Avoid an Unnecessary Buffer Copy

Author: CodaFi

Sources/SwiftDriver/IncrementalCompilation/SourceFileDependencyGraph.swift

@@ -251,7 +251,14 @@ extension SourceFileDependencyGraph {
     }
 
     var visitor = Visitor(extractFromSwiftModule: extractFromSwiftModule)
-    try Bitcode.read(stream: Data(data.contents), using: &visitor)
+    try data.contents.withUnsafeBytes { buf in
+      // SAFETY: The bitcode reader does not mutate the data stream we give it.
+      // FIXME: Let's avoid this altogether and traffic in ByteString/[UInt8]
+      // if possible. There's no real reason to use `Data` in this API.
+      let baseAddr = UnsafeMutableRawPointer(mutating: buf.baseAddress!)
+      let data = Data(bytesNoCopy: baseAddr, count: buf.count, deallocator: .none)
+      try Bitcode.read(stream: data, using: &visitor)
+    }
     guard let major = visitor.majorVersion,
           let minor = visitor.minorVersion,
           let versionString = visitor.compilerVersionString else {

Tests/SwiftDriverTests/ModuleDependencyGraphTests.swift

@@ -1034,7 +1034,7 @@ fileprivate struct SourceFileDependencyGraphMocker {
 
   private mutating func mock() -> SourceFileDependencyGraph {
     buildNodes()
-    return SourceFileDependencyGraph(nodesForTesting: allNodes )
+    return SourceFileDependencyGraph(nodesForTesting: allNodes)
   }
 
   private mutating func buildNodes() {