Only selectively deoptimize call parameters

[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:

• resolves Simple reproductible issue with treeshaking #4582

Description

Before, any call to a function would deoptimize all call parameters meaning that they become "unknown", even if the function was pure.

This PR changes the logic to actually track what mutations happen with a function parameter and apply only the corresponding deoptimizations to the call parameters.
There are some limitations as the initial naive implementation had terrible performance: Deoptimizations are only tracked one level deep. If a nested property is reassigned, i.e.

function foo(arg) {
  arg.a.b = 'mutated'
}

then this is treated as if the top-level property, i.e. arg.a, was reassigned instead. This also applies to property accesses because we do not track if deeper properties are setters/getters. So in the example, just the line arg.a.b would deoptimize arg.a.

But I think this should be good enough for most purposes and is already a huge improvement over the previous "deoptimize everything" approach.

I also applied this optimization to known global functions: Now all known global functions except Object.freeze, Object.defineProperty and Object.defineProperties employ this logic and no longer deoptimize their arguments.

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[github-actions]

Thank you for your contribution! ❤️

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

npm install rollup/rollup#selective-parameter-deoptimization

or load it into the REPL:
https://deploy-preview-4892--rollupjs.netlify.app/repl/?pr=4892

[codecov]

Codecov Report

Merging #4892 (7a17845) into master (d985838) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4892      +/-   ##
==========================================
+ Coverage   98.98%   99.01%   +0.03%     
==========================================
  Files         219      221       +2     
  Lines        7968     8025      +57     
  Branches     2194     2209      +15     
==========================================
+ Hits         7887     7946      +59     
  Misses         26       26              
+ Partials       55       53       -2     

Impacted Files	Coverage Δ
src/ast/NodeInteractions.ts	100.00% <ø> (ø)
src/ast/nodes/Literal.ts	100.00% <ø> (ø)
src/ast/nodes/TemplateLiteral.ts	100.00% <ø> (ø)
src/ast/scopes/BlockScope.ts	100.00% <ø> (ø)
src/ast/scopes/CatchScope.ts	100.00% <ø> (ø)
src/ast/scopes/Scope.ts	84.61% <ø> (ø)
src/ast/scopes/TrackingScope.ts	100.00% <ø> (ø)
src/ast/nodes/ArrayExpression.ts	100.00% <100.00%> (ø)
src/ast/nodes/CallExpression.ts	100.00% <100.00%> (ø)
src/ast/nodes/ConditionalExpression.ts	100.00% <100.00%> (ø)
... and 30 more

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[lukastaegert]

I tested this on several larger projects and both performance and output seem to be fine. Will release this soon.

[rollup-bot]

This PR has been released as part of rollup@3.19.0. You can test it via npm install rollup.