Feat: add syntheticNamedExports (#3295)

* Feat: add syntheticNamedExports

* fix namespaces

* remove getDefaultExport

* enable tests

* live references

* add tests

* fix test

* fix dead reference

* apply fixes

* add test for syntheticNamedExports in  transform

* apply feedback

* enable tests

* fix test

* unneeded exports check

* rename SyntheticNamedExport

* add docs

* Slightly clarify docs.

Co-authored-by: Lukas Taegert-Atkinson <lukastaegert@users.noreply.github.com>

docs/05-plugin-development.md

@@ -169,15 +169,33 @@ Phase: `generate`
 Cf. [`output.intro/output.outro`](guide/en/#outputintrooutputoutro).
 
 #### `load`
-Type: `(id: string) => string | null | { code: string, map?: string | SourceMap, ast? : ESTree.Program, moduleSideEffects?: boolean | null }`<br>
+Type: `(id: string) => string | null | { code: string, map?: string | SourceMap, ast? : ESTree.Program, moduleSideEffects?: boolean | null, syntheticNamedExports?: boolean | null }`<br>
 Kind: `async, first`<br>
 Phase: `build`
 
 Defines a custom loader. Returning `null` defers to other `load` functions (and eventually the default behavior of loading from the file system). To prevent additional parsing overhead in case e.g. this hook already used `this.parse` to generate an AST for some reason, this hook can optionally return a `{ code, ast, map }` object. The `ast` must be a standard ESTree AST with `start` and `end` properties for each node. If the transformation does not move code, you can preserve existing sourcemaps by setting `map` to `null`. Otherwise you might need to generate the source map. See [the section on source code transformations](#source-code-transformations).
 
 If `false` is returned for `moduleSideEffects` and no other module imports anything from this module, then this module will not be included in the bundle without checking for actual side-effects inside the module. If `true` is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If `null` is returned or the flag is omitted, then `moduleSideEffects` will be determined by the first `resolveId` hook that resolved this module, the `treeshake.moduleSideEffects` option, or eventually default to `true`. The `transform` hook can override this.
 
-You can use [`this.getModuleInfo`](guide/en/#thisgetmoduleinfomoduleid-string--moduleinfo) to find out the previous value of `moduleSideEffects` inside this hook.
+If `true` is returned for `syntheticNamedExports`, this module will fallback the resolution of any missing named export to properties of the `default` export. The `transform` hook can override this. This option allows to have dynamic named exports that might not be declared in the module, such as in this example:
+
+**dep.js: (`{syntheticNamedExports: true}`)**
+
+```
+export default {
+  foo: 42,
+  bar: 'hello'
+}
+```
+
+**main.js: (entry point)**
+
+```js
+import { foo, bar } from './dep.js'
+console.log(foo, bar);
+```
+
+You can use [`this.getModuleInfo`](guide/en/#thisgetmoduleinfomoduleid-string--moduleinfo) to find out the previous values of `moduleSideEffects` and `syntheticNamedExports` inside this hook.
 
