Improve Resolver Determinism (#3862)

* Added test.

* Implemented fix.

Author: SDGGiesbrecht

Sources/PackageGraph/PackageGraph+Loading.swift

@@ -230,7 +230,7 @@ private func createResolvedPackages(
             metadata: package.diagnosticsMetadata
         )
 
-        var dependencies = [PackageIdentity: ResolvedPackageBuilder]()
+        var dependencies = OrderedDictionary<PackageIdentity, ResolvedPackageBuilder>()
         var dependenciesByNameForTargetDependencyResolution = [String: ResolvedPackageBuilder]()
 
         // Establish the manifest-declared package dependencies.
@@ -257,7 +257,7 @@ private func createResolvedPackages(
                 // check if this resolved package already listed in the dependencies
                 // this means that the dependencies share the same identity
                 // FIXME: this works but the way we find out about this is based on a side effect, need to improve it
-                guard !dependencies.keys.contains(resolvedPackage.package.identity) else {
+                guard dependencies[resolvedPackage.package.identity] == nil else {
                     let error = PackageGraphError.dependencyAlreadySatisfiedByIdentifier(
                         package: package.identity.description,
                         dependencyLocation: dependencyLocation,

Tests/PackageGraphTests/PackageGraphTests.swift

@@ -1269,6 +1269,98 @@ class PackageGraphTests: XCTestCase {
         })
     }
 
+    func testResolutionDeterminism() throws {
+        let fileSystem = InMemoryFileSystem(
+            emptyFiles: [
+                "/A/Sources/A/A.swift",
+                "/B/Sources/B/B.swift",
+                "/C/Sources/C/C.swift",
+                "/D/Sources/D/D.swift",
+                "/E/Sources/E/E.swift",
+                "/F/Sources/F/F.swift",
+            ]
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+        _ = try loadPackageGraph(
+            fs: fileSystem,
+            manifests: [
+                Manifest.createRootManifest(
+                    name: "A",
+                    path: .init("/A"),
+                    dependencies: [
+                        .localSourceControl(path: .init("/B"), requirement: .upToNextMajor(from: "1.0.0")),
+                        .localSourceControl(path: .init("/C"), requirement: .upToNextMajor(from: "1.0.0")),
+                        .localSourceControl(path: .init("/D"), requirement: .upToNextMajor(from: "1.0.0")),
+                        .localSourceControl(path: .init("/E"), requirement: .upToNextMajor(from: "1.0.0")),
+                        .localSourceControl(path: .init("/F"), requirement: .upToNextMajor(from: "1.0.0")),
+                    ],
+                    targets: [
+                        TargetDescription(name: "A", dependencies: []),
+                    ]),
+                Manifest.createFileSystemManifest(
+                    name: "B",
+                    path: .init("/B"),
+                    products: [
+                        ProductDescription(name: "B", type: .library(.automatic), targets: ["B"])
+                    ],
+                    targets: [
+                        TargetDescription(name: "B"),
+                    ]
+                ),
+                Manifest.createFileSystemManifest(
+                    name: "C",
+                    path: .init("/C"),
+                    products: [
+                        ProductDescription(name: "C", type: .library(.automatic), targets: ["C"])
+                    ],
+                    targets: [
+                        TargetDescription(name: "C"),
+                    ]
+                ),
+                Manifest.createFileSystemManifest(
+                    name: "D",
+                    path: .init("/D"),
+                    products: [
+                        ProductDescription(name: "D", type: .library(.automatic), targets: ["D"])
+                    ],
+                    targets: [
+                        TargetDescription(name: "D"),
+                    ]
+                ),
+                Manifest.createFileSystemManifest(
+                    name: "E",
+                    path: .init("/E"),
+                    products: [
+                        ProductDescription(name: "E", type: .library(.automatic), targets: ["E"])
+                    ],
+                    targets: [
+                        TargetDescription(name: "E"),
+                    ]
+                ),
+                Manifest.createFileSystemManifest(
+                    name: "F",
+                    path: .init("/F"),
+                    products: [
+                        ProductDescription(name: "F", type: .library(.automatic), targets: ["F"])
+                    ],
+                    targets: [
+                        TargetDescription(name: "F"),
+                    ]
+                ),
+            ],
+            observabilityScope: observability.topScope
+        )
+
+        testDiagnostics(observability.diagnostics) { result in
+            result.check(diagnostic: "dependency 'b' is not used by any target", severity: .warning)
+            result.check(diagnostic: "dependency 'c' is not used by any target", severity: .warning)
+            result.check(diagnostic: "dependency 'd' is not used by any target", severity: .warning)
+            result.check(diagnostic: "dependency 'e' is not used by any target", severity: .warning)
+            result.check(diagnostic: "dependency 'f' is not used by any target", severity: .warning)
+        }
+    }
+
     func testTargetDependencies_Pre52() throws {
         let fs = InMemoryFileSystem(emptyFiles:
             "/Foo/Sources/Foo/foo.swift",