Add table.copy_with_metatables

[appgurueu]

Implements what I have suggested in #15744.

How to test

There are basic unit tests. Feel free to add more.

[y5nw]

Looks good aside from some minor things.

(Edit: For some reason Github seems to show batched follow-up comments without much context on previous posts.)

[SmallJoker]

This might need another remark:

Due to implementation details, specifying `preserve_metatables` on engine versions
without `core.features.table_copy_preserve_metatables` is likely to cause errors.

[appgurueu]

I think I will turn this into table.copy (old behavior) and table.copy_with_metatables. Saves everyone involved the headache:

• More readable than a true parameter.
• No need for a core.features entry; it'll just be nil and cause errors when calling it is attempted when unavailable, as it should be.
• No concerns about the true being passed as the seen table.

Re-reviewing

[y5nw]

IMO we don't need the metatable-related behavior to change in the future since we have table.copy_with_metatable() as a separate function now.

[sfan5]

shorter name suggestion: table.metacopy

[Zughy]

Wouldn't be better to have a boolean as an extra parameter in the original table.copy?

[y5nw]

Wouldn't be better to have a boolean as an extra parameter in the original table.copy?

The issue is that table.copy from the current master takes an undocumented second argument seen, which - as a table (or nil/false, in which case the argument is then initialized with an empty table) - keeps tracks of references that table.copy has seen to handle references (in particular, recursive data structures) correctly. This prevents the use of a preserve_metatables flag as the second argument since it would break compatibility on older versions, where table.copy(t, true) causes an error： #15754 (comment)

(Adding the option as a third parameter would be doable, but it is IMO annoying since the second parameter would almost alwasy be {} or nil with some very few exceptions.)

[nauta-turbidus]

I'd support metacopy. It makes sense and if anyone is confused, docs are there.