Support `@nosideeffects` all code (not just externs)

There are many scenarios where the developer knows that code has no side-effects but the CC fails to detect it and retains the code.

CC already has @nosideeffects annotation, but that one only works in externs file. Could we make it work in normal code as well?

[NMinhNguyen]

Sorry to be commenting on an old thread, but this is the only one I’ve found to be remotely relevant to the question I am about to ask. Babel 7 now emits #__PURE__ annotations in order to improve tree-shake-ability when the transpiled output is consumed by Uglify. References: babel/babel#6209, babel/babel#5632, https://github.com/mishoo/UglifyJS2/commit/1e51586996ae4fdac68a8ea597c20ab170809c43, and the side_effects compiler option in Uglify’s README. I was wondering if Google Closure Compiler also supports or plans to support this annotation at some point in the future?

[tadeegan]

The compiler side effects computation could definitely use some improvement.

As an alternative to adding “@nosideeffects” someone could invest time into making the PureFunctionIdentifier more accurate. For example, the following function “f” would be marked as having side effects:

function f() {
  var a = 3;
  a += 5;
  return a;
}

Note that the severity of this is probably reduced by the inline variables pass.

See: https://github.com/google/closure-compiler/blob/de36c96e070dc2f9ad69fb814925af34c4d6c91d/src/com/google/javascript/jscomp/PureFunctionIdentifier.java#L58

And: https://github.com/google/closure-compiler/blob/26b48d2562494a46c74a193fd65f6be872933285/src/com/google/javascript/jscomp/NodeUtil.java#L4238

Tests to be improved: https://github.com/google/closure-compiler/blob/de36c96e070dc2f9ad69fb814925af34c4d6c91d/test/com/google/javascript/jscomp/PureFunctionIdentifierTest.java#L947

https://github.com/google/closure-compiler/blob/de36c96e070dc2f9ad69fb814925af34c4d6c91d/test/com/google/javascript/jscomp/PureFunctionIdentifierTest.java#L1321

https://github.com/google/closure-compiler/blob/de36c96e070dc2f9ad69fb814925af34c4d6c91d/test/com/google/javascript/jscomp/PureFunctionIdentifierTest.java#L697

Perhaps I should open a new bug for that, or maybe there already is one…