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.

Start with the TransformationContext and CompilerHost APIs, then trace how custom transformers receive context and how ts.resolveModuleName uses a host. Define the intended API shape and check its compatibility with existing transformers. Done means a transformer can access the relevant CompilerHost for module resolution without creating a redundant host.