Add toString method to BitPat (#1819)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

src/main/scala/chisel3/util/BitPat.scala

@@ -119,4 +119,11 @@ sealed class BitPat(val value: BigInt, val mask: BigInt, width: Int) extends Sou
     !(this === that)
   }
 
+  override def toString = {
+    "BitPat(" +
+      (0 until width).map(i =>
+        if (((mask >> i) & 1) == 1) if (((value >> i) & 1) == 1)  "1" else "0" else "?"
+      ).reverse.reduce(_ + _) +
+    ")"
+  }
 }

src/test/scala/chiselTests/util/BitPatSpec.scala

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chiselTests.util
+
+import chisel3.util.BitPat
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+
+class BitPatSpec extends AnyFlatSpec with Matchers {
+  behavior of classOf[BitPat].toString
+
+  it should "convert a BitPat to readable form" in {
+    val testPattern = "0" * 32 + "1" * 32 + "?" * 32 + "?01" * 32
+    BitPat("b" + testPattern).toString should be (s"BitPat($testPattern)")
+  }
+}