exportInfoMap.ts

import {
    __String,
    addToSeen,
    append,
    arrayIsEqualTo,
    CancellationToken,
    CompilerOptions,
    consumesNodeCoreModules,
    createMultiMap,
    Debug,
    emptyArray,
    ensureTrailingDirectorySeparator,
    findIndex,
    forEachAncestorDirectory,
    forEachEntry,
    FutureSourceFile,
    getBaseFileName,
    GetCanonicalFileName,
    getDefaultLikeExportNameFromDeclaration,
    getDirectoryPath,
    getEmitScriptTarget,
    getLocalSymbolForExportDefault,
    getNamesForExportedSymbol,
    getNodeModulePathParts,
    getPackageNameFromTypesPackageName,
    getRegexFromPattern,
    getSubPatternFromSpec,
    getSymbolId,
    hostGetCanonicalFileName,
    hostUsesCaseSensitiveFileNames,
    InternalSymbolName,
    isExternalModuleNameRelative,
    isExternalModuleSymbol,
    isExternalOrCommonJsModule,
    isKnownSymbol,
    isNonGlobalAmbientModule,
    isPrivateIdentifierSymbol,
    LanguageServiceHost,
    mapDefined,
    ModuleSpecifierCache,
    ModuleSpecifierResolutionHost,
    moduleSpecifiers,
    moduleSymbolToValidIdentifier,
    nodeModulesPathPart,
    PackageJsonImportFilter,
    Path,
    pathContainsNodeModules,
    Program,
    skipAlias,
    SourceFile,
    startsWith,
    Statement,
    stripQuotes,
    Symbol,
    SymbolFlags,
    timestamp,
    TypeChecker,
    unescapeLeadingUnderscores,
    unmangleScopedPackageName,
    UserPreferences,
} from "./_namespaces/ts.js";

/** @internal */
export const enum ImportKind {
    Named,
    Default,
    Namespace,
    CommonJS,
}

/** @internal */
export const enum ExportKind {
    Named,
    Default,
    ExportEquals,
    UMD,
}

/** @internal */
export interface SymbolExportInfo {
    readonly symbol: Symbol;
    readonly moduleSymbol: Symbol;
    /** Set if `moduleSymbol` is an external module, not an ambient module */
    moduleFileName: string | undefined;
    exportKind: ExportKind;
    targetFlags: SymbolFlags;
    /** True if export was only found via the package.json AutoImportProvider (for telemetry). */
    isFromPackageJson: boolean;
}

/**
 * @internal
 * ExportInfo for an export that does not exist yet, so does not have a symbol.
 */
export type FutureSymbolExportInfo = Omit<SymbolExportInfo, "symbol"> & { readonly symbol?: undefined; };

interface CachedSymbolExportInfo {
    // Used to rehydrate `symbol` and `moduleSymbol` when transient
    id: number;
    symbolName: string;
    capitalizedSymbolName: string | undefined;
    symbolTableKey: __String;
    moduleName: string;
    moduleFile: SourceFile | undefined;
    packageName: string | undefined;

    // SymbolExportInfo, but optional symbols
    readonly symbol: Symbol | undefined;
    readonly moduleSymbol: Symbol | undefined;
    moduleFileName: string | undefined;
    exportKind: ExportKind;
    targetFlags: SymbolFlags;
    isFromPackageJson: boolean;
}

/** @internal */
export interface ExportInfoMap {
    isUsableByFile(importingFile: Path): boolean;
    clear(): void;
    add(importingFile: Path, symbol: Symbol, key: __String, moduleSymbol: Symbol, moduleFile: SourceFile | undefined, exportKind: ExportKind, isFromPackageJson: boolean, checker: TypeChecker): void;
    get(importingFile: Path, key: ExportMapInfoKey): readonly SymbolExportInfo[] | undefined;
    search<T>(importingFile: Path, preferCapitalized: boolean, matches: (name: string, targetFlags: SymbolFlags) => boolean, action: (info: readonly SymbolExportInfo[], symbolName: string, isFromAmbientModule: boolean, key: ExportMapInfoKey) => T | undefined): T | undefined;
    releaseSymbols(): void;
    isEmpty(): boolean;
    /** @returns Whether the change resulted in the cache being cleared */
    onFileChanged(oldSourceFile: SourceFile, newSourceFile: SourceFile, typeAcquisitionEnabled: boolean): boolean;
}

