ir: make HashCode.str public

This will allow chiseltest to save
the hash code to disk for the purpose
of caching simulation binaries.

src/main/scala/firrtl/ir/StructuralHash.scala

@@ -115,7 +115,9 @@ object StructuralHash {
 }
 
 trait HashCode {
-  protected val str: String
+
+  /** string representation of the hash code */
+  def str: String
   override def hashCode(): Int = str.hashCode
   override def equals(obj: Any): Boolean = obj match {
     case hashCode: HashCode => this.str.equals(hashCode.str)
@@ -124,7 +126,7 @@ trait HashCode {
 }
 
 private class MDHashCode(code: Array[Byte]) extends HashCode {
-  protected override val str: String = code.map(b => f"${b.toInt & 0xff}%02x").mkString("")
+  override val str: String = code.map(b => f"${b.toInt & 0xff}%02x").mkString("")
 }
 
 /** Generic hashing interface which allows us to use different backends to trade of speed and collision resistance */

src/test/scala/firrtl/ir/StructuralHashSpec.scala

@@ -38,6 +38,16 @@ class StructuralHashSpec extends AnyFlatSpec {
     assert(hash(b1) != hash(UIntLiteral(1, IntWidth(2))))
   }
 
+  it should "generate the same hash String if the objects are structurally the same" in {
+    assert(hash(b0).str == hash(UIntLiteral(0, IntWidth(1))).str)
+    assert(hash(b0).str != hash(UIntLiteral(1, IntWidth(1))).str)
+    assert(hash(b0).str != hash(UIntLiteral(1, IntWidth(2))).str)
+
+    assert(hash(b1).str == hash(UIntLiteral(1, IntWidth(1))).str)
+    assert(hash(b1).str != hash(UIntLiteral(0, IntWidth(1))).str)
+    assert(hash(b1).str != hash(UIntLiteral(1, IntWidth(2))).str)
+  }
+
   it should "ignore expression types" in {
     assert(hash(add) == hash(DoPrim(Add, Seq(b0, b1), Seq(), UnknownType)))
     assert(hash(add) == hash(DoPrim(Add, Seq(b0, b1), Seq(), UIntType(UnknownWidth))))