Prevent CSS from being added to pages not using it (#2918)

* Prevent CSS from being added to pages not using it

* Adds a changeset

* Add clarification when the CSS is appended to the pageData

* Move addStyles up a level

12 files changed, 153 insertions, 7 deletions

diff --git a/.changeset/tiny-snails-grab.md b/.changeset/tiny-snails-grab.md
new file mode 100644
index 000000000..2f7d50651
--- /dev/null
+++ b/.changeset/tiny-snails-grab.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Prevent CSS from being added to the wrong pages
diff --git a/packages/astro/src/core/build/internal.ts b/packages/astro/src/core/build/internal.ts
index c436b9c5c..674a657b6 100644
--- a/packages/astro/src/core/build/internal.ts
+++ b/packages/astro/src/core/build/internal.ts
@@ -105,6 +105,10 @@ export function getPageDataByViteID(internals: BuildInternals, viteid: ViteID):
 	return undefined;
 }
 
+export function hasPageDataByViteID(internals: BuildInternals, viteid: ViteID): boolean {
+	return internals.pagesByViteID.has(viteid);
+}
+
 export function* eachPageData(internals: BuildInternals) {
 	yield* internals.pagesByComponent.values();
 }
diff --git a/packages/astro/src/core/build/scan-based-build.ts b/packages/astro/src/core/build/scan-based-build.ts
index 1a4cf16e4..ec473ca79 100644
--- a/packages/astro/src/core/build/scan-based-build.ts
+++ b/packages/astro/src/core/build/scan-based-build.ts
@@ -76,6 +76,7 @@ export async function build(opts: ScanBasedBuildOptions): Promise<RollupOutput |
 			}),
 			rollupPluginAstroBuildCSS({
 				internals,
+				legacy: true
 			}),
 			...(viteConfig.plugins || []),
 		],
diff --git a/packages/astro/src/core/build/static-build.ts b/packages/astro/src/core/build/static-build.ts
index 2e54891d3..086d0b82e 100644
--- a/packages/astro/src/core/build/static-build.ts
+++ b/packages/astro/src/core/build/static-build.ts
@@ -132,7 +132,6 @@ async function ssrBuild(opts: StaticBuildOptions, internals: BuildInternals, inp
 					entryFileNames: opts.buildConfig.serverEntry,
 					chunkFileNames: 'chunks/chunk.[hash].mjs',
 					assetFileNames: 'assets/asset.[hash][extname]',
-					inlineDynamicImports: true,
 				},
 			},
 			// must match an esbuild target
@@ -147,6 +146,7 @@ async function ssrBuild(opts: StaticBuildOptions, internals: BuildInternals, inp
 			vitePluginPages(opts, internals),
 			rollupPluginAstroBuildCSS({
 				internals,
+				legacy: false
 			}),
 			...(viteConfig.plugins || []),
 			// SSR needs to be last
@@ -200,6 +200,7 @@ async function clientBuild(opts: StaticBuildOptions, internals: BuildInternals,
 			vitePluginHoistedScripts(astroConfig, internals),
 			rollupPluginAstroBuildCSS({
 				internals,
+				legacy: false
 			}),
 			...(viteConfig.plugins || []),
 		],
diff --git a/packages/astro/src/core/build/vite-plugin-pages.ts b/packages/astro/src/core/build/vite-plugin-pages.ts
index 22991c8ae..ea7fe1600 100644
--- a/packages/astro/src/core/build/vite-plugin-pages.ts
+++ b/packages/astro/src/core/build/vite-plugin-pages.ts
@@ -6,7 +6,7 @@ import { eachPageData } from './internal.js';
 import { isBuildingToSSR } from '../util.js';
 
 export const virtualModuleId = '@astrojs-pages-virtual-entry';
-const resolvedVirtualModuleId = '\0' + virtualModuleId;
+export const resolvedVirtualModuleId = '\0' + virtualModuleId;
 
 export function vitePluginPages(opts: StaticBuildOptions, internals: BuildInternals): VitePlugin {
 	return {
diff --git a/packages/astro/src/vite-plugin-build-css/index.ts b/packages/astro/src/vite-plugin-build-css/index.ts
index 7de50af4e..94be52405 100644
--- a/packages/astro/src/vite-plugin-build-css/index.ts
+++ b/packages/astro/src/vite-plugin-build-css/index.ts
@@ -1,10 +1,12 @@
-import type { BuildInternals } from '../core/build/internal';
+import { BuildInternals } from '../core/build/internal';
+import type { ModuleInfo, PluginContext } from 'rollup';
 
