[stdlib] Substring.UnicodeScalarView: Add _invariantCheck

Author: lorentey

stdlib/public/core/StringGuts.swift

@@ -357,6 +357,7 @@ extension _StringGuts {
   /// index, but it is guaranteed to never incorrectly return false. If all
   /// loaded binaries were built in 5.7+, then this method is guaranteed to
   /// always return the correct value.
+  @_alwaysEmitIntoClient
   internal func hasMatchingEncoding(_ i: String.Index) -> Bool {
     (isForeign && i._canBeUTF16) || (!isForeign && i._canBeUTF8)
   }

stdlib/public/core/Substring.swift

@@ -1050,6 +1050,7 @@ extension Substring {
       _unchecked base: String.UnicodeScalarView, bounds: Range<Index>
     ) {
       _slice = Slice(base: base, bounds: bounds)
+      _invariantCheck()
     }
 
     /// Creates an instance that slices `base` at `_bounds`.
@@ -1073,7 +1074,25 @@ extension Substring.UnicodeScalarView {
   @_alwaysEmitIntoClient
   @inline(__always)
   internal var _bounds: Range<Index> { _slice._bounds }
+}
+
+extension Substring.UnicodeScalarView {
+  #if !INTERNAL_CHECKS_ENABLED
+  @_alwaysEmitIntoClient @inline(__always)
+  internal func _invariantCheck() {}
+  #else
+  @_alwaysEmitIntoClient
+  @inline(never) @_effects(releasenone)
+  internal func _invariantCheck() {
+    _internalInvariant(endIndex <= _wholeGuts.endIndex)
+    _internalInvariant(
+      _wholeGuts.hasMatchingEncoding(startIndex) &&
+      _wholeGuts.hasMatchingEncoding(endIndex))
+    _internalInvariant(
+      startIndex._isScalarAligned && endIndex._isScalarAligned)
+    _slice._base._invariantCheck()
   }
+  #endif // INTERNAL_CHECKS_ENABLED
 }
 
 extension Substring.UnicodeScalarView: BidirectionalCollection {