Add Sendable conformances to all public types

[lorentey]

Resolves #166.

List of conformances added:

(The conformances are currently protected by #if swift(>=5.5) clauses that may get removed if we bump the minimum toolchain version to 5.5 in the 1.1.0 tag.)

extension BitArray: Sendable {}

extension BitSet: Sendable {}
extension BitSet.Iterator: Sendable {}
extension BitSet.Index: Sendable {}
extension BitSet.Counted: Sendable {}

extension Deque: @unchecked Sendable where Element: Sendable {}
extension Deque.Iterator: Sendable where Element: Sendable {}

extension Heap: Sendable where Element: Sendable {}

extension OrderedSet: @unchecked Sendable where Element: Sendable {}
extension OrderedSet.SubSequence: Sendable where Element: Sendable {}
extension OrderedSet.UnorderedView: Sendable where Element: Sendable {}
extension OrderedSet._UnstableInternals: Sendable where Element: Sendable {}

extension OrderedDictionary: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension OrderedDictionary.Iterator: Sendable where Key: Sendable, Value: Sendable {}
extension OrderedDictionary.Values: Sendable where Key: Sendable, Value: Sendable {}
extension OrderedDictionary.Elements: Sendable where Key: Sendable, Value: Sendable {}
extension OrderedDictionary.Elements.SubSequence: Sendable where Key: Sendable, Value: Sendable {}
extension OrderedDictionary.Elements.SubSequence.Iterator: Sendable where Key: Sendable, Value: Sendable {}

extension PersistentSet: @unchecked Sendable where Element: Sendable {}
extension PersistentSet.Iterator: @unchecked Sendable where Element: Sendable {}
extension PersistentSet.Index: @unchecked Sendable where Element: Sendable {}

extension PersistentDictionary: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension PersistentDictionary.Iterator: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension PersistentDictionary.Index: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension PersistentDictionary.Keys: Sendable where Key: Sendable, Value: Sendable {}
extension PersistentDictionary.Keys.Iterator: Sendable where Key: Sendable, Value: Sendable {}
extension PersistentDictionary.Values: Sendable where Key: Sendable, Value: Sendable {}
extension PersistentDictionary.Values.Iterator: Sendable where Key: Sendable, Value: Sendable {}

// The ones below are provisional -- SortedSet/SortedDictionary aren't quite ready yet, 
// and they may or may not make it into the 1.1.0 release.
extension SortedSet: @unchecked Sendable where Element: Sendable {}
extension SortedSet.Iterator: @unchecked Sendable where Element: Sendable {}
extension SortedSet.Index: @unchecked Sendable where Element: Sendable {}
extension SortedSet.SubSequence: @unchecked Sendable where Element: Sendable {}
extension SortedSet.SubSequence.Iterator: @unchecked Sendable where Element: Sendable {}

extension SortedDictionary: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.Iterator: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.Index: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.SubSequence: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.SubSequence.Iterator: @unchecked Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.Keys: Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.Keys.Iterator: Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.Values: Sendable where Key: Sendable, Value: Sendable {}
extension SortedDictionary.Values.Iterator: Sendable where Key: Sendable, Value: Sendable {}

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[natecook1000]

AFAICT this looks good! Anything that's problematic for Sendable in theory would be in the unchanged parts of the implementations. It would be good for us to document what kinds of things would cause a problem for concurrent accesses — like for instance, the Regex struct stores a reference type that, on first usage, compiles the regular expression and stores it, which would cause concurrent writes if not protected.

[stephentyrone]

@natecook1000 Not just what to look out for, but the patterns for how to guard them.

[lorentey]

It would be good for us to document what kinds of things would cause a problem for concurrent accesses

Indeed, but this package is probably not the right place for that documentation -- it better belongs in the docs of Sendable itself. (The current API docs are suspiciously quiet about spelling out the actual requirements.)

Related: it took me quite a while to figure out what exactly Swift means by "value semantics". (AIUI it's mostly/entirely about eliminating action-at-a-distance effects, rather than having anything to do with textbook definitions of referential transparency etc.) AIUI, all types with value semantics can be Sendable, but not all Sendable types are values. The distinction ought to be clearly explained somewhere.

like for instance, the Regex struct stores a reference type that, on first usage, compiles the regular expression and stores it, which would cause concurrent writes if not protected.

Hm, this strongly feels like an issue with the Regex type. As a baseline expectation, updating the reference ought to be an atomic operation, along the lines of AtomicLazyReference. (Unless of course this is done in a mutating operation, in which case the Law of Exclusivity applies, and no synchronization is necessary.)

(Update: I think I found the relevant code. It does try to do the correct thing, but it stops short of fully achieving it -- I filed swiftlang/swift-experimental-string-processing#609 to fix the remaining Law of Exclusivity violation.)

[msteindorfer]

LGTM

[flowtoolz]

Looks like release/1.1 has been merged into main on December 1st. But the latest release from December 8 is still 1.0.4 and does not offer the Sendable conformances (I get warnings when using OrderedDictionary in Sendable types). How am I to understand this?

[benrimmington]

@flowtoolz OrderedDictionary not Sendable?

[qusc]

Any news about when this will be released? Thanks!