/** @internal */
export interface CacheableExportInfoMapHost {
    getCurrentProgram(): Program | undefined;
    getPackageJsonAutoImportProvider(): Program | undefined;
    getGlobalTypingsCacheLocation(): string | undefined;
}

export type ExportMapInfoKey = string & { __exportInfoKey: void; };
/** @internal */
export function createCacheableExportInfoMap(host: CacheableExportInfoMapHost): ExportInfoMap {
    let exportInfoId = 1;
    const exportInfo = createMultiMap<ExportMapInfoKey, CachedSymbolExportInfo>();
    const symbols = new Map<number, [symbol: Symbol, moduleSymbol: Symbol]>();
    /**
     * Key: node_modules package name (no @types).
     * Value: path to deepest node_modules folder seen that is
     * both visible to `usableByFileName` and contains the package.
     *
     * Later, we can see if a given SymbolExportInfo is shadowed by
     * a another installation of the same package in a deeper
     * node_modules folder by seeing if its path starts with the
     * value stored here.
     */
    const packages = new Map<string, string>();
    let usableByFileName: Path | undefined;
    const cache: ExportInfoMap = {
        isUsableByFile: importingFile => importingFile === usableByFileName,
        isEmpty: () => !exportInfo.size,
        clear: () => {
            exportInfo.clear();
            symbols.clear();
            usableByFileName = undefined;
        },
        add: (importingFile, symbol, symbolTableKey, moduleSymbol, moduleFile, exportKind, isFromPackageJson, checker) => {
            if (importingFile !== usableByFileName) {
                cache.clear();
                usableByFileName = importingFile;
            }

            let packageName;
            if (moduleFile) {
                const nodeModulesPathParts = getNodeModulePathParts(moduleFile.fileName);
                if (nodeModulesPathParts) {
                    const { topLevelNodeModulesIndex, topLevelPackageNameIndex, packageRootIndex } = nodeModulesPathParts;
                    packageName = unmangleScopedPackageName(getPackageNameFromTypesPackageName(moduleFile.fileName.substring(topLevelPackageNameIndex + 1, packageRootIndex)));
                    if (startsWith(importingFile, moduleFile.path.substring(0, topLevelNodeModulesIndex))) {
                        const prevDeepestNodeModulesPath = packages.get(packageName);
                        const nodeModulesPath = moduleFile.fileName.substring(0, topLevelPackageNameIndex + 1);
                        if (prevDeepestNodeModulesPath) {
                            const prevDeepestNodeModulesIndex = prevDeepestNodeModulesPath.indexOf(nodeModulesPathPart);
                            if (topLevelNodeModulesIndex > prevDeepestNodeModulesIndex) {
                                packages.set(packageName, nodeModulesPath);
                            }
                        }
                        else {
                            packages.set(packageName, nodeModulesPath);
                        }
                    }
                }
            }

            const isDefault = exportKind === ExportKind.Default;
            const namedSymbol = isDefault && getLocalSymbolForExportDefault(symbol) || symbol;
            // 1. A named export must be imported by its key in `moduleSymbol.exports` or `moduleSymbol.members`.
            // 2. A re-export merged with an export from a module augmentation can result in `symbol`
            //    being an external module symbol; the name it is re-exported by will be `symbolTableKey`
            //    (which comes from the keys of `moduleSymbol.exports`.)
            // 3. Otherwise, we have a default/namespace import that can be imported by any name, and
            //    `symbolTableKey` will be something undesirable like `export=` or `default`, so we try to
            //    get a better name.
            const names = exportKind === ExportKind.Named || isExternalModuleSymbol(namedSymbol)
                ? unescapeLeadingUnderscores(symbolTableKey)
                : getNamesForExportedSymbol(namedSymbol, /*scriptTarget*/ undefined);

