[stdlib] name your constants

Author: glessard

stdlib/public/core/SmallString.swift

@@ -93,6 +93,15 @@ extension _SmallString {
 #endif
   }
 
+  @_alwaysEmitIntoClient @inline(__always)
+  internal static func contiguousCapacity() -> Int {
+#if _pointerBitWidth(_32) && os(watchOS)
+    return capacity &- 2
+#else
+    return capacity
+#endif
+  }
+
   // Get an integer equivalent to the _StringObject.discriminatedObjectRawBits
   // computed property.
   @inlinable @inline(__always)

stdlib/public/core/StringUTF8View.swift

@@ -409,7 +409,7 @@ extension String.UTF8View {
   public var _span: Span<UTF8.CodeUnit>? {
     @lifetime(borrow self)
     borrowing get {
-      if _guts.isSmall && _guts.count > (_SmallString.capacity &- 2) {
+      if _guts.isSmall, _guts.count > _SmallString.contiguousCapacity() {
         return nil
       }
       return _underlyingSpan()

stdlib/public/core/Substring.swift

@@ -871,7 +871,8 @@ extension Substring.UTF8View {
   public var _span: Span<UTF8.CodeUnit>? {
     @lifetime(borrow self)
     borrowing get {
-      if _wholeGuts.isSmall && _wholeGuts.count > (_SmallString.capacity &- 2) {
+      if _wholeGuts.isSmall,
+         _wholeGuts.count > _SmallString.contiguousCapacity() {
         // substring is spannable only when the whole string is spannable.
         return nil
       }

stdlib/public/core/UTF8Span.swift

@@ -291,7 +291,7 @@ extension String {
   public var _utf8Span: UTF8Span? {
     @lifetime(borrow self)
     borrowing get {
-      if _guts.isSmall && _guts.count > (_SmallString.capacity &- 2) {
+      if _guts.isSmall, _guts.count > _SmallString.contiguousCapacity() {
         return nil
       }
       return unsafe UTF8Span(
@@ -426,7 +426,8 @@ extension Substring {
   public var _utf8Span: UTF8Span? {
     @lifetime(borrow self)
     borrowing get {
-      if _wholeGuts.isSmall && _wholeGuts.count > (_SmallString.capacity &- 2) {
+      if _wholeGuts.isSmall,
+         _wholeGuts.count > _SmallString.contiguousCapacity() {
         // substring is spannable only when the whole string is spannable.
         return nil
       }