Avoid adding unnecessary closure for block scoping

[sophiebits]

When you write

for (const x of l) {
  setTimeout(() => x);
}

we need to add a closure because the variable is meant to be block-scoped and recreated each time the block runs. We do this.

However, we also add the closure when a block is present but no loop is:

function foo() {
  {
    const x;
    setTimeout(() => x);
  }
}

This isn't necessary, because if no loop is present then each piece of code runs at most once. I changed the transform to only add a closure if a variable is referenced from within a loop.

(Builds on #5236.)

[codecov-io]

Codecov Report

Merging #5246 into master will decrease coverage by -0.2%.

@@           Coverage Diff            @@
##           master   #5246     +/-   ##
========================================
- Coverage    89.4%   89.2%   -0.2%     
========================================
  Files         204     203      -1     
  Lines        9916    9851     -65     
  Branches     2670    2628     -42     
========================================
- Hits         8865    8788     -77     
- Misses       1051    1063     +12

Impacted Files	Coverage Δ
...plugin-transform-es2015-block-scoping/src/index.js	93.45% <100%> (-0.85%)	❌
packages/babel-register/src/cache.js	27.27% <ø> (-54.55%)	❌
packages/babel-traverse/src/path/family.js	84.37% <ø> (-4.42%)	❌
packages/babel-traverse/src/path/replacement.js	79.62% <ø> (-1.41%)	❌
packages/babel-traverse/src/path/modification.js	85.43% <ø> (-0.68%)	❌
packages/babel-traverse/src/path/evaluation.js	77.15% <ø> (-0.57%)	❌
packages/babel-traverse/src/index.js	93.47% <ø> (-0.28%)	❌
...bel-plugin-transform-es2015-classes/src/vanilla.js	95.67% <ø> (-0.18%)	❌
packages/babel-generator/src/node/parentheses.js	92.24% <ø> (-0.14%)	❌
... and 40 more

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[sophiebits]

cc @gaearon @hzoo @existentialism – Any thoughts on this?

[wtgtybhertgeghgtwtg]

Is this gonna make it into babel@7.0.0?

[loganfsmyth]

This will land in 6, I'm investigating one last issue before we publish the next 6.x minor, so next few days probably.

7.x likely won't be for a few more weeks as a minimum, we've still got things we want to address for that.

[hzoo]

Thanks again @spicyj 👌

[sophiebits]

Thank you all!!