Do not tear apart declarations in loop or if bodies

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

As it turns out, partially tree-shaking a declaration while keeping side-effects, i.e.

// input
var foo = 1, unused = sideEffect(), bar = 2;

// output
var foo = 1; sideEffect(); var bar = 2;

will wreak havoc if the declaration is the body of a loop or if statement, e.g.

// input
if (condition) var foo = 1, unused = sideEffect(), bar = 2;

// broken output
if (condition) var foo = 1; sideEffect(); var bar = 2;

as it will replace the conditional side-effect with an unconditional one. Similar things apply to loops. The solution is to always include the identifier together with the initialiser in such a situation.

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#partially-tree-shaken-loop-body-declarations

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14182

[codecov]

Codecov Report

Merging #3947 (6911a37) into master (683cf48) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3947      +/-   ##
==========================================
- Coverage   97.18%   97.18%   -0.01%     
==========================================
  Files         188      188              
  Lines        6681     6678       -3     
  Branches     1946     1948       +2     
==========================================
- Hits         6493     6490       -3     
  Misses         99       99              
  Partials       89       89              

Impacted Files	Coverage Δ
src/ast/nodes/VariableDeclarator.ts	100.00% <ø> (ø)
src/ast/nodes/shared/Node.ts	100.00% <ø> (ø)
src/ast/nodes/DoWhileStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/ForInStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/ForOfStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/ForStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/IfStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/VariableDeclaration.ts	98.00% <100.00%> (+0.02%)	⬆️
src/ast/nodes/WhileStatement.ts	100.00% <100.00%> (ø)

---

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[yoyo837]

How is it different from this.left.include(context, true);?

[lukastaegert]

There is a special string value for includeChildrenRecursively that is like true but also includes "related" code. Something we use to deoptimize try-catch statements.

[lukastaegert]

You would need to do something rather nonsensical like this to get a change from that:
https://rollupjs.org/repl/?version=2.38.4&shareable=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