importHelpers + module: es2015/esnext emits unused helper imports

[d-fischer]

TypeScript Version: 4.2.0-dev.20201112

Search Terms: esm, importHelpers, unused

Code

// @importHelpers: true
// @module: es2015
// @target: es5
// @downlevelIteration: true
const a = [1, 2];
const b = [3, 4]
export const c = [...a, ...b];

Expected behavior:
Imports only the used helpers:

import { __spread } from "tslib";
var a = [1, 2];
var b = [3, 4];
export var c = __spread(a, b);

Actual behavior:
Imports the used helpers and also the helpers they depend on:

import { __read, __spread } from "tslib";
var a = [1, 2];
var b = [3, 4];
export var c = __spread(a, b);

Note the extra unused import in the first line.

Playground Link: https://www.typescriptlang.org/play?downlevelIteration=true&importHelpers=true&target=1&module=5&ts=4.2.0-dev.20201112#code/MYewdgzgLgBAhjAvDA2gRgDQwEwF0DcAUKJLAEZKoDMWALLoQKYAeADiAE6wnQzCUoAdMLhZhgsgSA

Additional info: I already have a fix for this ready; basically I'm just following protocol, waiting for approval.

[ExE-Boss]

I believe it’s because __spread uses __read under the hood: https://github.com/microsoft/tslib/blob/76bedfddc2cad38aa11060a32f56e9484c279c98/tslib.es6.js#L147-L151

And the way the helper emit code is currently written doesn’t distinguish between helpers that are only needed by other helpers, and helpers that are actually needed by transpiled code:

TypeScript/src/compiler/factory/emitHelpers.ts

Lines 287 to 295 in 566faa7

	function createSpreadHelper(argumentList: readonly Expression[]) {
	context.requestEmitHelper(readHelper);
	context.requestEmitHelper(spreadHelper);
	return factory.createCallExpression(
	getUnscopedHelperName("__spread"),
	/*typeArguments*/ undefined,
	argumentList
	);
	}

[d-fischer]

As I already noted in the original post, a fix is ready to be PRd on my machine. I'm just waiting for the approval to match the process outlined in the guidelines and the PR template.