warn about missing svelte export condition

index.js

@@ -1,5 +1,6 @@
 const path = require('path');
 const fs = require('fs');
+const { resolve } = require('resolve.exports');
 const { createFilter } = require('@rollup/pluginutils');
 const { compile, preprocess } = require('svelte/compiler');
 
@@ -14,6 +15,8 @@ const plugin_options = new Set([
 	'preprocess'
 ]);
 
+let warned = false;
+
 /**
  * @param [options] {Partial<import('.').Options>}
  * @returns {import('rollup').Plugin}
@@ -51,7 +54,7 @@ module.exports = function (options = {}) {
 		/**
 		 * Resolve an import's full filepath.
 		 */
-		resolveId(importee, importer) {
+		async resolveId(importee, importer) {
 			if (cache_emit.has(importee)) return importee;
 			if (
 				!importer ||
@@ -69,17 +72,42 @@ module.exports = function (options = {}) {
 				name += `/${parts.shift()}`;
 			}
 
-			if (parts.length > 0) return;
+			const entry = parts.join('/') || '.';
+
+			let pkg;
+			let dir;
 
 			let search_dir = importer;
 			while (search_dir !== (search_dir = path.dirname(search_dir))) {
-				const dir = path.join(search_dir, 'node_modules', name);
+				dir = path.join(search_dir, 'node_modules', name);
 				const file = path.join(dir, 'package.json');
 				if (fs.existsSync(file)) {
-					const pkg = JSON.parse(fs.readFileSync(file, 'utf-8'));
-					if (pkg.svelte) {
-						return path.resolve(dir, pkg.svelte);
+					pkg = JSON.parse(fs.readFileSync(file, 'utf-8'));
+					break;
+				}
+			}
+
+			if (!pkg) return;
+
+			// resolve pkg.svelte first for backwards compatibility
+			// we should resolve it after exports longer-term
+			if (entry === '.' && pkg.svelte) {
+				return path.resolve(dir, pkg.svelte);
+			}
+
+			const resolved = await this.resolve(importee, importer, { skipSelf: true });
+
+			// if we can't resolve this import without the `svelte` condition, warn the user
+			if (!resolved) {
+				try {
+					resolve(pkg, entry, { conditions: ['svelte'] });
+
+					if (!warned) {
+						console.error('\n\u001B[1m\u001B[31mWARNING: Your @rollup/plugin-node-resolve configuration\'s \'exportConditions\' array should include \'svelte\'. See https://github.com/sveltejs/rollup-plugin-svelte#svelte-exports-condition for more information\u001B[39m\u001B[22m\n');
+						warned = true;
 					}
+				} catch (e) {
+					// do nothing, this isn't a Svelte library
 				}
 			}
 		},

package.json

@@ -25,11 +25,12 @@
 		"node": ">=10"
 	},
 	"dependencies": {
-		"@rollup/pluginutils": "^4.1.0"
+		"@rollup/pluginutils": "^4.1.0",
+		"resolve.exports": "^2.0.0"
 	},
 	"peerDependencies": {
-		"svelte": ">=3.5.0",
-		"rollup": ">=2.0.0"
+		"rollup": ">=2.0.0",
+		"svelte": ">=3.5.0"
 	},
 	"devDependencies": {
 		"eslint": "^7.14.0",

test/index.js

@@ -8,40 +8,44 @@ const sander = require('sander');
 
 const plugin = require('..');
 
-test('resolves using pkg.svelte', () => {
+const context = {
+	resolve: () => 'resolved'
+};
+
+test('resolves using pkg.svelte', async () => {
 	const p = plugin();
 	assert.is(
-		p.resolveId('widget', path.resolve('test/foo/main.js')),
+		await p.resolveId.call(context, 'widget', path.resolve('test/foo/main.js')),
 		path.resolve('test/node_modules/widget/src/Widget.svelte')
 	);
 });
 
-test('ignores built-in modules', () => {
+test('ignores built-in modules', async () => {
 	const p = plugin();
-	assert.ok(
-		p.resolveId('path', path.resolve('test/foo/main.js')) == null
+	assert.is(
+		await p.resolveId.call(context, 'path', path.resolve('test/foo/main.js')), undefined
 	);
 });
 
-test('ignores esm modules that do not export package.json', () => {
+test('ignores esm modules that do not export package.json', async () => {
 	const p = plugin();
-	assert.ok(
-		p.resolveId('esm-no-pkg-export', path.resolve('test/foo/main.js')) == null
+	assert.is(
+		await p.resolveId.call(context, 'esm-no-pkg-export', path.resolve('test/foo/main.js')), undefined
 	);
 });
 
-test('resolves esm module that exports package.json', () => {
+test('resolves esm module that exports package.json', async () => {
 	const p = plugin();
 	assert.is(
-		p.resolveId('esm-component', path.resolve('test/foo/main.js')),
+		await p.resolveId.call(context, 'esm-component', path.resolve('test/foo/main.js')),
 		path.resolve('test/node_modules/esm-component/src/Component.svelte')
 	);
 });
 
-test('ignores virtual modules', () => {
+test('ignores virtual modules', async () => {
 	const p = plugin();
-	assert.ok(
-		p.resolveId('path', path.resolve('\0some-plugin-generated-module')) == null
+	assert.is(
+		await p.resolveId.call(context, 'path', path.resolve('\0some-plugin-generated-module')), undefined
 	);
 });