module_resolution=none or =explicit

Discussed in-person with @DanielRosenwasser

In google we have performance issues with tsc and with editor plugins. We have tracked this down to the node moduleResolutionKind looking in many locations on a network-mounted disk.

We don't actually want the node moduleResolution, we only want the baseUrl/paths/rootDirs feature known as "path mapping". Put another way, we explicitly list locations for all inputs, and we never want the compiler to stat a file outside of the explicit input locations.

For tsc, we provide a custom CompilerHost that avoids touching the disk, we implement fileExists and directoryExists purely by consulting our file manifest. However, we can't do this trick for editors, which are becoming increasingly unusable.

As an example, we provide interop with Closure Compiler code that declares goog.module, by naming these goog:some.module. This results in the language services looking for a file like goog:some.module.d.ts all over the network-mounted filesystem, even though a later sourceFile in the program has declare module 'goog:some.module'

cc @vikerman @rkirov

まず tsc のモジュール解決の動作と、エディタープラグインで使用される言語サービスから始めます。モジュール検索中に CompilerHost fileExists と directoryExists がどのように参照されるかを確認し、そのうえで、明示的または無効化されたモジュール解決モードが baseUrl、paths、rootDirs、宣言されたモジュールに対してどう動作すべきかを定義します。明示的な入力の範囲外にある場所を検索で避けつつ、パスマッピングを壊さないことが完了の条件です。