 #### `options`
 Type: `(options: InputOptions) => InputOptions | null`<br>
@@ -268,7 +286,7 @@ resolveFileUrl({fileName}) {
 ```
 
 #### `resolveId`
-Type: `(source: string, importer: string) => string | false | null | {id: string, external?: boolean, moduleSideEffects?: boolean | null}`<br>
+Type: `(source: string, importer: string) => string | false | null | {id: string, external?: boolean, moduleSideEffects?: boolean | null, syntheticNamedExports?: boolean | null}`<br>
 Kind: `async, first`<br>
 Phase: `build`
 
@@ -289,6 +307,24 @@ Relative ids, i.e. starting with `./` or `../`, will **not** be renormalized whe
 
 If `false` is returned for `moduleSideEffects` in the first hook that resolves a module id and no other module imports anything from this module, then this module will not be included without checking for actual side-effects inside the module. If `true` is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If `null` is returned or the flag is omitted, then `moduleSideEffects` will be determined by the `treeshake.moduleSideEffects` option or default to `true`. The `load` and `transform` hooks can override this.
 
+If `true` is returned for `syntheticNamedExports`, this module will fallback the resolution of any missing named export to properties of the `default` export. The `load` and `transform` hooks can override this. This option allows to have dynamic named exports that might not be declared in the module, such as in this example:
+
+**dep.js: (`{syntheticNamedExports: true}`)**
+
+```
+export default {
+  foo: 42,
+  bar: 'hello'
+}
+```
+
+**main.js: (entry point)**
+
+```js
+import { foo, bar } from './dep.js'
+console.log(foo, bar);
+```
+
 #### `resolveImportMeta`
 Type: `(property: string | null, {chunkId: string, moduleId: string, format: string}) => string | null`<br>
 Kind: `sync, first`<br>
@@ -313,7 +349,7 @@ resolveImportMeta(property, {moduleId}) {
 Note that since this hook has access to the filename of the current chunk, its return value will not be considered when generating the hash of this chunk.
 
 #### `transform`
-Type: `(code: string, id: string) => string | null | { code: string, map?: string | SourceMap, ast? : ESTree.Program, moduleSideEffects?: boolean | null }`<br>
+Type: `(code: string, id: string) => string | null | { code: string, map?: string | SourceMap, ast? : ESTree.Program, moduleSideEffects?: boolean | null, syntheticNamedExports?: boolean | null }`<br>
 Kind: `async, sequential`<br>
 Phase: `build`
 
@@ -323,6 +359,24 @@ Note that in watch mode, the result of this hook is cached when rebuilding and t
 
 If `false` is returned for `moduleSideEffects` and no other module imports anything from this module, then this module will not be included without checking for actual side-effects inside the module. If `true` is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If `null` is returned or the flag is omitted, then `moduleSideEffects` will be determined by the first `resolveId` hook that resolved this module, the `treeshake.moduleSideEffects` option, or eventually default to `true`.
 
+If `true` is returned for `syntheticNamedExports`, this module will fallback the resolution of any missing named export to properties of the `default` export. This option allows to have dynamic named exports that might not be declared in the module, such as in this example:
+
+**dep.js: (`{syntheticNamedExports: true}`)**
+
+```
+export default {
+  foo: 42,
+  bar: 'hello'
+}
+```
+
+**main.js: (entry point)**
+
+```js
+import { foo, bar } from './dep.js'
+console.log(foo, bar);
+```
+
 You can use [`this.getModuleInfo`](guide/en/#thisgetmoduleinfomoduleid-string--moduleinfo) to find out the previous value of `moduleSideEffects` inside this hook.
 
 #### `watchChange`

src/Module.ts

@@ -22,9 +22,11 @@ import TemplateLiteral from './ast/nodes/TemplateLiteral';
 import VariableDeclaration from './ast/nodes/VariableDeclaration';
 import ModuleScope from './ast/scopes/ModuleScope';
 import { PathTracker, UNKNOWN_PATH } from './ast/utils/PathTracker';
+import ExportDefaultVariable from './ast/variables/ExportDefaultVariable';
 import ExportShimVariable from './ast/variables/ExportShimVariable';
 import ExternalVariable from './ast/variables/ExternalVariable';
 import NamespaceVariable from './ast/variables/NamespaceVariable';
+import SyntheticNamedExportVariable from './ast/variables/SyntheticNamedExportVariable';
 import Variable from './ast/variables/Variable';
 import Chunk from './Chunk';
 import ExternalModule from './ExternalModule';