            const symbolName = typeof names === "string" ? names : names[0];
            const capitalizedSymbolName = typeof names === "string" ? undefined : names[1];

            const moduleName = stripQuotes(moduleSymbol.name);
            const id = exportInfoId++;
            const target = skipAlias(symbol, checker);
            const storedSymbol = symbol.flags & SymbolFlags.Transient ? undefined : symbol;
            const storedModuleSymbol = moduleSymbol.flags & SymbolFlags.Transient ? undefined : moduleSymbol;
            if (!storedSymbol || !storedModuleSymbol) symbols.set(id, [symbol, moduleSymbol]);

            exportInfo.add(key(symbolName, symbol, isExternalModuleNameRelative(moduleName) ? undefined : moduleName, checker), {
                id,
                symbolTableKey,
                symbolName,
                capitalizedSymbolName,
                moduleName,
                moduleFile,
                moduleFileName: moduleFile?.fileName,
                packageName,
                exportKind,
                targetFlags: target.flags,
                isFromPackageJson,
                symbol: storedSymbol,
                moduleSymbol: storedModuleSymbol,
            });
        },
        get: (importingFile, key) => {
            if (importingFile !== usableByFileName) return;
            const result = exportInfo.get(key);
            return result?.map(rehydrateCachedInfo);
        },
        search: (importingFile, preferCapitalized, matches, action) => {
            if (importingFile !== usableByFileName) return;
            return forEachEntry(exportInfo, (info, key) => {
                const { symbolName, ambientModuleName } = parseKey(key);
                const name = preferCapitalized && info[0].capitalizedSymbolName || symbolName;
                if (matches(name, info[0].targetFlags)) {
                    const rehydrated = info.map(rehydrateCachedInfo);
                    const filtered = rehydrated.filter((r, i) => isNotShadowedByDeeperNodeModulesPackage(r, info[i].packageName));
                    if (filtered.length) {
                        const res = action(filtered, name, !!ambientModuleName, key);
                        if (res !== undefined) return res;
                    }
                }
            });
        },
        releaseSymbols: () => {
            symbols.clear();
        },
        onFileChanged: (oldSourceFile: SourceFile, newSourceFile: SourceFile, typeAcquisitionEnabled: boolean) => {
            if (fileIsGlobalOnly(oldSourceFile) && fileIsGlobalOnly(newSourceFile)) {
                // File is purely global; doesn't affect export map
                return false;
            }
            if (
                usableByFileName && usableByFileName !== newSourceFile.path ||
                // If ATA is enabled, auto-imports uses existing imports to guess whether you want auto-imports from node.
                // Adding or removing imports from node could change the outcome of that guess, so could change the suggestions list.
                typeAcquisitionEnabled && consumesNodeCoreModules(oldSourceFile) !== consumesNodeCoreModules(newSourceFile) ||
                // Module agumentation and ambient module changes can add or remove exports available to be auto-imported.
                // Changes elsewhere in the file can change the *type* of an export in a module augmentation,
                // but type info is gathered in getCompletionEntryDetails, which doesn't use the cache.
                !arrayIsEqualTo(oldSourceFile.moduleAugmentations, newSourceFile.moduleAugmentations) ||
                !ambientModuleDeclarationsAreEqual(oldSourceFile, newSourceFile)
            ) {
                cache.clear();
                return true;
            }
            usableByFileName = newSourceFile.path;
            return false;
        },
    };
    if (Debug.isDebugging) {
        Object.defineProperty(cache, "__cache", { value: exportInfo });
    }
    return cache;

