Build tree shaking do not remove every unused code

Which @angular/* package(s) are the source of the bug?

compiler-cli

Is this a regression?

Yes

Description

I have generated a new project - ng create test-ts - and added just a few unused variables and only one that is used.

(in grey, unused variables):

Extract from generated main.js:

The kpiResourceMap is in the final bundle (and unassigned) but never used in the app, so it should be removed.

Please provide a link to a minimal reproduction of the bug

https://github.com/MYRE-CORP/test-ts

Please provide the exception or error you saw

No response

Please provide the environment you discovered this bug in

Angular CLI: 12.2.4
Node: 14.15.3
Package Manager: npm 7.21.0
OS: Ubuntu

Anything else?

No response

[clydin]

This case is related to dead code elimination safety. In the above code, new Map is at the module level which will cause the expression to be executed at module evaluation time. Such code that can possibility have side effects cannot be safely removed since removal may alter the behavior of the application at runtime. Since the Map constructor has the possibility of throwing an exception it has possible side effects. Determining whether the Map construction is safe at build time is a non-trivial analysis effort for JavaScript and one which current optimizers do not perform. However, if the expression has been manually reviewed to be side effect free, the expression can be annotated to provide a hint to the optimizer that it is safe to be removed. This should be possible by adding a pure annotation as a leading comment similar to the following: const kpiResourceMap = /*#__PURE__*/ new Map(....)

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