@@ -39,7 +41,7 @@ import {
 	RollupWarning,
 	TransformModuleJSON
 } from './rollup/types';
-import { error } from './utils/error';
+import { error, Errors } from './utils/error';
 import getCodeFrame from './utils/getCodeFrame';
 import { getOriginalLocation } from './utils/getOriginalLocation';
 import { makeLegal } from './utils/identifierHelpers';
@@ -206,6 +208,7 @@ export default class Module {
 	scope!: ModuleScope;
 	sourcemapChain!: DecodedSourceMapOrMissing[];
 	sources = new Set<string>();
+	syntheticNamedExports: boolean;
 	transformFiles?: EmittedFile[];
 	userChunkNames = new Set<string>();
 	usesTopLevelAwait = false;
@@ -214,19 +217,28 @@ export default class Module {
 	private ast!: Program;
 	private astContext!: AstContext;
 	private context: string;
+	private defaultExport: ExportDefaultVariable | null | undefined = null;
 	private esTreeAst!: ESTree.Program;
 	private graph: Graph;
 	private magicString!: MagicString;
 	private namespaceVariable: NamespaceVariable | null = null;
+	private syntheticExports = new Map<string, SyntheticNamedExportVariable>();
 	private transformDependencies: string[] = [];
 	private transitiveReexports: string[] | null = null;
 
-	constructor(graph: Graph, id: string, moduleSideEffects: boolean, isEntry: boolean) {
+	constructor(
+		graph: Graph,
+		id: string,
+		moduleSideEffects: boolean,
+		syntheticNamedExports: boolean,
+		isEntry: boolean
+	) {
 		this.id = id;
 		this.graph = graph;
 		this.excludeFromSourcemap = /\0/.test(id);
 		this.context = graph.getModuleContext(id);
 		this.moduleSideEffects = moduleSideEffects;
+		this.syntheticNamedExports = syntheticNamedExports;
 		this.isEntryPoint = isEntry;
 	}
 
@@ -299,6 +311,21 @@ export default class Module {
 		return allExportNames;
 	}
 
+	getDefaultExport() {
+		if (this.defaultExport === null) {
+			this.defaultExport = undefined;
+			this.defaultExport = this.getVariableForExportName('default') as ExportDefaultVariable;
+		}
+		if (!this.defaultExport) {
+			return error({
+				code: Errors.SYNTHETIC_NAMED_EXPORTS_NEED_DEFAULT,
+				id: this.id,
+				message: `Modules with 'syntheticNamedExports' need a default export.`
+			});
+		}
+		return this.defaultExport;
+	}
+
 	getDynamicImportExpressions(): (string | Node)[] {
 		return this.dynamicImports.map(({ node }) => {
 			const importArgument = node.source;
@@ -342,7 +369,7 @@ export default class Module {
 
 	getOrCreateNamespace(): NamespaceVariable {
 		if (!this.namespaceVariable) {
-			this.namespaceVariable = new NamespaceVariable(this.astContext);
+			this.namespaceVariable = new NamespaceVariable(this.astContext, this.syntheticNamedExports);
 			this.namespaceVariable.initialise();
 		}
 		return this.namespaceVariable;
@@ -439,9 +466,22 @@ export default class Module {
 
 		// we don't want to create shims when we are just
 		// probing export * modules for exports
-		if (this.graph.shimMissingExports && !isExportAllSearch) {
-			this.shimMissingExport(name);
-			return this.exportShimVariable;
+		if (!isExportAllSearch) {
+			if (this.syntheticNamedExports) {
+				let syntheticExport = this.syntheticExports.get(name);
+				if (!syntheticExport) {
+					const defaultExport = this.getDefaultExport();
+					syntheticExport = new SyntheticNamedExportVariable(this.astContext, name, defaultExport);
+					this.syntheticExports.set(name, syntheticExport);
+					return syntheticExport;
+				}
+				return syntheticExport;
+			}
+
+			if (this.graph.shimMissingExports) {
+				this.shimMissingExport(name);
+				return this.exportShimVariable;
+			}
 		}
 		return undefined as any;
 	}
@@ -534,6 +574,7 @@ export default class Module {
 		originalSourcemap,
 		resolvedIds,
 		sourcemapChain,
+		syntheticNamedExports,
 		transformDependencies,
 		transformFiles
 	}: TransformModuleJSON & {
@@ -551,6 +592,9 @@ export default class Module {
 		if (typeof moduleSideEffects === 'boolean') {
 			this.moduleSideEffects = moduleSideEffects;
 		}
+		if (typeof syntheticNamedExports === 'boolean') {
+			this.syntheticNamedExports = syntheticNamedExports;
+		}
 
 		timeStart('generate ast', 3);
 
@@ -633,6 +677,7 @@ export default class Module {
 			originalSourcemap: this.originalSourcemap,
 			resolvedIds: this.resolvedIds,
 			sourcemapChain: this.sourcemapChain,
+			syntheticNamedExports: this.syntheticNamedExports,
 			transformDependencies: this.transformDependencies,
 			transformFiles: this.transformFiles
 		};

src/ModuleLoader.ts

@@ -17,6 +17,7 @@ import {
 	errBadLoader,
