Thoroughly improve import resolution (#2584)

* Add modules to variables to make resolution easier

* Improve handling of missing exports, add module references to variables
to simplify tracing

* Simplify namespace handling and prevent name conflicts with other imports
when creating a namespace object

* Directly register imported variables during tree-shaking to only import
variables into modules that actually use them. Get rid of variable
tracing logic on chunks.

* Fix dynamic namespace imports when preserving modules
Also, do not inline the execution list when preserving modules

* Ensure proper execution order by always adding all direct dependencies
and sorting only once all dependencies have been added

src/ExternalModule.ts

@@ -1,5 +1,4 @@
 import ExternalVariable from './ast/variables/ExternalVariable';
-import Variable from './ast/variables/Variable';
 import Graph from './Graph';
 import { OutputOptions } from './rollup/types';
 import { makeLegal } from './utils/identifierHelpers';
@@ -87,7 +86,7 @@ export default class ExternalModule {
 		});
 	}
 
-	traceExport(name: string, _isExportAllSearch?: boolean): Variable {
+	getVariableForExportName(name: string, _isExportAllSearch?: boolean): ExternalVariable {
 		if (name !== 'default' && name !== '*') this.exportsNames = true;
 		if (name === '*') this.exportsNamespace = true;
 

src/Graph.ts

@@ -4,7 +4,6 @@ import injectImportMeta from 'acorn-import-meta/inject';
 import { Program } from 'estree';
 import GlobalScope from './ast/scopes/GlobalScope';
 import { EntityPathTracker } from './ast/utils/EntityPathTracker';
-import GlobalVariable from './ast/variables/GlobalVariable';
 import Chunk from './Chunk';
 import ExternalModule from './ExternalModule';
 import Module, { defaultAcornOptions } from './Module';
@@ -32,6 +31,7 @@ import { createPluginDriver, PluginDriver } from './utils/pluginDriver';
 import relativeId, { getAliasName } from './utils/relativeId';
 import { timeEnd, timeStart } from './utils/timers';
 import transform from './utils/transform';
+import { MISSING_EXPORT_SHIM_VARIABLE } from './utils/variableNames';
 
 function makeOnwarn() {
 	const warned = Object.create(null);
@@ -61,9 +61,9 @@ export default class Graph {
 	deoptimizationTracker: EntityPathTracker;
 	scope: GlobalScope;
 	shimMissingExports: boolean;
-	exportShimVariable: GlobalVariable;
 	treeshakingOptions: TreeshakingOptions;
 	varOrConst: 'var' | 'const';
+	preserveModules: boolean;
 
 	isExternal: IsExternal;
 
@@ -97,6 +97,7 @@ export default class Graph {
 				for (const key of Object.keys(cache)) cache[key][0]++;
 			}
 		}
+		this.preserveModules = options.experimentalPreserveModules;
 
 		this.cacheExpiry = options.experimentalCacheExpiry;
 
@@ -155,11 +156,9 @@ export default class Graph {
 		this.shimMissingExports = options.shimMissingExports;
 
 		this.scope = new GlobalScope();
-		// Strictly speaking, this only applies with non-ES6, non-default-only bundles
-		for (const name of ['module', 'exports', '_interopDefault']) {
+		for (const name of ['module', 'exports', '_interopDefault', MISSING_EXPORT_SHIM_VARIABLE]) {
 			this.scope.findVariable(name); // creates global variable as side-effect
 		}
-		this.exportShimVariable = this.scope.findVariable('_missingExportShim');
 
 		this.context = String(options.context);
 
@@ -335,8 +334,7 @@ export default class Graph {
 	build(
 		entryModules: string | string[] | Record<string, string>,
 		manualChunks: Record<string, string[]> | void,
-		inlineDynamicImports: boolean,
-		preserveModules: boolean
+		inlineDynamicImports: boolean
 	): Promise<Chunk[]> {
 		// Phase 1 – discovery. We load the entry module and find which
 		// modules it imports, and import those, until we have all
@@ -401,7 +399,7 @@ export default class Graph {
 				// entry point graph colouring, before generating the import and export facades
 				timeStart('generate chunks', 2);
 
-				if (!preserveModules && !inlineDynamicImports) {
+				if (!this.preserveModules && !inlineDynamicImports) {
 					assignChunkColouringHashes(entryModules, manualChunkModules);
 				}
 
@@ -416,7 +414,7 @@ export default class Graph {
 				//       either as a namespace import reexported or top-level export *)
 				//       should be made to be its own entry point module before chunking
 				let chunks: Chunk[] = [];
-				if (preserveModules) {
+				if (this.preserveModules) {
 					for (const module of orderedModules) {
 						const chunk = new Chunk(this, [module], inlineDynamicImports);
 						if (module.isEntryPoint || !chunk.isEmpty) {
@@ -457,14 +455,14 @@ export default class Graph {
 
 				// then go over and ensure all entry chunks export their variables
 				for (const chunk of chunks) {
-					if (preserveModules || chunk.entryModules.length > 0) {
+					if (this.preserveModules || chunk.entryModules.length > 0) {
 						chunk.generateEntryExportsOrMarkAsTainted();
 					}
 				}
 
 				// create entry point facades for entry module chunks that have tainted exports
 				const facades = [];
-				if (!preserveModules) {
+				if (!this.preserveModules) {
 					for (const chunk of chunks) {
 						for (const entryModule of chunk.entryModules) {
 							if (chunk.facadeModule !== entryModule) {
@@ -687,12 +685,12 @@ export default class Graph {
 								});
 							}
 
-							// add external declarations so we can detect which are never used
-							for (const name in module.imports) {
-								const importDeclaration = module.imports[name];
+							for (const name in module.importDescriptions) {
+								const importDeclaration = module.importDescriptions[name];
 								if (importDeclaration.source !== source) return;
 
-								externalModule.traceExport(importDeclaration.name);
+								// this will trigger a warning for unused external imports
+								externalModule.getVariableForExportName(importDeclaration.name);
 							}
 						} else {
 							module.resolvedIds[source] = <string>resolvedId;