[PackageBuilder] Error on invalid manifest config

Sources/PackageLoading/PackageBuilder.swift

@@ -31,6 +31,9 @@ public enum ModuleError: Swift.Error {
 
     /// A module was marked as being dependent on an executable.
     case executableAsDependency(module: String, dependency: String)
+
+    /// The manifest has invalid configuration wrt type of the module.
+    case invalidManifestConfig(String, String)
 }
 
 extension ModuleError: FixableError {
@@ -42,6 +45,8 @@ extension ModuleError: FixableError {
             return "the package has an unsupported layout, \(type.error)"
         case .executableAsDependency(let module, let dependency):
             return "the target \(module) cannot have the executable \(dependency) as a dependency"
+        case .invalidManifestConfig(let package, let message):
+            return "invalid configuration in '\(package)': \(message)"
         }
     }
 
@@ -53,6 +58,8 @@ extension ModuleError: FixableError {
             return type.fix
         case .executableAsDependency(_):
             return "move the shared logic inside a library, which can be referenced from both the target and the executable"
+        case .invalidManifestConfig(_):
+            return nil
         }
     }
 }
@@ -338,6 +345,15 @@ public struct PackageBuilder {
             return [try CModule(name: manifest.name, sources: sources, path: packagePath, pkgConfig: pkgConfigPath, providers: manifest.package.providers)]
         }
 
+        // At this point the module can't be a system module, make sure manifest doesn't contain
+        // system module specific configuration.
+        guard manifest.package.pkgConfig == nil else {
+            throw ModuleError.invalidManifestConfig(manifest.name, "pkgConfig should only be used with a System Module Package")
+        }
+        guard manifest.package.providers == nil else {
+            throw ModuleError.invalidManifestConfig(manifest.name, "providers should only be used with a System Module Package")
+        }
+
         // If everything is excluded, just return an empty array.
         if manifest.package.exclude.contains(".") {
             return []

Tests/PackageLoadingTests/ConventionTests.swift

@@ -840,6 +840,26 @@ class ConventionTests: XCTestCase {
         }
     }
 
+    func testInvalidManifestConfigForNonSystemModules() {
+        var fs = InMemoryFileSystem(emptyFiles:
+            "/Sources/main.swift"
+        )
+        var package = PackageDescription.Package(name: "pkg", pkgConfig: "foo")
+
+        PackageBuilderTester(package, in: fs) { result in
+            result.checkDiagnostic("invalid configuration in 'pkg': pkgConfig should only be used with a System Module Package")
+        }
+
+        fs = InMemoryFileSystem(emptyFiles:
+            "/Sources/Foo/main.c"
+        )
+        package = PackageDescription.Package(name: "pkg", providers: [.Brew("foo")])
+
+        PackageBuilderTester(package, in: fs) { result in
+            result.checkDiagnostic("invalid configuration in 'pkg': providers should only be used with a System Module Package")
+        }
+    }
+
     static var allTests = [
         ("testCInTests", testCInTests),
         ("testDotFilesAreIgnored", testDotFilesAreIgnored),
@@ -873,6 +893,7 @@ class ConventionTests: XCTestCase {
         ("testBadProducts", testBadProducts),
         ("testVersionSpecificManifests", testVersionSpecificManifests),
         ("testTestsProduct", testTestsProduct),
+        ("testInvalidManifestConfigForNonSystemModules", testInvalidManifestConfigForNonSystemModules),
     ]
 }