enforce treeshakeability

Author: Rich-Harris

packages/svelte/package.json

@@ -90,7 +90,7 @@
     "templating"
   ],
   "scripts": {
-    "build": "rollup -c && node scripts/build.js",
+    "build": "rollup -c && node scripts/build.js && node scripts/check-treeshakeability.js",
     "watch": "rollup -cw",
     "check": "tsc && cd ./tests/types && tsc",
     "check:watch": "tsc --watch",
@@ -103,6 +103,7 @@
     "@rollup/plugin-commonjs": "^25.0.7",
     "@rollup/plugin-node-resolve": "^15.2.3",
     "@rollup/plugin-terser": "^0.4.4",
+    "@rollup/plugin-virtual": "^3.0.2",
     "@types/aria-query": "^5.0.3",
     "@types/estree": "^1.0.5",
     "dts-buddy": "^0.4.0",

packages/svelte/scripts/check-treeshakeability.js

@@ -0,0 +1,38 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import { rollup } from 'rollup';
+import virtual from '@rollup/plugin-virtual';
+
+const pkg = JSON.parse(fs.readFileSync('package.json', 'utf8'));
+
+for (const key in pkg.exports) {
+	for (const type of ['browser', 'default']) {
+		if (!pkg.exports[key][type]) continue;
+
+		const resolved = path.resolve(pkg.exports[key][type]);
+
+		const bundle = await rollup({
+			input: '__entry__',
+			plugins: [
+				virtual({
+					__entry__: `import ${JSON.stringify(resolved)}`
+				})
+			],
+			onwarn: (warning, handle) => {
+				// if (warning.code !== 'EMPTY_BUNDLE') handle(warning);
+			}
+		});
+
+		const { output } = await bundle.generate({});
+
+		if (output.length > 1) {
+			throw new Error('errr what');
+		}
+
+		const { code } = output[0];
+		if (code.trim()) {
+			console.error(code);
+			throw new Error(`${path.join(pkg.name, key)} ${type} export is not tree-shakeable`);
+		}
+	}
+}