Add support for more complex selectors for sideEffects

[TheLarkInn]

Do you want to request a feature or report a bug?
Feature

What is the current behavior?
webpack searched for "sideEffects": false in package.json to perform the extra optimizations for esm modules. However if false is a string, the optimization isn't detected.

We should probably (in the case that package authors might mistake this as "false" vs false) put the wrong value in, and theres no simple way to detect (as a dev) why the 'deopt' occurs.
If the current behavior is a bug, please provide the steps to reproduce.

What is the expected behavior?
"sideEffects": "false" in package.json works the same as "sideEffects": false

If this is a feature request, what is motivation or use case for changing the behavior?
Motivation described above.

Please mention other relevant information such as the browser version, Node.js version, webpack version and Operating System.
webpack 4.0.0-alpha

[codywl]

I'm not quite sure how to architect the test case(s) for this issue. However, I am looking into existing tests to see if I can reuse anything.

[sokra]

"sideEffects": "false" should mark every module as side effect free except a file named false

[codywl]

Oh, I may have completely misunderstood the issue, then. Am I just to implement the case where specifying a String instead of a Boolean value tells webpack which files are known to have effects? If so, should we allow Arrays of strings as well?

[sokra]

yep Array of string should be allowed too.

It should support glob expressions too.

"sideEffects": [
  "src/**/*.js"
  "./lib/abc.js"
]

[codywl]

Got it - I'll work on that, and inquire further if I have any difficulties.

[codywl]

I've been trying to decipher some segments of this particular Plugin instance, but I might be lacking some context. The first ambiguous thing in here (to me) is the usage of harmony, so I ran off to check out examples. As far as I can tell, this is part of how webpack keeps collisions out of imports. I can tell that it will take some time before I grasp the larger web of interaction, so I'll proceed with that assumption (unless somebody corrects me, which would be welcome anyways).

In any case, do I have access to the name of the module or the filename when normal-module-factory returns the data object? I'm assuming that's how I will be comparing it to the array of strings the user provides. It looks like I may be able to call __filename from the Node API, but it's unclear to me whether that would report what I want inside of this context.

[sokra]

https://github.com/webpack/webpack/blob/next/lib/optimize/SideEffectsFlagPlugin.js#L19

resolveData.relativePath is the relative path of the file relative to the package.json.