[stdlib][cosmetics] Clean up unused/underused interfaces, update naming

There is little point to having `isUTF16` properties when they simply
return `!isUTF8`; remove them.

Rename `String.Index._copyEncoding(from:)` to
`_copyingEncoding(from:)`.

Author: lorentey

stdlib/public/core/StringGuts.swift

@@ -301,19 +301,13 @@ extension _StringGuts {
 // Encoding
 extension _StringGuts {
   /// Returns whether this string has a UTF-8 storage representation.
+  /// If this returns false, then the string is encoded in UTF-16.
   ///
   /// This always returns a value corresponding to the string's actual encoding.
   @_alwaysEmitIntoClient
   @inline(__always)
   internal var isUTF8: Bool { _object.isUTF8 }
 
-  /// Returns whether this string has a UTF-16 storage representation.
-  ///
-  /// This always returns a value corresponding to the string's actual encoding.
-  @_alwaysEmitIntoClient
-  @inline(__always)
-  internal var isUTF16: Bool { _object.isUTF16 }
-
   @_alwaysEmitIntoClient // Swift 5.7
   @inline(__always)
   internal func markEncoding(_ i: String.Index) -> String.Index {

stdlib/public/core/StringGutsRangeReplaceable.swift

@@ -466,10 +466,9 @@ extension _StringGuts {
     _internalInvariant(
       subrange.lowerBound >= startIndex && subrange.upperBound <= endIndex)
 
-    if _slowPath(isUTF16) {
-      // UTF-16 (i.e., foreign) string. The mutation will convert this to the
-      // native UTF-8 encoding, so we need to do some extra work to preserve our
-      // bounds.
+    guard _slowPath(isUTF8) else {
+      // UTF-16 string. The mutation will convert this to the native UTF-8
+      // encoding, so we need to do some extra work to preserve our bounds.
       let utf8StartOffset = String(self).utf8.distance(
         from: self.startIndex, to: startIndex)
       let oldUTF8Count = String(self).utf8.distance(

stdlib/public/core/StringIndex.swift

@@ -481,24 +481,12 @@ extension String.Index {
   }
 
   @_alwaysEmitIntoClient // Swift 5.7
-  internal func _copyEncoding(from index: Self) -> Self {
+  internal func _copyingEncoding(from index: Self) -> Self {
     let mask = Self.__utf8Bit | Self.__utf16Bit
     return Self((_rawBits & ~mask) | (index._rawBits & mask))
   }
 }
 
-extension String.Index {
-  @_alwaysEmitIntoClient @inline(__always) // Swift 5.7
-  internal var _isUTF8CharacterIndex: Bool {
-    _canBeUTF8 && _isCharacterAligned
-  }
-
-  @_alwaysEmitIntoClient @inline(__always) // Swift 5.7
-  internal var _isUTF8ScalarIndex: Bool {
-    _canBeUTF8 && _isScalarAligned
-  }
-}
-
 extension String.Index: Equatable {
   @inlinable @inline(__always)
   public static func == (lhs: String.Index, rhs: String.Index) -> Bool {

stdlib/public/core/StringObject.swift

@@ -1009,6 +1009,7 @@ extension _StringObject {
   }
 
   /// Returns whether this string has a UTF-8 storage representation.
+  /// If this returns false, then the string is encoded in UTF-16.
   ///
   /// This always returns a value corresponding to the string's actual encoding.
   @_alwaysEmitIntoClient
@@ -1030,15 +1031,6 @@ extension _StringObject {
     providesFastUTF8 || _countAndFlags.isForeignUTF8
   }
 
-  /// Returns whether this string has a UTF-16 storage representation.
-  ///
-  /// This always returns a value corresponding to the string's actual encoding.
-  @_alwaysEmitIntoClient
-  @inline(__always) // Swift 5.7
-  internal var isUTF16: Bool {
-    !isUTF8
-  }
-
   // Get access to fast UTF-8 contents for large strings which provide it.
   @inlinable @inline(__always)
   internal var fastUTF8: UnsafeBufferPointer<UInt8> {

stdlib/public/core/UnicodeHelpers.swift

@@ -167,7 +167,7 @@ extension _StringGuts {
       result = idx
     } else {
       // TODO(String performance): isASCII check
-      result = scalarAlignSlow(idx)._scalarAligned._copyEncoding(from: idx)
+      result = scalarAlignSlow(idx)._scalarAligned._copyingEncoding(from: idx)
     }
 
     _internalInvariant(isOnUnicodeScalarBoundary(result),