    function rehydrateCachedInfo(info: CachedSymbolExportInfo): SymbolExportInfo {
        if (info.symbol && info.moduleSymbol) return info as SymbolExportInfo;
        const { id, exportKind, targetFlags, isFromPackageJson, moduleFileName } = info;
        const [cachedSymbol, cachedModuleSymbol] = symbols.get(id) || emptyArray;
        if (cachedSymbol && cachedModuleSymbol) {
            return {
                symbol: cachedSymbol,
                moduleSymbol: cachedModuleSymbol,
                moduleFileName,
                exportKind,
                targetFlags,
                isFromPackageJson,
            };
        }
        const checker = (isFromPackageJson
            ? host.getPackageJsonAutoImportProvider()!
            : host.getCurrentProgram()!).getTypeChecker();
        const moduleSymbol = info.moduleSymbol || cachedModuleSymbol || Debug.checkDefined(
            info.moduleFile
                ? checker.getMergedSymbol(info.moduleFile.symbol)
                : checker.tryFindAmbientModule(info.moduleName),
        );
        const symbol = info.symbol || cachedSymbol || Debug.checkDefined(
            exportKind === ExportKind.ExportEquals
                ? checker.resolveExternalModuleSymbol(moduleSymbol)
                : checker.tryGetMemberInModuleExportsAndProperties(unescapeLeadingUnderscores(info.symbolTableKey), moduleSymbol),
            `Could not find symbol '${info.symbolName}' by key '${info.symbolTableKey}' in module ${moduleSymbol.name}`,
        );
        symbols.set(id, [symbol, moduleSymbol]);
        return {
            symbol,
            moduleSymbol,
            moduleFileName,
            exportKind,
            targetFlags,
            isFromPackageJson,
        };
    }

    function key(importedName: string, symbol: Symbol, ambientModuleName: string | undefined, checker: TypeChecker) {
        const moduleKey = ambientModuleName || "";
        return `${importedName.length} ${getSymbolId(skipAlias(symbol, checker))} ${importedName} ${moduleKey}` as ExportMapInfoKey;
    }

    function parseKey(key: ExportMapInfoKey) {
        const firstSpace = key.indexOf(" ");
        const secondSpace = key.indexOf(" ", firstSpace + 1);
        const symbolNameLength = parseInt(key.substring(0, firstSpace), 10);

        const data = key.substring(secondSpace + 1);
        const symbolName = data.substring(0, symbolNameLength);
        const moduleKey = data.substring(symbolNameLength + 1);
        const ambientModuleName = moduleKey === "" ? undefined : moduleKey;
        return { symbolName, ambientModuleName };
    }

    function fileIsGlobalOnly(file: SourceFile) {
        return !file.commonJsModuleIndicator && !file.externalModuleIndicator && !file.moduleAugmentations && !file.ambientModuleNames;
    }

    function ambientModuleDeclarationsAreEqual(oldSourceFile: SourceFile, newSourceFile: SourceFile) {
        if (!arrayIsEqualTo(oldSourceFile.ambientModuleNames, newSourceFile.ambientModuleNames)) {
            return false;
        }
        let oldFileStatementIndex = -1;
        let newFileStatementIndex = -1;
        for (const ambientModuleName of newSourceFile.ambientModuleNames) {
            const isMatchingModuleDeclaration = (node: Statement) => isNonGlobalAmbientModule(node) && node.name.text === ambientModuleName;
            oldFileStatementIndex = findIndex(oldSourceFile.statements, isMatchingModuleDeclaration, oldFileStatementIndex + 1);
            newFileStatementIndex = findIndex(newSourceFile.statements, isMatchingModuleDeclaration, newFileStatementIndex + 1);
            if (oldSourceFile.statements[oldFileStatementIndex] !== newSourceFile.statements[newFileStatementIndex]) {
                return false;
            }
        }
        return true;
    }

    function isNotShadowedByDeeperNodeModulesPackage(info: SymbolExportInfo, packageName: string | undefined) {
        if (!packageName || !info.moduleFileName) return true;
        const typingsCacheLocation = host.getGlobalTypingsCacheLocation();
        if (typingsCacheLocation && startsWith(info.moduleFileName, typingsCacheLocation)) return true;
        const packageDeepestNodeModulesPath = packages.get(packageName);
        return !packageDeepestNodeModulesPath || startsWith(info.moduleFileName, packageDeepestNodeModulesPath);
    }
}

