Include CompilerHost in TransformationContext

Search Terms

compiler host, CompilerHost,
transformation context, TransformationContext,
transformer, custom transformer

Suggestion

I'd like to access the CompilerHost instance from my custom transformers (via TransformationContext).

As far as I can see this wouldn't expose any "risky" patterns like #25147.

Use Cases

My specific use-case is resolving import declarations. This can be done using ts.resolveModuleName (https://github.com/microsoft/TypeScript/issues/28276#issuecomment-435016356), but this requires a CompilerHost instance.

Example

An example for my specific use-case would be:

const transformer: ts.TransformerFactory<ts.SourceFile> = context => {
    return file => {
        return ts.visitEachChild(
            file,
            node => {
                if (ts.isImportDeclaration(node)) {
                    const resolved = ts.resolveModuleName(
                        (node.moduleSpecifier as ts.StringLiteral).text,
                        file.fileName,
                        context.getCompilerOptions(),
                        context.getCompilerHost() // this is currently impossible
                    )
                    
                    // do something interesting with resolved
                }
                return node;
            },
            context
        )
    }
}

Workaround

Luckily there is a workaround for this:

const host = ts.createCompilerHost(context.getCompilerOptions());

But this feels redundant and somewhat "wrong".

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

TransformationContext と CompilerHost の API から始め、カスタム transformer がどのように context を受け取り、ts.resolveModuleName がどのように host を使用するかを追跡します。意図する API の形を定義し、既存の transformer との互換性を確認します。transformer が冗長な host を作成せずに、モジュール解決に関連する CompilerHost にアクセスできれば完了です。