[String] Assertion logic and isASCII bug fix.

Fix bugs in assertion logic and properly update the isASCII bit on
RRC. RRC tests added.

Author: milseman

stdlib/public/core/StringStorage.swift

@@ -446,7 +446,7 @@ extension _StringStorage {
   ) {
     _sanityCheck(lower <= upper)
     let replCount = replacement.count
-    _sanityCheck((upper - lower) + replCount <= unusedCapacity)
+    _sanityCheck(replCount - (upper - lower) <= unusedCapacity)
 
     // Position the tail
     let lowerPtr = mutableStart + lower
@@ -459,9 +459,8 @@ extension _StringStorage {
         mutating: replacement.baseAddress._unsafelyUnwrappedUnchecked),
       count: replCount)
 
-    _postRRCAdjust(
-      newCount: lower + replCount + tailCount,
-      newIsASCII: self.isASCII && isASCII)
+    let isASCII = self.isASCII && _allASCII(replacement)
+    _postRRCAdjust(newCount: lower + replCount + tailCount, newIsASCII: isASCII)
   }
 
 
@@ -473,15 +472,15 @@ extension _StringStorage {
     replacementCount replCount: Int
   ) where C.Element == UInt8 {
     _sanityCheck(lower <= upper)
-    _sanityCheck((upper - lower) + replCount <= unusedCapacity)
+    _sanityCheck(replCount - (upper - lower) <= unusedCapacity)
 
     // Position the tail
     let lowerPtr = mutableStart + lower
     let tailCount = _slideTail(
       src: mutableStart + upper, dst: lowerPtr + replCount)
 
     // Copy in the contents
-    var isASCII = true
+    var isASCII = self.isASCII
     var srcCount = 0
     for cu in replacement {
       if cu >= 0x80 { isASCII = false }
@@ -491,8 +490,7 @@ extension _StringStorage {
     _sanityCheck(srcCount == replCount)
 
     _postRRCAdjust(
-      newCount: lower + replCount + tailCount,
-      newIsASCII: self.isASCII && isASCII)
+      newCount: lower + replCount + tailCount, newIsASCII: isASCII)
   }
 }
 

validation-test/stdlib/String.swift

@@ -1047,6 +1047,24 @@ StringTests.test("UnicodeScalarViewReplace") {
   }
 }
 
+StringTests.test("StringRRC") {
+  let narrow = "01234567890"
+  let wide = "ⅠⅡⅢⅣⅤⅥⅦⅧⅨⅩⅪ"
+  for s1 in [narrow, wide] {
+    for s2 in [narrow, wide] {
+      let doubleS2 = Array(String(makeStringGuts(s2 + s2)).utf16)
+      checkRangeReplaceable(
+        { () -> String in String(makeStringGuts(s1)) },
+        { String(decoding: doubleS2[0..<$0], as: UTF16.self) }
+      )
+      checkRangeReplaceable(
+        { String(makeStringGuts(s1)) },
+        { Array(String(makeStringGuts(s2 + s2)))[0..<$0] }
+      )
+    }
+  }
+}
+
 StringTests.test("reserveCapacity") {
   var s = ""
   let id0 = s.bufferID