Treeshake chained assignment expressions

[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 tree-shaking parts of chained assignment expressions. Example:

// input
let a, b, c, d;
a = b = 1;
c = d = 2;
console.log(a, d);

// output
let a, d;
a = 1;
d = 2;
console.log(a, d);

Special care is taken so that

• Non-"=" Assignment operators such as "+=" are not tree-shaken
• Side-effects on the left side are still observed

This will be helpful with some planned updates for the commonjs plugin as it will produce nicer output.

[rollup-bot]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#tree-shake-chained-assignments

or load it into the REPL:
https://rollupjs.org/repl/?circleci=13910

[codecov]

Codecov Report

Merging #3919 (0ce5295) into master (1378cae) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3919   +/-   ##
=======================================
  Coverage   97.08%   97.08%           
=======================================
  Files         187      187           
  Lines        6546     6551    +5     
  Branches     1905     1909    +4     
=======================================
+ Hits         6355     6360    +5     
  Misses        101      101           
  Partials       90       90           

Impacted Files	Coverage Δ
src/ast/nodes/AssignmentExpression.ts	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1378cae...0ce5295. Read the comment docs.