web: Allow building an MVP vanilla WASM module

Author: danielhjacobs

web/README.md

@@ -106,6 +106,7 @@ In this project, you may run the following commands to build all packages:
     -   There is `npm run build:dual-wasm` as well, to build a second WebAssembly module that makes use of some WebAssembly extensions,
         potentially resulting in better performance in browsers that support them, at the expense of longer build time.
     -   `npm run build:repro` enables reproducible builds. Note that this also requires a `version_seal.json`, which is not provided in the normal Git repository - only specially-marked reproducible source archives. Running this without a version seal will generate one based on the current state of your environment.
+    -   With either of the prior two commands, you can set the environment variable `BUILD_WASM_MVP=1`. This will build the vanilla WASM module using nightly Rust and disable all WebAssembly features that Rust enables by default. You will first need to run the command `rustup target add wasm32-unknown-unknown --toolchain nightly` so nightly Rust can also output WebAssembly.
 
 From here, you may follow the instructions to [use Ruffle on your website](packages/selfhosted/README.md),
 run a demo locally with `npm run demo`, or [install the extension in your browser](https://github.com/ruffle-rs/ruffle/wiki/Using-Ruffle#browser-extension).

web/packages/core/tools/build_wasm.ts

@@ -42,12 +42,25 @@ function cargoBuild({
     profile,
     features,
     rustFlags,
+    extensions,
 }: {
     profile?: string;
     features?: string[];
     rustFlags?: string[];
+    extensions?: boolean;
 }) {
-    let args = ["build", "--locked", "--target", "wasm32-unknown-unknown"];
+    let args =
+        !extensions && process.env["BUILD_WASM_MVP"]
+            ? [
+                  "+nightly",
+                  "build",
+                  "--locked",
+                  "-Z",
+                  "build-std=std,panic_abort",
+                  "--target",
+                  "wasm32-unknown-unknown",
+              ]
+            : ["build", "--locked", "--target", "wasm32-unknown-unknown"];
     if (profile) {
         args.push("--profile", profile);
     }
@@ -99,9 +112,13 @@ function buildWasm(
     let originalWasmPath;
     if (wasmSource === "cargo" || wasmSource === "cargo_and_store") {
         console.log(`Building ${flavor} with cargo...`);
+        if (process.env["BUILD_WASM_MVP"]) {
+            rustFlags.push("-C", "target-cpu=mvp");
+        }
         cargoBuild({
             profile,
             rustFlags,
+            extensions,
         });
         originalWasmPath = `../../../target/wasm32-unknown-unknown/${profile}/ruffle_web.wasm`;
         if (wasmSource === "cargo_and_store") {