Enable custom transforms for declaration emit

[zerkalica]

I try to use absolute paths aliases for libraries. Output d.ts with aliased paths breaks type resolving, when package usesd as external package. Problem not resolved in typescript (see #18972).

Example:

my-lib/src/index.ts

export * from 'my-lib/some'

my-lib/dist/index.d.ts

export * from 'my-lib/some'

my-app.ts

 import some from 'my-lib'
 // ts does not check types here:
 some.bla-bla

I wrote ts-transform-paths transfomer, which rewrites aliased import paths accordingly tsconfig baseUrl and paths.

With my trasformer my-lib/dist/index.d.ts

export * from './some'

It's works for npm packages and applications.

Problem:

Ts language service service.getEmitOutput(key, true) ignores transformers in dts generation mode:

performance.mark("beforeEmit");
const transformers = emitOnlyDtsFiles ? [] : getTransformers(options, customTransformers);

In my transformer chain helper i use a emitFiles hack to keep some dts-related transformers. I use dts flag to indicate that plugin works for d.ts.

export interface CustomTransformer {
    dts?: boolean
    before?: ts.TransformerFactory<ts.SourceFile>
    after?: ts.TransformerFactory<ts.SourceFile>
}

May be add posibility to filter dts related trasformers in typescript if emitOnlyDtsFiles given?

Something like this:

performance.mark("beforeEmit");
const transformers = getTransformers(options, customTransformers, emitOnlyDtsFiles);

[alexeagle]

Angular needs this also for the Ivy renderer compiler to be a ts.CustomTransformers

Discussed with @mhegazy a proposal where we add a property named afterDeclaration:

interface CustomTransformers {
        /** Custom transformers to evaluate before built-in transformations on .js emit. */
        before?: TransformerFactory<SourceFile>[];
        /** Custom transformers to evaluate after built-in transformations on .js emit. */
        after?: TransformerFactory<SourceFile>[];
        /** Custom transformers to evaluate after built-in transformations on .d.ts emit. */
        afterDeclaration?: TransformerFactory<SourceFile>[];
    }