Introducing Swift MMIO

[fumoboy007]

@rauhul

@Register(bitWidth: 32)
struct CR1 {
  @ReadWrite(bits: 0..<1, as: Bool.self)
  var en: EN
  @ReadWrite(bits: 1..<2, as: Bool.self)
  var clken: CLKEN
  @ReadWrite(bits: 2..<3, as: Bool.self)
  var rst: RST
}

@RegisterBank
struct Control {
  @RegisterBank(offset: 0x0)
  var cr1: Register<CR1>
  @RegisterBank(offset: 0x4)
  var cr2: Register<CR2>
}

I wonder if these bit field macros could be more general purpose than MMIO. Perhaps they should be a Swift language feature?

@mgriebling previously brought up a use case unrelated to MMIO and the Swift evolution pitch by @Douglas_Gregor and @Ben_Cohen for an @OptionSet macro feels like a subset of a general bit field macro.