[stdlib] Should the `RangeSet` types be `@frozen`?

[benrimmington]

Should the following types from SE-0270 be @frozen in Swift 6?

• RangeSet
• RangeSet.Ranges
• DiscontiguousSlice
• DiscontiguousSlice.Index

Cc: @swiftlang/standard-librarians

[jmschonfeld]

When we landed those types in the stdlib, we decided against making RangeSet.Ranges @frozen so that we'd have flexibility in the future to update the underlying storage of the RangeSet type (for example to special case certain sets like the empty or single element sets, or to use a tree-like structure rather than a flat array). IIRC, when testing performance of RangeSet I determined that marking the surrounding types as @frozen had insignificant performance impact because the underlying storage type was still non-frozen, and would make it easier to adapt to new storage within RangeSet.Ranges by leaving them unfrozen. Making these all @frozen certainly would help with performance, but in the end if I'm remembering correctly we determined that for this type the flexibility was the better choice.

[benrimmington]

@jmschonfeld Thanks for the detailed explanation. I should have read the final review of your pull request:

The big caveat is that this is an opaque Collection type, a big deviation from most previous stdlib additions.