[stdlib] Handle String's 32-bit inline representation for Span

[glessard]

String's inline representation is currently non-contiguous on 32-bit platforms. Furthermore, that representation is baked in ABI for watchOS's 32-bit variant. This PR implements mitigations.

A. For 32-bit platforms without ABI stability, we reduce the size of the inline representation to 8 UTF-8 code units (from 10).

B. For 32-bit watchOS, we add an optional property var _span: Span<UTF8.CodeUnit>? that returns nil when the representation of the String is non-contiguous, that is when it is 9 or 10 UTF-8 code units.

All platforms will get this underscored property, so that code that supports 32-bit watchOS doesn't have to be specific to it.

[glessard]

@swift-ci please test

[lorentey]

Nice! (But also: ugh 😣)

[lorentey]

Do we have platforms with 16-bit pointers? 😮

[glessard]

They're not impossible!

[glessard]

@swift-ci please test

[glessard]

@swift-ci please test

[glessard]

@swift-ci please test linux platform

[atrick]

This patch works for me... The getter's 'self' is non-copyable (no-implicit-copy). I'm not actually sure how to avoid that.
At any rate, non-copyable values can't be captured, and boolean operators capture their operands...

diff --git a/stdlib/public/core/Substring.swift b/stdlib/public/core/Substring.swift
index ae55c6fd9a0..b9dc2299827 100644
--- a/stdlib/public/core/Substring.swift
+++ b/stdlib/public/core/Substring.swift
@@ -871,10 +871,13 @@ extension Substring.UTF8View {
   public var _span: Span<UTF8.CodeUnit>? {
     @lifetime(borrow self)
     borrowing get {
-      if _wholeGuts.isSmall &&
-         _wholeGuts.count > _SmallString.contiguousCapacity() {
-        // substring is spannable only when the whole string is spannable.
-        return nil
+      let isSmall = _wholeGuts.isSmall
+      if isSmall {
+        let count = _wholeGuts.count
+        if count > _SmallString.contiguousCapacity() {
+          // substring is spannable only when the whole string is spannable.
+          return nil
+        }
       }
       return _underlyingSpan()
     }
diff --git a/stdlib/public/core/UTF8Span.swift b/stdlib/public/core/UTF8Span.swift
index a7f5aaeb5d3..99a8effb0bb 100644
--- a/stdlib/public/core/UTF8Span.swift
+++ b/stdlib/public/core/UTF8Span.swift
@@ -426,10 +426,13 @@ extension Substring {
   public var _utf8Span: UTF8Span? {
     @lifetime(borrow self)
     borrowing get {
-      if _wholeGuts.isSmall &&
-          _wholeGuts.count > _SmallString.contiguousCapacity() {
-        // substring is spannable only when the whole string is spannable.
-        return nil
+      let isSmall = _wholeGuts.isSmall
+      if isSmall {
+        let count = _wholeGuts.count
+        if count > _SmallString.contiguousCapacity() {
+          // substring is spannable only when the whole string is spannable.
+          return nil
+        }
       }
       return unsafe UTF8Span(
         unchecked: _underlyingSpan(), isKnownASCII: base._guts.isASCII

[atrick]

Alternatively, remove the ‘borrowing’ keyword from the _span getters:

diff --git a/stdlib/public/core/Substring.swift b/stdlib/public/core/Substring.swift
index ae55c6fd9a0..afae49f9f2f 100644
--- a/stdlib/public/core/Substring.swift
+++ b/stdlib/public/core/Substring.swift
@@ -870,7 +870,7 @@ extension Substring.UTF8View {
   @available(SwiftStdlib 6.2, *)
   public var _span: Span<UTF8.CodeUnit>? {
     @lifetime(borrow self)
-    borrowing get {
+    get {
       if _wholeGuts.isSmall &&
          _wholeGuts.count > _SmallString.contiguousCapacity() {
         // substring is spannable only when the whole string is spannable.
diff --git a/stdlib/public/core/UTF8Span.swift b/stdlib/public/core/UTF8Span.swift
index a7f5aaeb5d3..e567e1c274a 100644
--- a/stdlib/public/core/UTF8Span.swift
+++ b/stdlib/public/core/UTF8Span.swift
@@ -425,7 +425,7 @@ extension Substring {
   @available(SwiftStdlib 6.2, *)
   public var _utf8Span: UTF8Span? {
     @lifetime(borrow self)
-    borrowing get {
+    get {
       if _wholeGuts.isSmall &&
           _wholeGuts.count > _SmallString.contiguousCapacity() {
         // substring is spannable only when the whole string is spannable.

[glessard]

I circumvented the autoclosure by using the condition-list syntax, rather than combining expressions with &&. Thanks for the reminder @atrick!

[glessard]

@swift-ci please clean test