Fix issue with older compilers not handling MutableSpan code

The use of SendableCompletionHandlers here is completely incidental,
because it happened to be introduced at roughly the same time as the
bits we needed for MutableSpan. Fixes rdar://148072153.

Author: DougGregor

stdlib/public/core/Span/MutableRawSpan.swift

@@ -588,10 +588,14 @@ extension MutableRawSpan {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(first maxLength: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(maxLength >= 0, "Can't have a prefix of negative length")
     let newCount = min(maxLength, byteCount)
     let newSpan = unsafe Self(_unchecked: _pointer, byteCount: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 
   /// Returns a span over all but the given number of trailing elements.
@@ -611,11 +615,15 @@ extension MutableRawSpan {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(droppingLast k: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(k >= 0, "Can't drop a negative number of elements")
     let droppedCount = min(k, byteCount)
     let newCount = byteCount &- droppedCount
     let newSpan = unsafe Self(_unchecked: _pointer, byteCount: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 
   /// Returns a span containing the final elements of the span,
@@ -660,11 +668,15 @@ extension MutableRawSpan {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(droppingFirst k: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(k >= 0, "Can't drop a negative number of bytes")
     let droppedCount = min(k, byteCount)
     let newStart = unsafe _pointer?.advanced(by: droppedCount)
     let newCount = byteCount &- droppedCount
     let newSpan = unsafe Self(_unchecked: newStart, byteCount: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 }

stdlib/public/core/Span/MutableSpan.swift

@@ -825,10 +825,14 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(first maxLength: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(maxLength >= 0, "Can't have a prefix of negative length")
     let newCount = min(maxLength, count)
     let newSpan = unsafe Self(_unchecked: _pointer, count: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 
   /// Returns a span over all but the given number of trailing elements.
@@ -848,11 +852,15 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(droppingLast k: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(k >= 0, "Can't drop a negative number of elements")
     let droppedCount = min(k, count)
     let newCount = count &- droppedCount
     let newSpan = unsafe Self(_unchecked: _pointer, count: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 
   /// Returns a span containing the final elements of the span,
@@ -873,12 +881,16 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(last maxLength: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(maxLength >= 0, "Can't have a suffix of negative length")
     let newCount = min(maxLength, count)
     let offset = (count &- newCount) * MemoryLayout<Element>.stride
     let newStart = unsafe _pointer?.advanced(by: offset)
     let newSpan = unsafe Self(_unchecked: newStart, count: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 
   /// Returns a span over all but the given number of initial elements.
@@ -898,12 +910,16 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(droppingFirst k: Int) -> Self {
+#if compiler(>=5.3) && hasFeature(SendableCompletionHandlers)
     _precondition(k >= 0, "Can't drop a negative number of elements")
     let droppedCount = min(k, count)
     let offset = droppedCount * MemoryLayout<Element>.stride
     let newStart = unsafe _pointer?.advanced(by: offset)
     let newCount = count &- droppedCount
     let newSpan = unsafe Self(_unchecked: newStart, count: newCount)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
+#else
+    fatalError("Unsupported compiler")
+#endif
   }
 }