ChiselTest Compatibility Layer for Chisel 7

docs/src/appendix/migrating-from-chiseltest.md

@@ -125,3 +125,45 @@ println("`"*3)
 <!-- KEEP THE CODE BLOCKS ABOVE IN SYNC -->
 
 ChiselSim also does not currently have any support for `fork`-`join`, so any tests using those constructs will need to be rewritten in a single-threaded manner.
+
+## ChiselTest Compatibility Layer
+
+For projects with a large test suite, Chisel now includes a **ChiselTest compatibility layer** at `chiseltest._` that provides a drop-in replacement for most ChiselTest APIs. This allows you to keep your existing test code while running on ChiselSim.
+
+### Using the Compatibility Layer
+
+Simply keep your existing imports and test structure:
+
+```scala mdoc
+import chisel3._
+import chiseltest._
+import org.scalatest.flatspec.AnyFlatSpec
+
+class MyModuleSpec extends AnyFlatSpec with ChiselScalatestTester {
+  behavior of "MyModule"
+  it should "do something" in {
+    test(new MyModule) { c =>
+      c.io.in.poke(42.U)
+      c.clock.step()
+      c.io.out.expect(42.U)
+    }
+  }
+}
+```
+
+The compatibility layer provides:
+- `poke`, `peek`, `expect` methods on Data types
+- `step()` methods on Clock
+- `ChiselScalatestTester` trait for ScalaTest integration
+- Automatic reset before each test (mimicking ChiselTest behavior)
+- Decoupled interface utilities (`enqueueNow`, `expectDequeueNow`, etc.)
+- Annotation stubs (`WriteVcdAnnotation`, etc.) for compatibility
+
+### Limitations
+
+- Annotations are ignored (waveforms always generated with Verilator)
+- Formal verification support is limited to stubs
+- `fork`/`join` patterns execute sequentially
+- Some advanced features may require migration to ChiselSim APIs
+
+For full details, see `src/main/scala/chiseltest/README.md` in the Chisel repository.

