Emit duplicate target dependency warnings (swiftlang#1903)

Fixes: https://bugs.swift.org/browse/SR-9402

Author: michaelmcguire

Sources/PackageLoading/Diagnostics.swift

@@ -77,6 +77,20 @@ public enum PackageBuilderDiagnostics {
         public let product: Product
     }
 
+    public struct DuplicateTargetDependencyDiagnostic: DiagnosticData {
+        public static let id = DiagnosticID(
+            type: DuplicateTargetDependencyDiagnostic.self,
+            name: "org.swift.diags.pkg-builder.dup-target-dependency",
+            defaultBehavior: .warning,
+            description: {
+                $0 <<< "invalid duplicate target dependency declaration" <<< { "'\($0.dependency)'" }
+                    <<< "in target" <<< { "'\($0.target)'" }
+        })
+
+        public let dependency: String
+        public let target: String
+    }
+
     struct SystemPackageDeprecatedDiagnostic: DiagnosticData {
         static let id = DiagnosticID(
             type: SystemPackageDeprecatedDiagnostic.self,

Sources/PackageLoading/PackageBuilder.swift

@@ -595,6 +595,12 @@ public final class PackageBuilder {
                 providers: manifestTarget?.providers
             )
         }
+        
+        // Check for duplicate target dependencies by name
+        let combinedDependencyNames = moduleDependencies.map { $0.name } + productDeps.map { $0.0 }
+        combinedDependencyNames.spm_findDuplicates().forEach {
+            diagnostics.emit(data: PackageBuilderDiagnostics.DuplicateTargetDependencyDiagnostic(dependency: $0, target: potentialModule.name))
+        }
 
         // Compute the path to public headers directory.
         let publicHeaderComponent = manifestTarget?.publicHeadersPath ?? ClangTarget.defaultPublicHeadersComponent

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -1576,6 +1576,40 @@ class PackageBuilderTests: XCTestCase {
             result.checkDiagnostic("invalid header search path '../../..'; header search path should not be outside the package root")
         }
     }
+
+    func testDuplicateTargetDependencies() throws {
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/Foo/Sources/Foo/foo.swift",
+            "/Foo/Sources/Foo2/foo.swift",
+            "/Bar/Sources/Bar/bar.swift"
+        )
+
+        let manifest1 = Manifest.createManifest(
+            name: "Foo",
+            v: .v5,
+            dependencies: [
+                PackageDependencyDescription(url: "/Bar", requirement: .upToNextMajor(from: "1.0.0")),
+            ],
+            targets: [
+                TargetDescription(
+                    name: "Foo",
+                    dependencies: [
+                        "Bar",
+                        "Bar",
+                        "Foo2",
+                        "Foo2",
+                    ]),
+                TargetDescription(name: "Foo2"),
+            ]
+        )
+
+        PackageBuilderTester(manifest1, path: AbsolutePath("/Foo"), in: fs) { result in
+            result.checkModule("Foo")
+            result.checkModule("Foo2")
+            result.checkDiagnostic("invalid duplicate target dependency declaration 'Bar' in target 'Foo'")
+            result.checkDiagnostic("invalid duplicate target dependency declaration 'Foo2' in target 'Foo'")
+        }
+    }
 }
 
 extension PackageModel.Product: ObjectIdentifierProtocol {}

Tests/PackageLoadingTests/XCTestManifests.swift

@@ -33,6 +33,7 @@ extension PackageBuilderTests {
         ("testDeclaredExecutableProducts", testDeclaredExecutableProducts),
         ("testDotFilesAreIgnored", testDotFilesAreIgnored),
         ("testDuplicateProducts", testDuplicateProducts),
+        ("testDuplicateTargetDependencies", testDuplicateTargetDependencies),
         ("testDuplicateTargets", testDuplicateTargets),
         ("testExcludes", testExcludes),
         ("testExecutableAsADep", testExecutableAsADep),