Address Karoy's comments

Author: Azoy

stdlib/public/core/StringWordBreaking.swift

@@ -21,14 +21,6 @@ extension _StringGuts {
       return i
     }
 
-    return _slowRoundDownToNearestWord(i)
-  }
-
-  @inline(never)
-  internal func _slowRoundDownToNearestWord(
-    _ i: String.Index
-  ) -> String.Index {
-    let offset = i._encodedOffset
     let start = previousWordIndex(endingAt: offset)
     let end = nextWordIndex(startingAt: start)
     _internalInvariant(offset <= end, "Word breaking inconsistency")
@@ -104,7 +96,7 @@ extension _StringGuts {
 
   @inline(never)
   internal func _foreignPreviousWordIndex(endingAt i: Int) -> Int {
-    #if _runtime(_ObjC)
+#if _runtime(_ObjC)
     return previousWordBoundary(endingAt: i) {
       _internalInvariant($0 <= count)
 

stdlib/public/core/UnicodeSPI.swift

@@ -208,8 +208,6 @@ extension String {
   @_spi(_Unicode)
   @available(SwiftStdlib 5.7, *)
   public func _wordIndex(after i: String.Index) -> String.Index {
-    _precondition(i < endIndex)
-
     let i = _guts.validateWordIndex(i)
 
     let next = _guts.nextWordIndex(startingAt: i._encodedOffset)
@@ -219,12 +217,17 @@ extension String {
   @_spi(_Unicode)
   @available(SwiftStdlib 5.7, *)
   public func _wordIndex(before i: String.Index) -> String.Index {
-    _precondition(i > startIndex)
-    _precondition(i <= endIndex)
-
     let i = _guts.validateInclusiveWordIndex(i)
 
+    _precondition(i > startIndex, "String index is out of bounds")
+
     let previous = _guts.previousWordIndex(endingAt: i._encodedOffset)
     return String.Index(_encodedOffset: previous)
   }
+
+  @_spi(_Unicode)
+  @available(SwiftStdlib 5.7, *)
+  public func _nearestWordIndex(atOrBelow i: String.Index) -> String.Index {
+    _guts.roundDownToNearestWord(i)
+  }
 }