Include CompilerHost in TransformationContext

[HoldYourWaffle]

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 (#28276 (comment)), 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.

[RyanCavanaugh]

@rbuckton thoughts?

[rbuckton]

I don't see why not.