Add tests and incorporate review

Sources/Spezi/Dependencies/DependencyCollection.swift

@@ -24,7 +24,7 @@ public struct DependencyCollection: DependencyDeclaration {
     /// - Parameters:
     ///   - type: The generic type resulting from the passed closure, has to conform to ``Module``.
     ///   - singleEntry: Closure returning a dependency conforming to ``Module``, stored within the ``DependencyCollection``.
-    public init<Dependency: Module>(for type: Dependency.Type = Dependency.self, singleEntry: (() -> Dependency)? = nil) {
+    public init<Dependency: Module>(for type: Dependency.Type = Dependency.self, singleEntry: @escaping (() -> Dependency)) {
         self.init(DependencyContext(for: type, defaultValue: singleEntry))
     }
 

Tests/SpeziTests/DependenciesTests/DependencyBuilderTests.swift

@@ -0,0 +1,47 @@
+//
+// This source file is part of the Stanford Spezi open-source project
+//
+// SPDX-FileCopyrightText: 2023 Stanford University and the project authors (see CONTRIBUTORS.md)
+//
+// SPDX-License-Identifier: MIT
+//
+
+import Spezi
+import XCTRuntimeAssertions
+
+private protocol ExampleTypeConstraint: Module {}
+
+private final class ExampleDependencyModule: ExampleTypeConstraint {}
+
+@resultBuilder
+private enum ExampleDependencyBuilder: DependencyCollectionBuilder {
+    /// An auto-closure expression, providing the default dependency value, building the ``DependencyCollection``.
+    public static func buildExpression<L: ExampleTypeConstraint>(_ expression: @escaping @autoclosure () -> L) -> DependencyCollection {
+        DependencyCollection(singleEntry: expression)
+    }
+}
+
+private class ExampleModule: Module {
+    @Dependency var dependencies: [any Module]
+
+
+    public init(
+        @ExampleDependencyBuilder _ dependencies: () -> DependencyCollection
+    ) {
+        self._dependencies = Dependency(dependencies)
+    }
+}
+
+private class ExampleConfiguration {
+    static let exampleModule = ExampleModule {
+        ExampleDependencyModule()
+    }
+}
+
+
+final class DependencyBuilderTests: XCTestCase {
+    func testDependencyBuilder() throws {
+        XCTAssertEqual(ExampleConfiguration.exampleModule.dependencies.count, 1)
+        _ = try XCTUnwrap(ExampleConfiguration.exampleModule.dependencies[0] as? ExampleDependencyModule)
+    }
+}