[Proposal] AttributedString UTF-8 and UTF-16 Views (#1067)

* [Proposal] AttributedString UTF-8 and UTF-16 Views

* Remove discontiguous slices

* Add default implementations for new AttributedStringProtocol requirements

* Apply suggestions from code review

Co-authored-by: Tina Liu <[email protected]>

---------

Co-authored-by: Tina Liu <[email protected]>

Author: jmschonfeld

Proposals/NNNN-attributedstring-utf8-utf16-views.md

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+# `AttributedString` UTF-8 and UTF-16 Views
+
+* Proposal: [SF-0012](0012-attributedstring-utf8-utf16-views.md)
+* Authors: [Jeremy Schonfeld](https://github.com/jmschonfeld)
+* Review Manager: [Tina Liu](https://github.com/itingliu)
+* Status: **Accepted**
+* Implementation: [swiftlang/swift-foundation#1066](https://github.com/swiftlang/swift-foundation/pull/1066)
+
+## Introduction/Motivation
+
+In macOS 12-aligned releases, Foundation added the `AttributedString` type as a new API representing rich/attributed text. `AttributedString` itself is not a collection, but rather a type that offers various views into its contents where each view represents a `Collection` over a different type of element. Today, `AttributedString` offers three views: the character view (`.characters`) which provides a collection of grapheme clusters using the `Character` element type, the unicode scalar view (`.unicodeScalars`) which provides a collection of `Unicode.Scalar`s, and the attribute runs view (`.runs`) which provides a collection of attribute runs present across the text using the `AttributedString.Runs.Run` element type. These three views form the critical APIs required to interact with an `AttributedString` via its text (either at the visual, grapheme cluster level or the underlying scalar level) and its runs. However, more advanced use cases require other ways to view an `AttributedString`'s text.
+
+When working with the text content of an `AttributedString`, sometimes it is necessary to view not only the characters or unicode scalars, but the underlying UTF-8 or UTF-16 contents that make up that text. This can be especially useful when interoperating with other types that use UTF-8 or UTF-16 encoded units as their currency types (for example, `NSAttributedString` and `NSString` which use UTF-16 offsets and UTF-16 scalars as their index and element types). Today, `String` itself has a UTF-8 and UTF-16 view that can be used to perform these encoding-specific operations, however `AttributedString` offers no equivalent. This proposal seeks to remedy this by adding equivalent UTF-8 and UTF-16 views to `AttributedString`, offering easy access to the encoded forms of the text.
+
+## Proposed solution
+
+Just like `String`, `AttributedString` will offer new, immutable UTF-8 and UTF-16 character views via the `.utf8` and `.utf16` properties. Developers will be able to use these new views like the following example:
+
+```swift
+var attrStr: AttributedString
+
+// Iterate over the UTF-8 scalars
+for scalar in attrStr.utf8 {
+    print(scalar)
+}
+
+// Determine the UTF-8 offset of a particular index
+let offset = attrStr.utf8.distance(from: attrStr.startIndex, to: someOtherIndex)
+```
+
+## Detailed design
+
+We propose adding the following API surface:
+
+```swift
+@available(FoundationPreview 6.2, *)
+extension AttributedString {
+    public struct UTF8View : BidirectionalCollection, CustomStringConvertible, Sendable {
+        public typealias Element = UTF8.CodeUnit
+        public typealias Index = AttributedString.Index
+        public typealias SubSequence = AttributedString.UTF8View
+    }
+    
+    public struct UTF16View : BidirectionalCollection, CustomStringConvertible, Sendable {
+        public typealias Element = UTF16.CodeUnit
+        public typealias Index = AttributedString.Index
+        public typealias SubSequence = AttributedString.UTF16View
+    }
+    
+    public var utf8: UTF8View { get }
+    public var utf16: UTF16View { get }
+}
+
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+protocol AttributedStringProtocol {
+    // ...
+    
+    @available(FoundationPreview 6.2, *)
+    var utf8: AttributedString.UTF8View { get }
+    @available(FoundationPreview 6.2, *)
+    var utf16: AttributedString.UTF16View { get }
+}
+
+
+@available(FoundationPreview 6.2, *)
+extension AttributedStringProtocol {
+    public var utf8: AttributedString.UTF8View { get }
+    public var utf16: AttributedString.UTF16View { get }
+}
+
+@available(FoundationPreview 6.2, *)
+extension AttributedSubstring {
+    public var utf8: AttributedString.UTF8View { get }
+    public var utf16: AttributedString.UTF16View { get }
+}
+```
+
+_Note: omitted here for brevity, `AttributedString.UTF8View` and `AttributedString.UTF16View` must implement all relevant, optional protocol requirements from `BidirectionalCollection` and `RangeReplaceableCollection` to ensure efficient operations over the underlying storage_
+
+## Source compatibility
+
+All of these changes are additive and have no impact on source compatibility. The added requirements to `AttributedStringProtocol` have provided default implementations and as such are not ABI/API breaking changes.
+
+## Implications on adoption
+
+These new views will be annotated with `FoundationPreview 6.2` availability. On platforms where availability is relevant, these APIs may only be used on versions where these new views are present.
+
+## Future directions
+
+No future directions are considered at this time.
+
+## Alternatives considered
+
+No alternatives are considered at this time.