Use an associated type to provide the bit width of a register

[fumoboy007]

(I originally proposed this change here.)

---

Currently, the bit width of a register is specified as a parameter of the @Register macro:

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

I propose that we use an associated type to provide the bit width instead of a macro parameter. The associated type could be constrained to FixedWidthInteger & UnsignedInteger.

@Register
struct CR1 {
  typealias RawValue = UInt32

  @ReadWrite(bits: 0..<1, as: Bool.self)
  var en: EN
}

I think this would be more familiar to a Swift developer and it would also make the raw value type explicit.

---

I wonder if this could also allow the @RegisterBank macro to infer the offsets of the registers. (I haven’t developed a macro yet, so I don’t know the answer.)

Before:

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

After:

@RegisterBank
struct Control {
  var cr1: Register<CR1>
  var cr2: Register<CR2>
}

[fumoboy007]

The associated type could be constrained to FixedWidthInteger & UnsignedInteger.

Or even better, the associated type could be constrained to a hypothetical VolatileRepresentable protocol that was brought up here.

[fumoboy007]

I wonder if this could also allow the @RegisterBank macro to infer the offsets of the registers. (I haven’t developed a macro yet, so I don’t know the answer.)

Not feasible. A macro can only read the code that it is attached to.