 import * as path from 'path';
 import esbuild from 'esbuild';
 import { Plugin as VitePlugin } from 'vite';
 import { isCSSRequest } from '../core/render/dev/css.js';
-import { getPageDatasByChunk } from '../core/build/internal.js';
+import { getPageDatasByChunk, getPageDataByViteID, hasPageDataByViteID } from '../core/build/internal.js';
+import { resolvedVirtualModuleId as virtualPagesModuleId } from '../core/build/vite-plugin-pages.js';
 
 const PLUGIN_NAME = '@astrojs/rollup-plugin-build-css';
 
@@ -44,12 +46,64 @@ function isPageStyleVirtualModule(id: string) {
 
 interface PluginOptions {
 	internals: BuildInternals;
+	legacy: boolean;
 }
 
 export function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin {
-	const { internals } = options;
+	const { internals, legacy } = options;
 	const styleSourceMap = new Map<string, string>();
 
+	function * walkStyles(ctx: PluginContext, id: string, seen = new Set<string>()): Generator<[string, string], void, unknown> {
+		seen.add(id);
+		if(styleSourceMap.has(id)) {
+			yield [id, styleSourceMap.get(id)!];
+		}
+
+		const info = ctx.getModuleInfo(id);
+		if(info) {
+			for(const importedId of info.importedIds) {
+				if(!seen.has(importedId)) {
+					yield * walkStyles(ctx, importedId, seen);
+				}
+			}
+		}
+	}
+
+	/**
+	 * This walks the dependency graph looking for styles that are imported
+	 * by a page and then creates a chunking containing all of the styles for that page.
+	 * Since there is only 1 entrypoint for the entire app, we do this in order
+	 * to prevent adding all styles to all pages.
+	 */
+	async function addStyles(this: PluginContext) {
+		for(const id of this.getModuleIds()) {
+			if(hasPageDataByViteID(internals, id)) {
+				let pageStyles = '';
+				for(const [_styleId, styles] of walkStyles(this, id)) {
+					pageStyles += styles;
+				}
+
+				// Pages with no styles, nothing more to do
+				if(!pageStyles) continue;
+
+				const { code: minifiedCSS } = await esbuild.transform(pageStyles, {
+					loader: 'css',
+					minify: true,
+				});
+				const referenceId = this.emitFile({
+					name: 'entry' + '.css',
+					type: 'asset',
+					source: minifiedCSS,
+				});
+				const fileName = this.getFileName(referenceId);
+
+				// Add CSS file to the page's pageData, so that it will be rendered with
+				// the correct links.
+				getPageDataByViteID(internals, id)?.css.add(fileName);
+			}
+		}
+	}
+
 	return {
 		name: PLUGIN_NAME,
 
@@ -76,7 +130,6 @@ export function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin {
 				plugins.splice(viteCSSPostIndex - 1, 0, ourPlugin);
 			}
 		},
-
 		async resolveId(id) {
 			if (isPageStyleVirtualModule(id)) {
 				return id;
@@ -108,6 +161,8 @@ export function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin {
 		},
 
 		async renderChunk(_code, chunk) {
+			if(!legacy) return null;
+
 			let chunkCSS = '';
 			let isPureCSS = true;
 			for (const [id] of Object.entries(chunk.modules)) {
@@ -147,7 +202,7 @@ export function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin {
 		},
 
 		// Delete CSS chunks so JS is not produced for them.
-		generateBundle(opts, bundle) {
+		async generateBundle(opts, bundle) {
 			const hasPureCSSChunks = internals.pureCSSChunks.size;
 			const pureChunkFilenames = new Set([...internals.pureCSSChunks].map((chunk) => chunk.fileName));
 			const emptyChunkFiles = [...pureChunkFilenames]
@@ -156,6 +211,11 @@ export function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin {
 				.replace(/\./g, '\\.');
 			const emptyChunkRE = new RegExp(opts.format === 'es' ? `\\bimport\\s*"[^"]*(?:${emptyChunkFiles})";\n?` : `\\brequire\\(\\s*"[^"]*(?:${emptyChunkFiles})"\\);\n?`, 'g');
 
