Editorial: Simplify object snapshotting

[gibson042]

Introduces SnapshotOwnProperties to precede CopyDataProperties with creating the target object and follow it with returning that object, collapsing many common algorithm subsequences (including all of those using the CopyOptions operation, which is fully subsumed).

Once introduced to ECMA-262, it can also simplify RestDestructuringAssignmentEvaluation and RestBindingInitialization:

-          1. Let _restObj_ be OrdinaryObjectCreate(%Object.prototype%).
-          1. Perform ? CopyDataProperties(_restObj_, _value_, _excludedNames_).
+          1. Let _restObj_ be ? SnapshotOwnProperties(_value_, %Object.prototype%, _excludedNames_).

[codecov]

Codecov Report

Merging #2586 (8f31100) into main (dac765f) will increase coverage by 96.14%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           main    #2586       +/-   ##
=========================================
+ Coverage      0   96.14%   +96.14%     
=========================================
  Files         0       20       +20     
  Lines         0    11364    +11364     
  Branches      0     2164     +2164     
=========================================
+ Hits          0    10926    +10926     
- Misses        0      375      +375     
- Partials      0       63       +63     

Impacted Files	Coverage Δ
polyfill/lib/calendar.mjs	87.77% <100.00%> (ø)
polyfill/lib/ecmascript.mjs	98.28% <100.00%> (ø)
polyfill/lib/zoneddatetime.mjs	100.00% <100.00%> (ø)

... and 17 files with indirect coverage changes

[ptomato]

Looks like a good improvement!

Would you like to propose the change to rest parameter destructuring in 262 separately? That would help make the Temporal proposal a bit smaller and get some editor feedback on this operation.

[gibson042]

Good idea: tc39/ecma262#3081

[justingrant]

Why is it ToObject and not "If fields is not an Object, throw a TypeError" ?

And was it an accidentally normative change to remove it here?

[gibson042]

To answer the second question first, this change is not normative because the first observable consequence of SnapshotOwnProperties is ? ToObject(_source_) in step 3.

As for the first question, I don't know why mergeFields includes coercion to object but I would be in favor of replacing that with rejection of non-objects in concordance with #2574 (and such a change could be implemented in SnapshotOwnProperties itself).