Step 1: Isolate the map to be replaced

Author: David Ungar

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift

@@ -24,19 +24,7 @@ import SwiftOptions
   @_spi(Testing) public var nodeFinder = NodeFinder()
 
   /// Maps input files (e.g. .swift) to and from the DependencySource object.
-  ///
-  // FIXME: The map between swiftdeps and swift files is absolutely *not*
-  // a bijection. In particular, more than one swiftdeps file can be encountered
-  // in the course of deserializing priors *and* reading the output file map
-  // *and* re-reading swiftdeps files after frontends complete
-  // that correspond to the same swift file. These cause two problems:
-  // - overwrites in this data structure that lose data and
-  // - cache misses in `getInput(for:)` that cause the incremental build to
-  // turn over when e.g. entries in the output file map change. This should be
-  // replaced by a multi-map from swift files to dependency sources,
-  // and a regular map from dependency sources to swift files -
-  // since that direction really is one-to-one.
-  @_spi(Testing) public private(set) var inputDependencySourceMap = BidirectionalMap<TypedVirtualPath, DependencySource>()
+  @_spi(Testing) public private(set) var inputDependencySourceMap = InputDependencySourceMap()
 
   // The set of paths to external dependencies known to be in the graph
   public internal(set) var fingerprintedExternalDependencies = Set<FingerprintedExternalDependency>()
@@ -1111,7 +1099,7 @@ extension Set where Element == ModuleDependencyGraph.Node {
   }
 }
 
-extension BidirectionalMap where T1 == TypedVirtualPath, T2 == DependencySource {
+extension InputDependencySourceMap {
   fileprivate func matches(_ other: Self) -> Bool {
     self == other
   }

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraphParts/InputDependencySourceMap.swift

@@ -0,0 +1,46 @@
+//===------------- InputDependencySourceMap.swift ---------------- --------===//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+import Foundation
+
+@_spi(Testing) public struct InputDependencySourceMap: Equatable, Sequence {
+  
+  /// Maps input files (e.g. .swift) to and from the DependencySource object.
+  ///
+  // FIXME: The map between swiftdeps and swift files is absolutely *not*
+  // a bijection. In particular, more than one swiftdeps file can be encountered
+  // in the course of deserializing priors *and* reading the output file map
+  // *and* re-reading swiftdeps files after frontends complete
+  // that correspond to the same swift file. These cause two problems:
+  // - overwrites in this data structure that lose data and
+  // - cache misses in `getInput(for:)` that cause the incremental build to
+  // turn over when e.g. entries in the output file map change. This should be
+  // replaced by a multi-map from swift files to dependency sources,
+  // and a regular map from dependency sources to swift files -
+  // since that direction really is one-to-one.
+  
+  public typealias BiMap = BidirectionalMap<TypedVirtualPath, DependencySource>
+  @_spi(Testing) public var biMap = BiMap()
+  
+  @_spi(Testing) public subscript(input: TypedVirtualPath) -> DependencySource? {
+    get { biMap[input] }
+    set { biMap[input] = newValue }
+  }
+  
+  @_spi(Testing) public private(set) subscript(dependencySource: DependencySource) -> TypedVirtualPath? {
+    get { biMap[dependencySource] }
+    set { biMap[dependencySource] = newValue }
+  }
+
+  public func makeIterator() -> BiMap.Iterator {
+    biMap.makeIterator()
+  }
+
+}