src/main/scala/chiseltest/ChiselScalatestTester.scala

@@ -0,0 +1,191 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chiseltest
+
+import chisel3._
+import chisel3.simulator.EphemeralSimulator._
+import chisel3.simulator.ChiselSim
+import svsim._
+import svsim.verilator.Backend.CompilationSettings.{TraceKind, TraceStyle}
+import scala.language.implicitConversions
+
+/**
+ * ChiselTest-compatible API that delegates to ChiselSim (Chisel 7)
+ *
+ * This trait provides the ChiselTest API that users are familiar with from Chisel 6,
+ * but internally uses ChiselSim from Chisel 7 to perform the actual testing.
+ *
+ * VCD GENERATION SUPPORT:
+ * This compatibility layer supports VCD generation when WriteVcdAnnotation is used.
+ * VCD files are generated in: build/chiselsim/<timestamp>/workdir-verilator/trace.vcd
+ *
+ * AUTOMATIC RESET FEATURE:
+ * By default, this trait automatically resets the module before running tests,
+ * mimicking ChiselTest's behavior from Chisel 6.
+ *
+ * To disable auto-reset or customize the reset duration:
+ * {{{
+ * class MyTest extends AnyFlatSpec with ChiselScalatestTester {
+ *   override def autoResetEnabled: Boolean = false  // Disable auto-reset
+ *   override def resetCycles: Int = 5               // Or change duration
+ * }
+ * }}}
+ *
+ * Example usage with VCD:
+ * {{{
+ * import chiseltest._
+ * import org.scalatest.flatspec.AnyFlatSpec
+ *
+ * class MyModuleSpec extends AnyFlatSpec with ChiselScalatestTester {
+ *   behavior of "MyModule"
+ *
+ *   it should "generate waveforms" in {
+ *     test(new MyModule).withAnnotations(Seq(WriteVcdAnnotation)) { dut =>
+ *       dut.io.in.poke(42.U)
+ *       dut.clock.step()
+ *       dut.io.out.expect(42.U)
+ *     }
+ *     // VCD file: build/chiselsim/<timestamp>/workdir-verilator/trace.vcd
+ *   }
+ * }
+ * }}}
+ */
+trait ChiselScalatestTester {
+
+  /**
+   * Enable automatic reset before each test.
+   * Override this to disable auto-reset if needed.
+   */
+  def autoResetEnabled: Boolean = false
+
+  /**
+   * Number of clock cycles to assert reset.
+   * Override this to change reset duration.
+   */
+  def resetCycles: Int = 1
+
+  /**
+   * Test a module with the given stimulus
+   *
+   * This method provides the ChiselTest interface.
+   *
+   * @param dutGen A generator function that creates the device under test
+   * @tparam T The type of module being tested
+   */
+  def test[T <: Module](dutGen: => T): TestBuilder[T] =
+    new TestBuilder(dutGen, autoResetEnabled, resetCycles)
+
+  /**
+   * Builder class to support .withAnnotations() chaining
+   *
+   * @param dutGen Generator for the device under test
+   * @param autoReset Whether automatic reset is enabled
+   * @param resetCyc Number of reset cycles to apply
+   */
+  class TestBuilder[T <: Module](dutGen: => T, autoReset: Boolean, resetCyc: Int) {
+
+    /**
+     * Attach ChiselTest-style annotations before running the test.
+     *
+     * @param annotations Annotation list (for example WriteVcdAnnotation)
+     * @return A runner that executes the test with the provided annotations
+     */
+    def withAnnotations(annotations: Seq[Any]): TestRunner[T] = {
+      new TestRunner(dutGen, autoReset, resetCyc, annotations)
+    }
+
+    /**
+     * Run the test body without explicit annotations.
+     *
+     * @param body User-defined test body operating on the DUT instance
+     */
+    // Allow direct execution without annotations
+    def apply(body: T => Unit): Unit = {
+      val chiselSim = new ChiselSim {}
+      chiselSim.simulate(dutGen) { dut =>
+        if (autoReset) {
+          applyReset(dut, resetCyc)
+        }
+        body(dut)
+      }
+    }
+  }
+
+  /**
+   * Runner class that executes the test with VCD support
+   *
+   * @param dutGen Generator for the device under test
+   * @param autoReset Whether automatic reset is enabled
+   * @param resetCyc Number of reset cycles to apply
+   * @param annotations Annotation list used to configure execution behavior
+   */
+  class TestRunner[T <: Module](dutGen: => T, autoReset: Boolean, resetCyc: Int, annotations: Seq[Any] = Seq()) {
+
+    /**
+     * Execute the configured test run.
+     *
+     * @param body User-defined test body operating on the DUT instance
+     */
+    def apply(body: T => Unit): Unit = {
+      // Check if WriteVcdAnnotation is present
+      val hasVcd = annotations.exists {
+        case _: chiseltest.WriteVcdAnnotation.type => true
+        case _ => false
+      }
+
+      val chiselSim = new ChiselSim {}
+
+      if (hasVcd) {
+        println("[ChiselTest Compat] WriteVcdAnnotation detected, enabling VCD trace generation...")
+
+        // Create backend modification to enable VCD tracing
+        implicit val backendMod: BackendSettingsModifications =
+          (settings: Backend.Settings) =>
+            settings match {
+              case vs: verilator.Backend.CompilationSettings =>
+                val vcdStyle = TraceStyle(
+                  kind = TraceKind.Vcd,
+                  traceUnderscore = false,
+                  traceStructs = true,
+                  traceParams = true,
+                  maxWidth = None,
+                  maxArraySize = None,
+                  traceDepth = None
+                )
+                vs.withTraceStyle(Some(vcdStyle))
+              case other => other
+            }
+
+        // Simulate with VCD enabled
+        chiselSim.simulate(dutGen) { dut =>
+          chiselSim.enableWaves()
+
+          if (autoReset) {
+            applyReset(dut, resetCyc)
+          }
+          body(dut)
+        }
+      } else {
+        // Simulate without VCD
+        chiselSim.simulate(dutGen) { dut =>
+          if (autoReset) {
+            applyReset(dut, resetCyc)
+          }
+          body(dut)
+        }
+      }
+    }
+  }
+
+  /**
+   * Apply reset sequence to the DUT
+   *
+   * @param dut Device under test
+   * @param cycles Number of cycles reset stays asserted
+   */
+  private def applyReset[T <: Module](dut: T, cycles: Int): Unit = {
+    toTestableReset(dut.reset).poke(true.B)
+    toTestableClock(dut.clock).step(cycles)
+    toTestableReset(dut.reset).poke(false.B)
+  }
+}

src/main/scala/chiseltest/DecoupledDriver.scala

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chiseltest
+
+/**
+ * Compatibility placeholder for ChiselTest's DecoupledDriver class.
+ *
+ * Decoupled enqueue/dequeue helpers are provided via implicit extensions in
+ * `package object chiseltest`. Use `import chiseltest._` to access:
+ * - `enqueueNow`, `enqueueSeq`
+ * - `expectDequeueNow`, `expectDequeueSeq`
+ * - `initSource`, `initSink`
+ */
+final class DecoupledDriver