Dynamic arrays are used in FixedWidthInteger.bitRangesCoalesced

[kubamracek]

Dynamic arrays are used in FixedWidthInteger.bitRangesCoalesced. Swift MMIO should really be allocations free. This was identified while investigating https://forums.swift.org/t/building-swift-mmio-for-stm32c011-with-no-allocations-results-in-compilation-error/75124.

extension FixedWidthInteger {
  @inlinable @inline(__always)
  static func bitRangesCoalesced(bits bitRanges: [Range<Int>]) -> Bool {   // << receives an arrays
    let bitRanges = bitRanges.sorted { $0.lowerBound < $1.lowerBound } // << sorting is not constant-foldable
    var lowerBound = -1
    for bitRange in bitRanges {
      // Specifically ensure that the bit ranges dont overlap, e.g. the
      // following ranges are not valid: 0..<1, 0..<2. This is to ensure ranges
      // are coalesced before iterating reduce the number of mask and shift
      // operations needed.
      guard lowerBound <= bitRange.lowerBound else { return false }
      lowerBound = bitRange.upperBound
    }
    return true
  }

[rauhul]

cc @xtremekforever

[rauhul]

I already had done a lot of the work to move these checks into the macro and out of the runtime, with nice diagnostics. Ill try to land this soon:

  /// Walk the bit ranges forming error diagnostics for overlapping of ranges.
  ///
  /// Given the example bit field:
  /// ```
  /// @BitField(bits: 0..<24, 8..<32, 16..<48, 36..<44)
  /// var field: Field
  /// ```
  ///
  /// The ranges visually look like:
  /// ```
  /// 0       8       16      24      32  36      44  48
  /// ╎       ╎       ╎       ╎       ╎   ╎       ╎   ╎
  /// •───────────────────────◦       ╎   ╎       ╎   ╎
  /// ╎       •───────────────────────◦   ╎       ╎   ╎
  /// ╎       ╎       •───────────────────────────────◦
  /// ╎       ╎       ╎       ╎       ╎   •───────◦   ╎
  /// ╎       ╎       ╎       ╎       ╎   ╎       ╎   ╎
  /// 0       8       16      24      32  36      44  48
  /// ```
  ///
  /// The following diagnostics will be emitted:
  /// ```
  /// <location> error: overlapping subranges in '@BitField(bits: 0..<24, 8..<40, 16..<48, 36..<44)'
  /// @BitField(bits: 0..<24, 8..<40, 16..<48, 36..<44)
  ///  ^~~~~~~~
  ///
  /// <location> note: subrange '8..<24' of bit range '0..<24' overlaps bit ranges '8..<32' and '16..<48'
  /// @BitField(bits: 0..<24, 8..<40, 16..<48, 36..<44)
  ///                 ^~~~~~
  ///
  /// <location> note: subrange '8..<32' of bit range '8..<32' overlaps bit ranges '0..<24' and '16..<48'
  /// @BitField(bits: 0..<24, 8..<32, 16..<48, 36..<44)
  ///                         ^~~~~~
  ///
  /// <location> note: subranges '16..<32' and '36..<44' of bit range '16..<48' overlap bit ranges '0..<24', '8..<32', and '36..<44'
  /// @BitField(bits: 0..<24, 8..<40, 16..<48, 36..<44)
  ///                                 ^~~~~~~
  ///
  /// <location> note: subrange '36..<44' of bit range '36..<44' overlaps bit range '16..<48'
  /// @BitField(bits: 0..<24, 8..<40, 16..<48, 36..<44)
  ///                                          ^~~~~~~
  /// ```