[5.0] Cherry-pick important bug fix and ABI expunges (#20844)

* [String] Drop spurious @inlinable

* [String] Bug fix for empty-range getCharacters

* [String] Expunge _StringGutsSlice from ABI

_StringGutsSlice is an implementation helper that's no longer needed
from inlinable code. Internalize it.

Author: milseman

stdlib/public/core/StringGutsSlice.swift

@@ -16,22 +16,18 @@
 
 // A sliced _StringGuts, convenient for unifying String/Substring comparison,
 // hashing, and RRC.
-@_fixed_layout
-@usableFromInline
 internal struct _StringGutsSlice {
-  @usableFromInline
   internal var _guts: _StringGuts
 
-  @usableFromInline
   internal var _offsetRange: Range<Int>
 
-  @inlinable @inline(__always)
+  @inline(__always)
   internal init(_ guts: _StringGuts) {
     self._guts = guts
     self._offsetRange = 0..<self._guts.count
   }
 
-  @inlinable @inline(__always)
+  @inline(__always)
   internal init(_ guts: _StringGuts, _ offsetRange: Range<Int>) {
     self._guts = guts
     self._offsetRange = offsetRange
@@ -83,7 +79,7 @@ internal struct _StringGutsSlice {
     }
   }
 
-  @inlinable @inline(__always)
+  @inline(__always)
   internal func withFastUTF8<R>(
     _ f: (UnsafeBufferPointer<UInt8>) throws -> R
   ) rethrows -> R {

stdlib/public/core/StringHashable.swift

@@ -45,7 +45,6 @@ extension StringProtocol {
 }
 
 extension _StringGutsSlice {
-  @usableFromInline // @opaque
   @inline(never) // slow-path
   internal func _normalizedHash(into hasher: inout Hasher) {
     if self.isNFCFastUTF8 {

stdlib/public/core/StringProtocol.swift

@@ -132,7 +132,6 @@ extension StringProtocol {
     get { return String(self) }
   }
 
-  @inlinable // Eliminate for String, Substring
   internal var _gutsSlice: _StringGutsSlice {
     @_specialize(where Self == String)
     @_specialize(where Self == Substring)
@@ -152,7 +151,8 @@ extension StringProtocol {
     @inline(__always) get {
       let start = startIndex
       let end = endIndex
-      _internalInvariant(start.transcodedOffset == 0 && end.transcodedOffset == 0)
+      _internalInvariant(
+        start.transcodedOffset == 0 && end.transcodedOffset == 0)
       return Range(uncheckedBounds: (start.encodedOffset, end.encodedOffset))
     }
   }

stdlib/public/core/StringStorage.swift

@@ -189,7 +189,6 @@ final internal class _StringStorage: _AbstractStringStorage {
 
   internal var _reserved: UInt16
 
-  @inlinable
   override internal var count: Int {
     @inline(__always) get { return _count }
     @inline(__always) set { _count = newValue }

stdlib/public/core/StringUTF16View.swift

@@ -537,6 +537,8 @@ extension String {
   ) {
     _internalInvariant(_guts.isFastUTF8)
 
+    if _slowPath(range.isEmpty) { return }
+
     return _guts.withFastUTF8 { utf8 in
       var writeIdx = 0
       let writeEnd = buffer.count

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

@@ -483,6 +483,23 @@ Protocol _NSEnumerator has been removed
 Protocol _NSFastEnumeration has been removed
 Protocol _ShadowProtocol has been removed
 
+Constructor _StringGutsSlice.init(_:) has been removed
+Constructor _StringGutsSlice.init(_:_:) has been removed
+Func _StringGutsSlice._normalizedHash(into:) has been removed
+Func _StringGutsSlice.compare(with:expecting:) has been removed
+Func _StringGutsSlice.withFastUTF8(_:) has been removed
+Struct _StringGutsSlice is now without @_fixed_layout
+Var StringProtocol._gutsSlice has been removed
+Var _StringGutsSlice._guts has been removed
+Var _StringGutsSlice._offsetRange has been removed
+Var _StringGutsSlice.count has been removed
+Var _StringGutsSlice.end has been removed
+Var _StringGutsSlice.isASCII has been removed
+Var _StringGutsSlice.isFastUTF8 has been removed
+Var _StringGutsSlice.isNFCFastUTF8 has been removed
+Var _StringGutsSlice.range has been removed
+Var _StringGutsSlice.start has been removed
+
 Func ManagedBufferPointer._sanityCheckValidBufferClass(_:creating:) has been removed
 Func _sanityCheck(_:_:file:line:) has been removed
 Func _sanityCheckFailure(_:file:line:) has been removed

validation-test/stdlib/StringBreadcrumbs.swift

@@ -38,14 +38,17 @@ let StringBreadcrumbsTests = TestSuite("StringBreadcrumbsTests")
 
 func validateBreadcrumbs(_ str: String) {
   var utf16CodeUnits = Array(str.utf16)
-  var utf16Indices = Array(str.utf16.indices)
   var outputBuffer = Array<UInt16>(repeating: 0, count: utf16CodeUnits.count)
 
-  for i in 0..<(utf16CodeUnits.count-1) {
-    for j in (i+1)..<utf16CodeUnits.count {
+  // Include the endIndex, so we can test end conversions
+  var utf16Indices = Array(str.utf16.indices) + [str.utf16.endIndex]
+
+  for i in 0...utf16CodeUnits.count {
+    for j in i...utf16CodeUnits.count {
       let range = Range(uncheckedBounds: (i, j))
 
       let indexRange = str._toUTF16Indices(range)
+
       // Range<String.Index> <=> Range<Int>
       expectEqual(utf16Indices[i], indexRange.lowerBound)
       expectEqual(utf16Indices[j], indexRange.upperBound)
@@ -69,6 +72,13 @@ func validateBreadcrumbs(_ str: String) {
   }
 }
 
+StringBreadcrumbsTests.test("uniform strings") {
+  validateBreadcrumbs(smallASCII)
+  validateBreadcrumbs(largeASCII)
+  validateBreadcrumbs(smallUnicode)
+  validateBreadcrumbs(largeUnicode)
+}
+
 StringBreadcrumbsTests.test("largeString") {
   validateBreadcrumbs(largeString)
 }