+	/// Returns whether this version of the toolchain supports nightly features.
+	fn supports_nightly_features(&self) -> bool {
+		self.version.map(|version| version.is_nightly).unwrap_or(false) ||
+			env::var("RUSTC_BOOTSTRAP").is_ok()
+	}
+
 	/// Check if the supplied cargo command supports our Substrate wasm environment.
 	///
 	/// This means that either the cargo version is at minimum 1.68.0 or this is a nightly cargo.
@@ -332,3 +344,21 @@ impl std::ops::Deref for CargoCommandVersioned {
 fn color_output_enabled() -> bool {
 	env::var(crate::WASM_BUILD_NO_COLOR).is_err()
 }
+
+/// Fetches a boolean environment variable. Will exit the process if the value is invalid.
+fn get_bool_environment_variable(name: &str) -> Option<bool> {
+	let value = env::var_os(name)?;
+
+	// We're comparing `OsString`s here so we can't use a `match`.
+	if value == "1" {
+		Some(true)
+	} else if value == "0" {
+		Some(false)
+	} else {
+		build_helper::warning!(
+			"the '{}' environment variable has an invalid value; it must be either '1' or '0'",
+			name
+		);
+		std::process::exit(1);
+	}
+}

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

@@ -693,6 +693,25 @@ fn build_project(
 		build_cmd.arg("--offline");
 	}
 
+	// Our executor currently only supports the WASM MVP feature set, however nowadays
+	// when compiling WASM the Rust compiler has more features enabled by default.
+	//
+	// We do set the `-C target-cpu=mvp` flag to make sure that *our* code gets compiled
+	// in a way that is compatible with our executor, however this doesn't affect Rust's
+	// standard library crates (`std`, `core` and `alloc`) which are by default precompiled
+	// and still can make use of these extra features.
+	//
+	// So here we force the compiler to also compile the standard library crates for us
+	// to make sure that they also only use the MVP features.
+	if crate::get_bool_environment_variable(crate::WASM_BUILD_STD).unwrap_or(true) {
+		// Unfortunately this is still a nightly-only flag, but FWIW it is pretty widely used
+		// so it's unlikely to break without a replacement.
+		build_cmd.arg("-Z").arg("build-std");
+		if !cargo_cmd.supports_nightly_features() {
+			build_cmd.env("RUSTC_BOOTSTRAP", "1");
+		}
+	}
+
 	println!("{}", colorize_info_message("Information that should be included in a bug report."));
 	println!("{} {:?}", colorize_info_message("Executing build command:"), build_cmd);
 	println!("{} {}", colorize_info_message("Using rustc version:"), cargo_cmd.rustc_version());
@@ -877,6 +896,7 @@ fn generate_rerun_if_changed_instructions(
 	println!("cargo:rerun-if-env-changed={}", crate::WASM_BUILD_RUSTFLAGS_ENV);
 	println!("cargo:rerun-if-env-changed={}", crate::WASM_TARGET_DIRECTORY);
 	println!("cargo:rerun-if-env-changed={}", crate::WASM_BUILD_TOOLCHAIN);
+	println!("cargo:rerun-if-env-changed={}", crate::WASM_BUILD_STD);
 }
 
 /// Track files and paths related to the given package to rerun `build.rs` on any relevant change.