/** @internal */
export function isImportableFile(
    program: Program,
    from: SourceFile,
    to: SourceFile,
    preferences: UserPreferences,
    packageJsonFilter: PackageJsonImportFilter | undefined,
    moduleSpecifierResolutionHost: ModuleSpecifierResolutionHost,
    moduleSpecifierCache: ModuleSpecifierCache | undefined,
): boolean {
    if (from === to) return false;
    const cachedResult = moduleSpecifierCache?.get(from.path, to.path, preferences, {});
    if (cachedResult?.isBlockedByPackageJsonDependencies !== undefined) {
        return !cachedResult.isBlockedByPackageJsonDependencies;
    }

    const getCanonicalFileName = hostGetCanonicalFileName(moduleSpecifierResolutionHost);
    const globalTypingsCache = moduleSpecifierResolutionHost.getGlobalTypingsCacheLocation?.();
    const hasImportablePath = !!moduleSpecifiers.forEachFileNameOfModule(
        from.fileName,
        to.fileName,
        moduleSpecifierResolutionHost,
        /*preferSymlinks*/ false,
        toPath => {
            const toFile = program.getSourceFile(toPath);
            // Determine to import using toPath only if toPath is what we were looking at
            // or there doesnt exist the file in the program by the symlink
            return (toFile === to || !toFile) &&
                isImportablePath(from.fileName, toPath, getCanonicalFileName, globalTypingsCache);
        },
    );

    if (packageJsonFilter) {
        const isAutoImportable = hasImportablePath && packageJsonFilter.allowsImportingSourceFile(to, moduleSpecifierResolutionHost);
        moduleSpecifierCache?.setBlockedByPackageJsonDependencies(from.path, to.path, preferences, {}, !isAutoImportable);
        return isAutoImportable;
    }

    return hasImportablePath;
}

/**
 * Don't include something from a `node_modules` that isn't actually reachable by a global import.
 * A relative import to node_modules is usually a bad idea.
 */
function isImportablePath(fromPath: string, toPath: string, getCanonicalFileName: GetCanonicalFileName, globalCachePath?: string): boolean {
    // If it's in a `node_modules` but is not reachable from here via a global import, don't bother.
    const toNodeModules = forEachAncestorDirectory(toPath, ancestor => getBaseFileName(ancestor) === "node_modules" ? ancestor : undefined);
    const toNodeModulesParent = toNodeModules && getDirectoryPath(getCanonicalFileName(toNodeModules));
    return toNodeModulesParent === undefined
        || startsWith(getCanonicalFileName(fromPath), toNodeModulesParent)
        || (!!globalCachePath && startsWith(getCanonicalFileName(globalCachePath), toNodeModulesParent));
}

/** @internal */
export function forEachExternalModuleToImportFrom(
    program: Program,
    host: LanguageServiceHost,
    preferences: UserPreferences,
    useAutoImportProvider: boolean,
    cb: (module: Symbol, moduleFile: SourceFile | undefined, program: Program, isFromPackageJson: boolean) => void,
) {
    const useCaseSensitiveFileNames = hostUsesCaseSensitiveFileNames(host);
    const excludePatterns = preferences.autoImportFileExcludePatterns && mapDefined(preferences.autoImportFileExcludePatterns, spec => {
        // The client is expected to send rooted path specs since we don't know
        // what directory a relative path is relative to.
        const pattern = getSubPatternFromSpec(spec, "", "exclude");
        return pattern ? getRegexFromPattern(pattern, useCaseSensitiveFileNames) : undefined;
    });

