Revert "[temporary] disable small-string support"

This reverts commit ea44ff9.

Author: glessard

stdlib/public/core/StringUTF8View.swift

@@ -89,6 +89,7 @@ extension String {
   ///     print(String(s1.utf8.prefix(15))!)
   ///     // Prints "They call me 'B"
   @frozen
+  @_addressableForDependencies
   public struct UTF8View: Sendable {
     @usableFromInline
     internal var _guts: _StringGuts
@@ -345,15 +346,14 @@ extension String.UTF8View {
 #endif
       let count = _guts.count
       if _guts.isSmall {
-        fatalError("Span over the small string form is not supported yet.")
-//        let a = Builtin.addressOfBorrow(self)
-//        let address = unsafe UnsafePointer<UTF8.CodeUnit>(a)
-//        let span = unsafe Span(_unsafeStart: address, count: count)
-//        return unsafe _overrideLifetime(span, borrowing: self)
+        let a = Builtin.addressOfBorrow(self)
+        let address = unsafe UnsafePointer<UTF8.CodeUnit>(a)
+        let span = unsafe Span(_unsafeStart: address, count: count)
+        return unsafe _overrideLifetime(span, borrowing: self)
       }
       _precondition(_guts.isFastUTF8)
       let buffer = unsafe _guts._object.fastUTF8
-      _internalInvariant(count == buffer.count)
+      _precondition(count == buffer.count)
       let span = unsafe Span(_unsafeElements: buffer)
       return unsafe _overrideLifetime(span, borrowing: self)
     }

stdlib/public/core/Substring.swift

@@ -630,6 +630,7 @@ extension Substring: LosslessStringConvertible {
 
 extension Substring {
   @frozen
+  @_addressableForDependencies
   public struct UTF8View: Sendable {
     @usableFromInline
     internal var _slice: Slice<String.UTF8View>
@@ -789,14 +790,13 @@ extension Substring.UTF8View {
       let first = _slice._startIndex._encodedOffset
       let end = _slice._endIndex._encodedOffset
       if _wholeGuts.isSmall {
-        fatalError("Span over the small string form is not supported yet.")
-//        let a = Builtin.addressOfBorrow(self)
-//        let offset = first &+ (2 &* MemoryLayout<String.Index>.stride)
-//        let start = unsafe UnsafePointer<UTF8.CodeUnit>(a).advanced(by: offset)
-//        let span = unsafe Span(_unsafeStart: start, count: end &- first)
-//        return unsafe _overrideLifetime(span, borrowing: self)
+        let a = Builtin.addressOfBorrow(self)
+        let offset = first &+ (2 &* MemoryLayout<String.Index>.stride)
+        let start = unsafe UnsafePointer<UTF8.CodeUnit>(a).advanced(by: offset)
+        let span = unsafe Span(_unsafeStart: start, count: end &- first)
+        return unsafe _overrideLifetime(span, borrowing: self)
       }
-      _internalInvariant(_wholeGuts.isFastUTF8)
+      _precondition(_wholeGuts.isFastUTF8)
       var span = unsafe Span(_unsafeElements: _wholeGuts._object.fastUTF8)
       span = span._extracting(first..<end)
       return unsafe _overrideLifetime(span, borrowing: self)

test/stdlib/Span/StringUTF8SpanProperty.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// RUN: %target-run-stdlib-swift(-enable-experimental-feature LifetimeDependence -enable-experimental-feature AddressableTypes)
+// RUN: %target-run-stdlib-swift(-enable-experimental-feature LifetimeDependence -enable-experimental-feature Span -enable-experimental-feature AddressableTypes)
 
 // REQUIRES: executable_test
 // REQUIRES: swift_feature_LifetimeDependence
@@ -22,13 +22,11 @@ var suite = TestSuite("StringUTF8StorageProperty")
 defer { runAllTests() }
 
 suite.test("Span from Small String")
-.skip(.wasiAny(reason: "Trap tests aren't supported on WASI."))
 .require(.stdlib_6_2).code {
   guard #available(SwiftStdlib 6.2, *) else { return }
 
   let s = "A small string.".utf8
   let u = Array(s)
-  expectCrashLater()
   let span = s.span
 
   let count = span.count
@@ -56,13 +54,11 @@ suite.test("Span from Large Native String")
 }
 
 suite.test("Span from Small String's Substring")
-.skip(.wasiAny(reason: "Trap tests aren't supported on WASI."))
 .require(.stdlib_6_2).code {
   guard #available(SwiftStdlib 6.2, *) else { return }
 
   let s = "A small string.".dropFirst(8).utf8
   let u = Array("string.".utf8)
-  expectCrashLater()
   let span = s.span
 
   let count = span.count