[api] No replacement for ts.transform / TransformationContext / custom transformers in program.emit #64861

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Trying to collect what we need to migrate our custom ts build in ts 6 to new ts native.

In 6.0, a build could plug custom transformers into the compiler:

• program.emit(undefined, undefined, undefined, undefined, { before: [factory] })
• ts.transform(sourceFile, [factory]) with TransformationContext, ts.visitEachChild(node, visitor, context), context.readEmitHelpers(), ts.addEmitHelpers

In 7.1:

• Program.emit(emitOnly?) / emitToString(emitOnly?) don't accept transformers.
• There's no transform function and no TransformationContext type.
• visitEachChild(node, visitor) exists in typescript/ast without a context.

The only remaining pattern is: parse with api.createSourceFile, rewrite with visitEachChild + typescript/ast/factory, print with api.printer.printNode, then pass the text to another compiler or bundler. That loses:

• source-text-preserving emit and source maps, since the printed output is a new text
• comments: the bug in issue [API] Printer attaches source comments to synthesized nodes because the node encoder sends pos -1 as 0 #64859, plus the missing synthetic comment API in [api] Add back missing: setEmitFlags and addSynthetic*Comment #64429
• emit helpers and the TS → JS lowering step in the same pass

Our use case: build-time instrumentation of a browser agent (wrapping function bodies, await expressions and .apply() calls) during the TypeScript emit of a build.

Is support for custom transformers planned for the native API, either as JS callbacks during emit or as a supported "transform the AST, then emit" path with source maps? If it isn't planned, is the parse → visit → print → re-emit approach the intended long-term pattern?

Related: #14419, #54276 (transformer plugins in 6.x, not about the native API).