Tree-shake unused declarations while keeping initializer side-effects

[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

This will allow partial tree-shaking of declarations where the variable is unused but the right-hand-side has side-effects. This will be important for upcoming improvements for the commonjs plugin. Example:

// main.js
let x = globalFunction();
x = 'this is some reassignment that does not matter but may be retained because x is retained';

// previous output
let x = globalFunction();
x = 'this is some reassignment that does not matter but may be retained because x is retained';

// new output
globalFunction();

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#unused-side-effect-declarations

or load it into the REPL:
https://rollupjs.org/repl/?circleci=14028

[codecov]

Codecov Report

Merging #3933 (2bc3bc5) into master (e23bb35) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3933      +/-   ##
==========================================
+ Coverage   97.13%   97.18%   +0.05%     
==========================================
  Files         188      188              
  Lines        6669     6681      +12     
  Branches     1940     1944       +4     
==========================================
+ Hits         6478     6493      +15     
+ Misses        101       99       -2     
+ Partials       90       89       -1     

Impacted Files	Coverage Δ
src/ast/nodes/AssignmentExpression.ts	100.00% <ø> (ø)
src/ast/nodes/MemberExpression.ts	98.31% <ø> (ø)
src/ast/nodes/ForInStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/ForOfStatement.ts	100.00% <100.00%> (ø)
src/ast/nodes/Property.ts	92.30% <100.00%> (ø)
src/ast/nodes/VariableDeclaration.ts	97.95% <100.00%> (+0.95%)	⬆️
src/ast/nodes/VariableDeclarator.ts	100.00% <100.00%> (ø)
src/ast/nodes/shared/Node.ts	100.00% <100.00%> (+2.98%)	⬆️

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[thorn0]

@lukastaegert Please have a look at this comment
prettier/prettier#10199 (comment)
Is my guess there correct?

[lukastaegert]

I fear it is. Damn JavaScript!

[lukastaegert]

Ok, fix at #3947, will release this once CI is done.