[stdlib] simplify spans of bridged substrings

Author: glessard

stdlib/public/core/Substring.swift

@@ -759,40 +759,28 @@ extension Substring.UTF8View {
 #if _runtime(_ObjC)
       // handle non-UTF8 Objective-C bridging cases here
       if !_wholeGuts.isFastUTF8 && _wholeGuts._object.hasObjCBridgeableObject {
-        let foreign = String(_wholeGuts)
-        let dts = foreign.distance(from: foreign.startIndex, to: startIndex)
-        let dte = foreign.distance(from: startIndex, to: endIndex)
-
-        let storage: __StringStorage
-        if let associated = _wholeGuts.getAssociatedStorage() {
-          storage = associated
-        }
-        else {
-          storage = _wholeGuts.getOrAllocateAssociatedStorage()
-        }
-        let native = storage.asString
-        let nativeStart = native.index(native.startIndex, offsetBy: dts)
-        let startOffset = nativeStart._encodedOffset
-        let endOffset = native.index(nativeStart, offsetBy: dte)._encodedOffset
-
-        let (start, count) = unsafe (storage.start, storage.count)
-        var span = unsafe Span(_unsafeStart: start, count: count)
-        span = unsafe span._extracting(unchecked: startOffset..<endOffset)
+        let base: String.UTF8View = self._base
+        let first = base._foreignDistance(from: base.startIndex, to: startIndex)
+        let count = base._foreignDistance(from: startIndex, to: endIndex)
+        let span = unsafe base.span._extracting(
+          unchecked: Range(_uncheckedBounds: (first, first &+ count))
+        )
         return unsafe _overrideLifetime(span, borrowing: self)
       }
 #endif
-      let start = _slice._startIndex._encodedOffset
+      let first = _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 = unsafe UnsafePointer<UTF8.CodeUnit>(a).advanced(by: offset)
-        let span = unsafe Span(_unsafeStart: address, count: end &- start)
+        let offset = first &+ (2 &* MemoryLayout<String.Index>.stride)
+        let start = unsafe UnsafePointer<UTF8.CodeUnit>(a).advanced(by: offset)
+        let span = unsafe Span(_unsafeStart: start, count: end &- first)
         return unsafe _overrideLifetime(span, borrowing: self)
       }
-      _precondition(_wholeGuts.isFastUTF8)
-      let buffer = unsafe _wholeGuts._object.fastUTF8.extracting(start..<end)
-      _internalInvariant((end &- start) == buffer.count)
+      _internalInvariant(_wholeGuts.isFastUTF8)
+      let buffer = unsafe _wholeGuts._object.fastUTF8.extracting(
+        Range(_uncheckedBounds: (first, end))
+      )
       let span = unsafe Span(_unsafeElements: buffer)
       return unsafe _overrideLifetime(span, borrowing: self)
     }