Require target is hardware for Vec.apply(a: UInt)

[seldridge]

Adds a check that a Vec being indexed by a UInt is, in fact, a
hardware type. This includes a test for this.

h/t @schoeberl
cc: @aswaterman

Consider the following where a Chisel type Vec is indexed by a hardware type:

  property("Indexing a Chisel type Vec(Vec) should give a sane error message") {
    assertThrows[ExpectedHardwareException] {
      elaborate{
        new Module {
          val io = IO(new Bundle{})
          val foo = Vec(2, Bool())
          foo(0.U) := false.B
        }}
    }
  }

On master, this will generate:

[info] - Indexing a Chisel type Vec(Vec) should give a sane error message *** FAILED ***
[info]   java.util.NoSuchElementException: None.get
[info]   at scala.None$.get(Option.scala:349)
[info]   at scala.None$.get(Option.scala:347)
[info]   at chisel3.Data.direction(Data.scala:343)
[info]   at chisel3.Vec.do_apply(Aggregate.scala:224)
[info]   at chiselTests.VecSpec$$anon$15.<init>(Vec.scala:281)
[info]   at chiselTests.VecSpec.$anonfun$new$64(Vec.scala:278)
[info]   at chisel3.Module$.do_apply(Module.scala:53)
[info]   at chisel3.Driver$.$anonfun$elaborate$1(Driver.scala:94)
[info]   at chisel3.internal.Builder$.$anonfun$build$2(Builder.scala:352)
[info]   at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
[info]   ...

This adds a check that the underlying Vec is a hardware type to prodcue:

[info] - Indexing a Chisel type Vec(Vec) should give a sane error message *** FAILED ***
[info]   chisel3.package$ExpectedHardwareException: vec 'Bool[2]' must be hardware, not a bare Chisel type. Perhaps you forgot to wrap it in Wire(_) or IO(_)?
[info]   at chisel3.internal.requireIsHardware$.apply(Binding.scala:20)
[info]   at chisel3.Vec.do_apply(Aggregate.scala:219)
[info]   at chiselTests.VecSpec$$anon$16.<init>(Vec.scala:281)
[info]   at chiselTests.VecSpec.$anonfun$new$64(Vec.scala:278)
[info]   at chisel3.Module$.do_apply(Module.scala:53)
[info]   at chisel3.Driver$.$anonfun$elaborate$1(Driver.scala:94)
[info]   at chisel3.internal.Builder$.$anonfun$build$2(Builder.scala:352)
[info]   at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
[info]   at chisel3.internal.Builder$.$anonfun$build$1(Builder.scala:350)
[info]   at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
[info]   ...

Related issue:

Type of change: other enhancement

Impact: no functional change

Development Phase: implementation

Release Notes

• Improved error message if doing UInt indexing on a Chisel Vec

[aswaterman]

LGTM, thanks @seldridge