BoringUtils: tapAndRead of literal throws error

[dtzSiFive]

Type of issue: Bug Report

Please provide the steps to reproduce the problem:

Use BoringUtils.tapAndRead on a literal, example:

//> using repository "sonatype-s01:snapshots"
//> using scala "2.13.14"
//> using dep "org.chipsalliance::chisel:7.0.0-M2+317-62ca9bdc-SNAPSHOT"
//> using plugin "org.chipsalliance:::chisel-plugin:7.0.0-M2+317-62ca9bdc-SNAPSHOT"
//> using options "-unchecked", "-deprecation", "-language:reflectiveCalls", "-feature", "-Xcheckinit", "-Xfatal-warnings", "-Ywarn-dead-code", "-Ywarn-unused", "-Ymacro-annotations"

import chisel3._
import circt.stage.ChiselStage
import chisel3.util.experimental.BoringUtils

class Foo extends RawModule {
  val x = 5.U
  val y = BoringUtils.tapAndRead(x)
}

object Main extends App {
  println(ChiselStage.emitCHIRRTL(new Foo, Array("--full-stacktrace")))
}

What is the current behavior?

Exception in thread "main" java.util.NoSuchElementException: None.get
	at scala.None$.get(Option.scala:627)
	at scala.None$.get(Option.scala:626)
	at chisel3.util.experimental.BoringUtils$.parent$1(BoringUtils.scala:230)
	at chisel3.util.experimental.BoringUtils$.boreOrTap(BoringUtils.scala:328)
	at chisel3.util.experimental.BoringUtils$.$anonfun$tapAndRead$3(BoringUtils.scala:423)
	at chisel3.experimental.skipPrefix$.apply(prefix.scala:102)
	at chisel3.util.experimental.BoringUtils$.$anonfun$tapAndRead$2(BoringUtils.scala:423)
	at chisel3.experimental.prefix$.apply(prefix.scala:50)
	at chisel3.util.experimental.BoringUtils$.$anonfun$tapAndRead$1(BoringUtils.scala:422)
	at chisel3.internal.plugin.package$.autoNameRecursively(package.scala:33)
	at chisel3.util.experimental.BoringUtils$.tapAndRead(BoringUtils.scala:422)
	at Foo.$anonfun$y$2(bore-literal.scala:13)
(snip)

What is the expected behavior?

Ability to use BoringUtils with literals.

Please tell us about your environment:

Other Information

What is the use case for changing the behavior?

Building library code using BoringUtils.

[dtzSiFive]

One strategy is to spill literals to wires and then tapAndRead that.

The wire would likely be created where tapAndRead is invoked, which is probably fine if maybe unexpected?

For example, if you tapAndRead something like val foo = Module(new Foo); val x = BoringUtils.tapAndRead(foo.x), the Wire might be expected to be in Foo not at the location of tapAndRead. Maybe that's fine though, looks a bit like an optimization?

[dtzSiFive]

Alternatively: Require non-literals and add a check with an error message explaining...

Or just pass through the literal? 🤔