+			// Crawl the module graph to find CSS chunks to create
+			if(!legacy) {
+				await addStyles.call(this);
+			}
+
 			for (const [chunkId, chunk] of Object.entries(bundle)) {
 				if (chunk.type === 'chunk') {
 					// This find shared chunks of CSS and adds them to the main CSS chunks,
diff --git a/packages/astro/test/fixtures/page-level-styles/src/layouts/Base.astro b/packages/astro/test/fixtures/page-level-styles/src/layouts/Base.astro
new file mode 100644
index 000000000..f900a1e88
--- /dev/null
+++ b/packages/astro/test/fixtures/page-level-styles/src/layouts/Base.astro
@@ -0,0 +1,13 @@
+---
+
+---
+<html lang="en">
+	<head>
+		<meta charset="utf-8" />
+		<meta name="viewport" content="width=device-width" />
+		<title>Astro</title>
+	</head>
+	<body>
+		<slot />
+	</body>
+</html>
diff --git a/packages/astro/test/fixtures/page-level-styles/src/layouts/Styled.astro b/packages/astro/test/fixtures/page-level-styles/src/layouts/Styled.astro
new file mode 100644
index 000000000..e32ade457
--- /dev/null
+++ b/packages/astro/test/fixtures/page-level-styles/src/layouts/Styled.astro
@@ -0,0 +1,13 @@
+---
+import "../styles/main.css";
+---
+<html lang="en">
+	<head>
+		<meta charset="utf-8" />
+		<meta name="viewport" content="width=device-width" />
+		<title>Astro</title>
+	</head>
+	<body>
+		<slot />
+	</body>
+</html>
diff --git a/packages/astro/test/fixtures/page-level-styles/src/pages/blog.astro b/packages/astro/test/fixtures/page-level-styles/src/pages/blog.astro
new file mode 100644
index 000000000..de9f896a9
--- /dev/null
+++ b/packages/astro/test/fixtures/page-level-styles/src/pages/blog.astro
@@ -0,0 +1,9 @@
+---
+import Layout from '../layouts/Styled.astro'
+---
+
+<Layout>
+	<h1>Astro (with main.css)</h1>
+	<p>This should have custom styles (white text on black background)</p>
+	<a href="/">Home (without main.css)</a>
+</Layout>
diff --git a/packages/astro/test/fixtures/page-level-styles/src/pages/index.astro b/packages/astro/test/fixtures/page-level-styles/src/pages/index.astro
new file mode 100644
index 000000000..d4ae35676
--- /dev/null
+++ b/packages/astro/test/fixtures/page-level-styles/src/pages/index.astro
@@ -0,0 +1,9 @@
+---
+import Layout from '../layouts/Base.astro'
+---
+
+<Layout>
+	<h1>Astro (without main.css)</h1>
+	<p>This should have default styles (white text on black background)</p>
+	<a href="/blog">Blog (with main.css)</a>
+</Layout>
diff --git a/packages/astro/test/fixtures/page-level-styles/src/styles/main.css b/packages/astro/test/fixtures/page-level-styles/src/styles/main.css
new file mode 100644
index 000000000..1bee9c028
--- /dev/null
+++ b/packages/astro/test/fixtures/page-level-styles/src/styles/main.css
@@ -0,0 +1,4 @@
+body {
+  background-color: black;
+  color: white;
+}
diff --git a/packages/astro/test/page-level-styles.test.js b/packages/astro/test/page-level-styles.test.js
new file mode 100644
index 000000000..ae06c6697
--- /dev/null
+++ b/packages/astro/test/page-level-styles.test.js
@@ -0,0 +1,27 @@
+import { expect } from 'chai';
+import { load as cheerioLoad } from 'cheerio';
+import { loadFixture } from './test-utils.js';
+
+// Asset bundling
+describe('Page-level styles', () => {
+	let fixture;
+
+	before(async () => {
+		fixture = await loadFixture({
+			projectRoot: './fixtures/page-level-styles/',
+		});
+		await fixture.build();
+	});
+
+	it('Doesn\'t add page styles for a page without style imports', async () => {
+		let html = await fixture.readFile('/index.html');
+		let $ = await cheerioLoad(html);
+		expect($('link').length).to.equal(0);
+	});
+
+	it('Does add page styles for pages with style imports (or deps)', async () => {
+		let html = await fixture.readFile('/blog/index.html');
+		let $ = await cheerioLoad(html);
+		expect($('link').length).to.equal(1);
+	});
+});