This release primarily exists to help get users ready for the coming Chisel 7 release--there are several advanced APIs that are now deprecated and will be removed in the Chisel 7.0 release.

API Deprecation

• Add ElaboratedCircuit and deprecate use of internal ir Circuit (backport #4683) (by @chiselbot in #4693)
  Change ChiselCircuitAnnotation and CircuitSerializationAnnotation to no longer be case classes to help with the transition to ElaboratedCircuit.
• Add safer Chisel annotation API, deprecate old ones (backport #4643) (by @chiselbot in #4697)
  Creating annotations in Chisel now requires reporting what InstanceIds are going to be annotated so that Chisel can do some safety checks.
• [testers] Deprecate everything the testers package (backport #4742) (by @chiselbot in #4743)
• Mass deprecations, to be removed in Chisel 7 (backport #4754) (by @chiselbot in #4756)
• Warn on .asUInt|.asSInt(_: Int) (backport #4764) (by @chiselbot in #4766)
  The user probably forgot .W. Apply the same technique as used for .U|.S.

Performance

• Speedup Computation for whether Aggregates containsProbe (backport #4656) (by @chiselbot in #4664)
  Add a private containsProbe var to Aggregate and use it to speed up containsProbe checks.

Fixes

• fix up extra waveform dump behavior in svsim (backport #4592) (by @mergify[bot] in #4600)
• Fixes sendBits()'s handling of signed numbers in svsim (backport #4599) (by @jackkoenig in #4606)
• Deterministic FibonacciLFSR reductions (backport #4688) (by @chiselbot in #4690)
• Escape special characters in Property String literals (backport #4564) (by @mergify[bot] in #4565)
  This technically breaks backwards compatibility for firrtl.ir.StringPropertyLiteral but firrtl.ir is considered an internal API.
• Dev/biancolin/fix module choice under di (backport #4569) (by @mergify[bot] in #4570)
• Support boring on original Module after .toInstance call (backport #4602) (by @mergify[bot] in #4604)

Documentation

• fix typo in SourceInfoTransform.scala (backport #4594) (by @mergify[bot] in #4597)
  Fix typo in scaladoc for sourceInfoTransform macro

Dependency Updates

• Cross-compile the plugin for Scala 2.13.16 (by @jackkoenig in #4692)

Build and Internal Changes

• Update CD permissions for uploading release artifacts (backport #4529) (by @mergify[bot] in #4531)
  Also bump softprops/action-gh-release to v2.1.0.
• [6.x] Enable MiMa for v6.6.0 (by @chiselbot in #4527)

Full Changelog: v6.6.0...v6.7.0