[stdlib] add storage property to Substring.UTF8View

Author: glessard

stdlib/public/core/Span/Properties/StringFamilyExtensions.swift

@@ -35,3 +35,31 @@ extension String.UTF8View {
     }
   }
 }
+
+extension Substring.UTF8View {
+
+  @available(SwiftStdlib 6.1, *)
+  public var storage: Span<UTF8.CodeUnit> {
+    @_alwaysEmitIntoClient
+    @lifetime(borrow self)
+    borrowing get {
+      let start = _slice._startIndex._encodedOffset
+      let end = _slice._endIndex._encodedOffset
+      if _wholeGuts.isSmall {
+        let a = Builtin.addressOfBorrow(self)
+        let offset = start &+ (MemoryLayout<String.Index>.stride &<< 1)
+        let address = UnsafePointer<UTF8.CodeUnit>(a).advanced(by: offset)
+        let span = Span(_unsafeStart: address, count: end &- start)
+        return _overrideLifetime(span, borrowing: self)
+      }
+      else if _wholeGuts.isFastUTF8 {
+        let buffer = _wholeGuts._object.fastUTF8.extracting(start..<end)
+        let count = end &- start
+        _internalInvariant(count == buffer.count)
+        let span = Span(_unsafeElements: buffer)
+        return _overrideLifetime(span, borrowing: self)
+      }
+      fatalError()
+    }
+  }
+}

test/stdlib/Span/StringUTF8StorageProperty.swift

@@ -58,3 +58,45 @@ suite.test("Span from Large Native String")
     expectEqual(span[i], u[i])
   }
 }
+
+suite.test("Span from Small String's Substring")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.1, *) { false } else { true } },
+  reason: "Requires Swift 6.1's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.1, *) else { return }
+
+  let s = "A small string.".dropFirst(8).utf8
+  let u = Array("string.".utf8)
+  let span = s.storage
+
+  let count = span.count
+  expectEqual(count, s.count)
+
+  for i in span.indices {
+    expectEqual(span[i], u[i])
+  }
+}
+
+suite.test("Span from Large Native String's Substring")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.1, *) { false } else { true } },
+  reason: "Requires Swift 6.1's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.1, *) else { return }
+
+  let t = "A long string that is altogether not smol."
+  let s = t.dropFirst(22).prefix(10).utf8
+  let u = Array("altogether".utf8)
+  let span = s.storage
+
+  let count = span.count
+  expectEqual(count, s.count)
+
+  for i in span.indices {
+    expectEqual(span[i], u[i])
+  }
+}
+