File altered by before transform does not remove new unused imports

[filipesilva]

Description of the bug:

When using a before transform that makes an import unused, the import will not be removed by TypeScript.

So if I have:

import { unused } from 'unused-module';
import { something } from 'some-module';
const value = something;

And replace const value = something with

import { somethingElse } from 'some-other-module';
const value = somethingElse;

The import { unused } from 'unused-module'; statement will be removed, but import { something } from 'some-module'; will remain:

import { something } from 'some-module';
import { somethingElse } from 'some-other-module';
const value = somethingElse;

TypeScript Version: 2.4.2

Code

import { unused } from 'unused-module';
import { something } from 'some-module';
const value = something;

Reproduction of this bug requires using the transforms API.

I have prepared a repository with a simple reproduction that runs this file: https://github.com/filipesilva/ts-transform-import-bug/blob/master/test.ts

git clone https://github.com/filipesilva/ts-transform-import-bug
cd ts-transform-decorator-bug
npm install
npm test

Expected behavior:
All unused imports are removed.

import { somethingElse } from 'some-other-module';
const value = somethingElse;

Actual behavior:
Only imports there were unused before the transformed are removed.

import { something } from 'some-module';
import { somethingElse } from 'some-other-module';
const value = somethingElse;

/cc @rbuckton

[rbuckton]

Currently, whether or not an import is referenced is determined by the checker at check time, prior to emit. We do not re-bind or re-check the AST between transformations, so all check-time state will always be based on the originally parsed AST.

The only way to elide an import in this case would be to have the "before" transformer explicitly elide the import. This is currently the intended behavior. We are considering ways of opening up the checker (and our internal EmitResolver) to custom transformations to allow more control, but that likely won't be ready in the 2.5 release timeframe.

[arciisine]

I'm actually facing the inverse of this problem. Something is being marked as unused (and so the import is elided) but I would like that import to stay. Is there anyway to manage this without checking the entire file and adding the import back in (if it truly is unused)