 	errCannotAssignModuleToChunk,
 	errEntryCannotBeExternal,
+	errExternalSyntheticExports,
 	errInternalIdCannotBeExternal,
 	errInvalidOption,
 	errNamespaceConflict,
@@ -243,7 +244,7 @@ export class ModuleLoader {
 					module.id,
 					(module.resolvedIds[source] =
 						module.resolvedIds[source] ||
-						this.handleMissingImports(await this.resolveId(source, module.id), source, module.id))
+						this.handleResolveId(await this.resolveId(source, module.id), source, module.id))
 				)
 			) as Promise<unknown>[]),
 			...module.getDynamicImportExpressions().map((specifier, index) =>
@@ -272,6 +273,7 @@ export class ModuleLoader {
 		id: string,
 		importer: string,
 		moduleSideEffects: boolean,
+		syntheticNamedExports: boolean,
 		isEntry: boolean
 	): Promise<Module> {
 		const existingModule = this.modulesById.get(id);
@@ -280,7 +282,13 @@ export class ModuleLoader {
 			return Promise.resolve(existingModule);
 		}
 
-		const module: Module = new Module(this.graph, id, moduleSideEffects, isEntry);
+		const module: Module = new Module(
+			this.graph,
+			id,
+			moduleSideEffects,
+			syntheticNamedExports,
+			isEntry
+		);
 		this.modulesById.set(id, module);
 		this.graph.watchFiles[id] = true;
 		const manualChunkAlias = this.getManualChunk(id);
@@ -321,6 +329,9 @@ export class ModuleLoader {
 				if (typeof sourceDescription.moduleSideEffects === 'boolean') {
 					module.moduleSideEffects = sourceDescription.moduleSideEffects;
 				}
+				if (typeof sourceDescription.syntheticNamedExports === 'boolean') {
+					module.syntheticNamedExports = sourceDescription.syntheticNamedExports;
+				}
 				return transform(this.graph, sourceDescription, module);
 			})
 			.then((source: TransformModuleJSON | ModuleJSON) => {
@@ -370,11 +381,17 @@ export class ModuleLoader {
 			}
 			return Promise.resolve(externalModule);
 		} else {
-			return this.fetchModule(resolvedId.id, importer, resolvedId.moduleSideEffects, false);
+			return this.fetchModule(
+				resolvedId.id,
+				importer,
+				resolvedId.moduleSideEffects,
+				resolvedId.syntheticNamedExports,
+				false
+			);
 		}
 	}
 
-	private handleMissingImports(
+	private handleResolveId(
 		resolvedId: ResolvedId | null,
 		source: string,
 		importer: string
@@ -387,8 +404,13 @@ export class ModuleLoader {
 			return {
 				external: true,
 				id: source,
-				moduleSideEffects: this.hasModuleSideEffects(source, true)
+				moduleSideEffects: this.hasModuleSideEffects(source, true),
+				syntheticNamedExports: false
 			};
+		} else {
+			if (resolvedId.external && resolvedId.syntheticNamedExports) {
+				this.graph.warn(errExternalSyntheticExports(source, importer));
+			}
 		}
 		return resolvedId;
 	}
@@ -407,7 +429,7 @@ export class ModuleLoader {
 					: resolveIdResult;
 
 			if (typeof id === 'string') {
-				return this.fetchModule(id, undefined as any, true, isEntry);
+				return this.fetchModule(id, undefined as any, true, false, isEntry);
 			}
 			return error(errUnresolvedEntry(unresolvedId));
 		});
@@ -420,6 +442,7 @@ export class ModuleLoader {
 		let id = '';
 		let external = false;
 		let moduleSideEffects = null;
+		let syntheticNamedExports = false;
 		if (resolveIdResult) {
 			if (typeof resolveIdResult === 'object') {
 				id = resolveIdResult.id;
@@ -429,6 +452,9 @@ export class ModuleLoader {
 				if (typeof resolveIdResult.moduleSideEffects === 'boolean') {
 					moduleSideEffects = resolveIdResult.moduleSideEffects;
 				}
+				if (typeof resolveIdResult.syntheticNamedExports === 'boolean') {
+					syntheticNamedExports = resolveIdResult.syntheticNamedExports;
+				}
 			} else {
 				if (this.isExternal(resolveIdResult, importer, true)) {
 					external = true;
@@ -448,7 +474,8 @@ export class ModuleLoader {
 			moduleSideEffects:
 				typeof moduleSideEffects === 'boolean'
 					? moduleSideEffects
-					: this.hasModuleSideEffects(id, external)
+					: this.hasModuleSideEffects(id, external),
+			syntheticNamedExports
 		};
 	}
 
@@ -478,13 +505,9 @@ export class ModuleLoader {
 		if (resolution == null) {
 			return (module.resolvedIds[specifier] =
 				module.resolvedIds[specifier] ||
-				this.handleMissingImports(
-					await this.resolveId(specifier, module.id),
-					specifier,
-					module.id
-				));
+				this.handleResolveId(await this.resolveId(specifier, module.id), specifier, module.id));
 		}
-		return this.handleMissingImports(
+		return this.handleResolveId(
 			this.normalizeResolveIdResult(resolution, importer, specifier),
 			specifier,
 			importer