[String] Add Substring.base

Adds Substring.base, analogous to Slice.base, to access the entire
String.

Tests added.

Author: milseman

stdlib/public/core/LegacyABI.swift

@@ -33,3 +33,9 @@ internal func _isContinuation(_ x: UInt8) -> Bool {
   return UTF8.isContinuation(x)
 }
 
+extension Substring {
+@available(*, unavailable, renamed: "Substring.base")
+  @inlinable
+  internal var _wholeString: String { return base }
+}
+

stdlib/public/core/StringCreate.swift

@@ -193,8 +193,8 @@ extension String {
   internal static func _fromSubstring(
     _ substring: __shared Substring
   ) -> String {
-    if substring._offsetRange == substring._wholeString._offsetRange {
-      return substring._wholeString
+    if substring._offsetRange == substring.base._offsetRange {
+      return substring.base
     }
 
     return String._copying(substring)

stdlib/public/core/Substring.swift

@@ -116,14 +116,12 @@ public struct Substring {
 }
 
 extension Substring {
-  @inlinable
-  internal var _wholeGuts: _StringGuts {
-    @inline(__always) get { return _slice.base._guts }
-  }
-  @inlinable
-  internal var _wholeString: String {
-    @inline(__always) get { return String(self._wholeGuts) }
-  }
+  /// Returns the underlying string from which this Substring was derived.
+  @_alwaysEmitIntoClient
+  public var base: String { return _slice.base }
+
+  @inlinable @inline(__always)
+  internal var _wholeGuts: _StringGuts { return base._guts }
 
   @inlinable
   internal var _offsetRange: Range<Int> {
@@ -141,7 +139,7 @@ extension Substring {
   #else
   @usableFromInline @inline(never) @_effects(releasenone)
   internal func _invariantCheck() {
-    self._wholeString._invariantCheck()
+    self.base._invariantCheck()
   }
   #endif // INTERNAL_CHECKS_ENABLED
 }
@@ -420,7 +418,7 @@ extension Substring {
   @inlinable
   public var utf8: UTF8View {
     get {
-      return _wholeString.utf8[startIndex..<endIndex]
+      return base.utf8[startIndex..<endIndex]
     }
     set {
       self = Substring(newValue)
@@ -546,7 +544,7 @@ extension Substring {
   @inlinable
   public var utf16: UTF16View {
     get {
-      return _wholeString.utf16[startIndex..<endIndex]
+      return base.utf16[startIndex..<endIndex]
     }
     set {
       self = Substring(newValue)
@@ -672,7 +670,7 @@ extension Substring {
   @inlinable
   public var unicodeScalars: UnicodeScalarView {
     get {
-      return _wholeString.unicodeScalars[startIndex..<endIndex]
+      return base.unicodeScalars[startIndex..<endIndex]
     }
     set {
       self = Substring(newValue)

test/stdlib/subString.swift

@@ -238,4 +238,13 @@ SubstringTests.test("Persistent Content") {
   expectEqual("bcdefg", (str.dropFirst(1) + "g") as String)
 }
 
+SubstringTests.test("Substring.base") {
+  let str = "abéÏ01😓🎃👨‍👨‍👧‍👦"
+  expectEqual(str, str.dropLast().base)
+  for idx in str.indices {
+    expectEqual(str, str[idx...].base)
+    expectEqual(str, str[...idx].base)
+  }
+}
+
 runAllTests()

validation-test/stdlib/String.swift

@@ -59,7 +59,7 @@ extension String {
 
 extension Substring {
   var bufferID: ObjectIdentifier? {
-    return _wholeString.bufferID
+    return base.bufferID
   }
 }