    forEachExternalModule(program.getTypeChecker(), program.getSourceFiles(), excludePatterns, host, (module, file) => cb(module, file, program, /*isFromPackageJson*/ false));
    const autoImportProvider = useAutoImportProvider && host.getPackageJsonAutoImportProvider?.();
    if (autoImportProvider) {
        const start = timestamp();
        const checker = program.getTypeChecker();
        forEachExternalModule(autoImportProvider.getTypeChecker(), autoImportProvider.getSourceFiles(), excludePatterns, host, (module, file) => {
            if (file && !program.getSourceFile(file.fileName) || !file && !checker.resolveName(module.name, /*location*/ undefined, SymbolFlags.Module, /*excludeGlobals*/ false)) {
                // The AutoImportProvider filters files already in the main program out of its *root* files,
                // but non-root files can still be present in both programs, and already in the export info map
                // at this point. This doesn't create any incorrect behavior, but is a waste of time and memory,
                // so we filter them out here.
                cb(module, file, autoImportProvider, /*isFromPackageJson*/ true);
            }
        });
        host.log?.(`forEachExternalModuleToImportFrom autoImportProvider: ${timestamp() - start}`);
    }
}

function forEachExternalModule(checker: TypeChecker, allSourceFiles: readonly SourceFile[], excludePatterns: readonly RegExp[] | undefined, host: LanguageServiceHost, cb: (module: Symbol, sourceFile: SourceFile | undefined) => void) {
    const realpathsWithSymlinks = host.getSymlinkCache?.().getSymlinkedDirectoriesByRealpath();
    const isExcluded = excludePatterns && (({ fileName, path }: SourceFile) => {
        if (excludePatterns.some(p => p.test(fileName))) return true;
        if (realpathsWithSymlinks?.size && pathContainsNodeModules(fileName)) {
            let dir = getDirectoryPath(fileName);
            return forEachAncestorDirectory(getDirectoryPath(path), dirPath => {
                const symlinks = realpathsWithSymlinks.get(ensureTrailingDirectorySeparator(dirPath));
                if (symlinks) {
                    return symlinks.some(s => excludePatterns.some(p => p.test(fileName.replace(dir, s))));
                }
                dir = getDirectoryPath(dir);
            }) ?? false;
        }
        return false;
    });

    for (const ambient of checker.getAmbientModules()) {
        if (!ambient.name.includes("*") && !(excludePatterns && ambient.declarations?.every(d => isExcluded!(d.getSourceFile())))) {
            cb(ambient, /*sourceFile*/ undefined);
        }
    }
    for (const sourceFile of allSourceFiles) {
        if (isExternalOrCommonJsModule(sourceFile) && !isExcluded?.(sourceFile)) {
            cb(checker.getMergedSymbol(sourceFile.symbol), sourceFile);
        }
    }
}

/** @internal */
export function getExportInfoMap(importingFile: SourceFile | FutureSourceFile, host: LanguageServiceHost, program: Program, preferences: UserPreferences, cancellationToken: CancellationToken | undefined): ExportInfoMap {
    const start = timestamp();
    // Pulling the AutoImportProvider project will trigger its updateGraph if pending,
    // which will invalidate the export map cache if things change, so pull it before
    // checking the cache.
    host.getPackageJsonAutoImportProvider?.();
    const cache = host.getCachedExportInfoMap?.() || createCacheableExportInfoMap({
        getCurrentProgram: () => program,
        getPackageJsonAutoImportProvider: () => host.getPackageJsonAutoImportProvider?.(),
        getGlobalTypingsCacheLocation: () => host.getGlobalTypingsCacheLocation?.(),
    });

    if (cache.isUsableByFile(importingFile.path)) {
        host.log?.("getExportInfoMap: cache hit");
        return cache;
    }

