Merge pull request #83775 from glessard/rdar158440246-62

[stdlib, 6.2.x] guard against deinitializing empty OutputSpan instances

stdlib/public/core/Span/OutputSpan.swift

@@ -315,6 +315,7 @@ extension OutputSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   @lifetime(self: copy self)
   public mutating func removeAll() {
+    guard count > 0 else { return }
     _ = unsafe _start().withMemoryRebound(to: Element.self, capacity: _count) {
       unsafe $0.deinitialize(count: _count)
     }

test/stdlib/Span/OutputSpanTests.swift

@@ -35,7 +35,9 @@ struct Allocation<T>: ~Copyable {
     _ body: (inout OutputSpan<T>) throws(E) -> Void
   ) throws(E) {
     if count != nil { fatalError() }
-    var outputBuffer = OutputSpan<T>(buffer: allocation, initializedCount: 0)
+    var allocation = allocation
+    if allocation.count == 0 { allocation = .init(start: nil, count: 0) }
+    var outputBuffer = OutputSpan(buffer: allocation, initializedCount: 0)
     do {
       try body(&outputBuffer)
       let initialized = outputBuffer.finalize(for: allocation)
@@ -188,6 +190,20 @@ suite.test("deinitialize buffer")
   catch {
     expectTrue(false)
   }
+
+  a = Allocation(of: 0, Int.self)
+  do {
+    try a.initialize {
+      expectEqual($0.freeCapacity, 0)
+      throw MyTestError.error
+    }
+  }
+  catch MyTestError.error {
+    expectEqual(a.isEmpty, true)
+  }
+  catch {
+    expectTrue(false)
+  }
 }
 
 suite.test("InlineArray initialization")