[String] NFC iterator fast-paths

Refactor and rename _StringGutsSlice, apply NFC-aware fast paths to a
new buffered iterator.

Also, fix bug in _typeName which used to assume ASCIIness and better
SIL optimizations on StringObject.

stdlib/public/core/StringGuts.swift

@@ -232,13 +232,13 @@ extension _StringGuts {
   @inlinable
   internal func copyUTF8(into mbp: UnsafeMutableBufferPointer<UInt8>) -> Int? {
     // TODO(UTF8 perf): minor perf win by avoiding slicing if fast...
-    return _SlicedStringGuts(self).copyUTF8(into: mbp)
+    return _StringGutsSlice(self).copyUTF8(into: mbp)
   }
 
   internal var utf8Count: Int {
     @inline(__always) get {
       if _fastPath(self.isFastUTF8) { return count }
-      return _SlicedStringGuts(self).utf8Count
+      return _StringGutsSlice(self).utf8Count
     }
   }
 
@@ -258,106 +258,3 @@ extension _StringGuts {
     @inline(__always) get { return Index(encodedOffset: self.count) }
   }
 }
-
-// A sliced _StringGuts, convenient for unifying String/Substring comparison,
-// hashing, and RRC.
-@_fixed_layout
-@usableFromInline
-internal struct _SlicedStringGuts {
-  @usableFromInline
-  internal var _guts: _StringGuts
-
-  @usableFromInline
-  internal var _offsetRange: Range<Int>
-
-  @inlinable @inline(__always)
-  internal init(_ guts: _StringGuts) {
-    self._guts = guts
-    self._offsetRange = 0..<self._guts.count
-  }
-
-  @inlinable @inline(__always)
-  internal init(_ guts: _StringGuts, _ offsetRange: Range<Int>) {
-    self._guts = guts
-    self._offsetRange = offsetRange
-  }
-
-  @inlinable
-  internal var count: Int {
-    @inline(__always) get { return _offsetRange.count }
-  }
-
-  @inlinable
-  internal var isNFCFastUTF8: Bool {
-    @inline(__always) get { return _guts.isNFCFastUTF8 }
-  }
-
-  @inlinable
-  internal var isASCII: Bool {
-    @inline(__always) get { return _guts.isASCII }
-  }
-
-  @inlinable
-  internal var isFastUTF8: Bool {
-    @inline(__always) get { return _guts.isFastUTF8 }
-  }
-
-  internal var utf8Count: Int {
-    @inline(__always) get {
-      if _fastPath(self.isFastUTF8) {
-        return _offsetRange.count
-      }
-      return Substring(self).utf8.count
-    }
-  }
-
-  @inlinable
-  internal var range: Range<String.Index> {
-    @inline(__always) get {
-      return String.Index(encodedOffset: _offsetRange.lowerBound)
-         ..< String.Index(encodedOffset: _offsetRange.upperBound)
-    }
-  }
-
-  @inlinable @inline(__always)
-  internal func withFastUTF8<R>(
-    _ f: (UnsafeBufferPointer<UInt8>) throws -> R
-  ) rethrows -> R {
-    return try _guts.withFastUTF8(range: _offsetRange, f)
-  }
-
-  // Copy UTF-8 contents. Returns number written or nil if not enough space.
-  // Contents of the buffer are unspecified if nil is returned.
-  @inlinable
-  internal func copyUTF8(into mbp: UnsafeMutableBufferPointer<UInt8>) -> Int? {
-    let ptr = mbp.baseAddress._unsafelyUnwrappedUnchecked
-    if _fastPath(self.isFastUTF8) {
-      return self.withFastUTF8 { utf8 in
-        guard utf8.count <= mbp.count else { return nil }
-
-        let utf8Start = utf8.baseAddress._unsafelyUnwrappedUnchecked
-        ptr.initialize(from: utf8Start, count: utf8.count)
-        return utf8.count
-      }
-    }
-
-    return _foreignCopyUTF8(into: mbp)
-  }
-
-  @_effects(releasenone)
-  @usableFromInline @inline(never) // slow-path
-  internal func _foreignCopyUTF8(
-    into mbp: UnsafeMutableBufferPointer<UInt8>
-  ) -> Int? {
-    var ptr = mbp.baseAddress._unsafelyUnwrappedUnchecked
-    var numWritten = 0
-    for cu in Substring(self).utf8 {
-      guard numWritten < mbp.count else { return nil }
-      ptr.initialize(to: cu)
-      ptr += 1
-      numWritten += 1
-    }
-
-    return numWritten
-  }
-}