[String] Custom iterator for UTF16View (#20929)

Defining a custom iterator for the UTF16View avoid some redundant
computation over the indexing model. This speeds up iteration by
around 40% on non-ASCII strings.

stdlib/public/core/StringCharacterView.swift

@@ -234,8 +234,8 @@ extension String {
 
     @inlinable
     internal init(_ guts: _StringGuts) {
-      self._guts = guts
       self._end = guts.count
+      self._guts = guts
     }
 
     @inlinable

stdlib/public/core/StringUTF16View.swift

@@ -260,6 +260,59 @@ extension String.UTF16View: BidirectionalCollection {
     }
   }
 }
+
+extension String.UTF16View {
+  @_fixed_layout
+  public struct Iterator: IteratorProtocol {
+    @usableFromInline
+    internal var _guts: _StringGuts
+
+    @usableFromInline
+    internal var _position: Int = 0
+
+    @usableFromInline
+    internal var _end: Int
+
+    // If non-nil, return this value for `next()` (and set it to nil).
+    //
+    // This is set when visiting a non-BMP scalar: the leading surrogate is
+    // returned, this field is set with the value of the trailing surrogate, and
+    // `_position` is advanced to the start of the next scalar.
+    @usableFromInline
+    internal var _nextIsTrailingSurrogate: UInt16? = nil
+
+    @inlinable
+    internal init(_ guts: _StringGuts) {
+      self._end = guts.count
+      self._guts = guts
+    }
+
+    @inlinable
+    public mutating func next() -> UInt16? {
+      if _slowPath(_nextIsTrailingSurrogate != nil) {
+        let trailing = self._nextIsTrailingSurrogate._unsafelyUnwrappedUnchecked
+        self._nextIsTrailingSurrogate = nil
+        return trailing
+      }
+      guard _fastPath(_position < _end) else { return nil }
+
+      let (scalar, len) = _guts.errorCorrectedScalar(startingAt: _position)
+      _position &+= len
+
+      if _slowPath(scalar.value > UInt16.max) {
+        self._nextIsTrailingSurrogate = scalar.utf16[1]
+        return scalar.utf16[0]
+      }
+      return UInt16(truncatingIfNeeded: scalar.value)
+    }
+  }
+  @inlinable
+  public __consuming func makeIterator() -> Iterator {
+    return Iterator(_guts)
+  }
+}
+
+
 extension String.UTF16View: CustomStringConvertible {
   @inlinable
   public var description: String {

stdlib/public/core/StringUnicodeScalarView.swift

@@ -180,8 +180,8 @@ extension String.UnicodeScalarView {
 
     @inlinable
     internal init(_ guts: _StringGuts) {
-      self._guts = guts
       self._end = guts.count
+      self._guts = guts
     }
 
     @inlinable

test/api-digester/Outputs/stability-stdlib-abi.swift.expected

@@ -499,6 +499,9 @@ Var _StringGutsSlice.isNFCFastUTF8 has been removed
 Var _StringGutsSlice.range has been removed
 Var _StringGutsSlice.start has been removed
 
+Struct String.UTF16View has type witness type for Collection.Iterator changing from IndexingIterator<String.UTF16View> to String.UTF16View.Iterator
+Struct String.UTF16View has type witness type for Sequence.Iterator changing from IndexingIterator<String.UTF16View> to String.UTF16View.Iterator
+
 Func ManagedBufferPointer._sanityCheckValidBufferClass(_:creating:) has been removed
 Func _sanityCheck(_:_:file:line:) has been removed
 Func _sanityCheckFailure(_:file:line:) has been removed