    host.log?.("getExportInfoMap: cache miss or empty; calculating new results");
    let moduleCount = 0;
    try {
        forEachExternalModuleToImportFrom(program, host, preferences, /*useAutoImportProvider*/ true, (moduleSymbol, moduleFile, program, isFromPackageJson) => {
            if (++moduleCount % 100 === 0) cancellationToken?.throwIfCancellationRequested();
            const seenExports = new Map<__String, true>();
            const checker = program.getTypeChecker();
            const defaultInfo = getDefaultLikeExportInfo(moduleSymbol, checker);
            // Note: I think we shouldn't actually see resolved module symbols here, but weird merges
            // can cause it to happen: see 'completionsImport_mergedReExport.ts'
            if (defaultInfo && isImportableSymbol(defaultInfo.symbol, checker)) {
                cache.add(
                    importingFile.path,
                    defaultInfo.symbol,
                    defaultInfo.exportKind === ExportKind.Default ? InternalSymbolName.Default : InternalSymbolName.ExportEquals,
                    moduleSymbol,
                    moduleFile,
                    defaultInfo.exportKind,
                    isFromPackageJson,
                    checker,
                );
            }
            checker.forEachExportAndPropertyOfModule(moduleSymbol, (exported, key) => {
                if (exported !== defaultInfo?.symbol && isImportableSymbol(exported, checker) && addToSeen(seenExports, key)) {
                    cache.add(
                        importingFile.path,
                        exported,
                        key,
                        moduleSymbol,
                        moduleFile,
                        ExportKind.Named,
                        isFromPackageJson,
                        checker,
                    );
                }
            });
        });
    }
    catch (err) {
        // Ensure cache is reset if operation is cancelled
        cache.clear();
        throw err;
    }

    host.log?.(`getExportInfoMap: done in ${timestamp() - start} ms`);
    return cache;
}

/** @internal */
export function getDefaultLikeExportInfo(moduleSymbol: Symbol, checker: TypeChecker) {
    const exportEquals = checker.resolveExternalModuleSymbol(moduleSymbol);
    if (exportEquals !== moduleSymbol) return { symbol: exportEquals, exportKind: ExportKind.ExportEquals };
    const defaultExport = checker.tryGetMemberInModuleExports(InternalSymbolName.Default, moduleSymbol);
    if (defaultExport) return { symbol: defaultExport, exportKind: ExportKind.Default };
}

function isImportableSymbol(symbol: Symbol, checker: TypeChecker) {
    return !checker.isUndefinedSymbol(symbol) && !checker.isUnknownSymbol(symbol) && !isKnownSymbol(symbol) && !isPrivateIdentifierSymbol(symbol);
}

/**
 * @internal
 * May call `cb` multiple times with the same name.
 * Terminates when `cb` returns a truthy value.
 */
export function forEachNameOfDefaultExport<T>(defaultExport: Symbol, checker: TypeChecker, compilerOptions: CompilerOptions, preferCapitalizedNames: boolean, cb: (name: string) => T | undefined): T | undefined {
    let chain: Symbol[] | undefined;
    let current: Symbol | undefined = defaultExport;

    while (current) {
        // The predecessor to this function also looked for a name on the `localSymbol`
        // of default exports, but I think `getDefaultLikeExportNameFromDeclaration`
        // accomplishes the same thing via syntax - no tests failed when I removed it.
        const fromDeclaration = getDefaultLikeExportNameFromDeclaration(current);
        if (fromDeclaration) {
            const final = cb(fromDeclaration);
            if (final) return final;
        }

        if (current.escapedName !== InternalSymbolName.Default && current.escapedName !== InternalSymbolName.ExportEquals) {
            const final = cb(current.name);
            if (final) return final;
        }

        chain = append(chain, current);
        current = current.flags & SymbolFlags.Alias ? checker.getImmediateAliasedSymbol(current) : undefined;
    }

    for (const symbol of chain ?? emptyArray) {
        if (symbol.parent && isExternalModuleSymbol(symbol.parent)) {
            const final = cb(moduleSymbolToValidIdentifier(symbol.parent, getEmitScriptTarget(compilerOptions), preferCapitalizedNames));
            if (final) return final;
        }
    }
}