How to implement conditional branch in constant generics

[Yandros]

See:

Conditional compilation based on generic constant

It's cumbersome (especially because the trait solver does not realize that if const B: bool, then either B == true, or B == false), but it can be done: trait RecordKeyType { type Key : ?Sized; } struct __<const UNICODE_KEYS: bool>; impl RecordKeyType for __<true> { type Key = String; } impl RecordKeyType for __<false> { type Key = Box<[u8]>; } struct Record<const UNICODE_KEYS: bool> where __<UNICODE_KEYS> : RecordKeyType, { key: <__<UNICODE_KEYS> as RecordKeyType>::Key, } f…

In it you can see you need to forward the bounds on the const bool, such as:

  impl<Left: IDataType, Right: IDataType> IDataType for MaxSizeType<Left, Right>
+ where
+     Condition<Left, Right, { Left::TYPE_SIZE > Right::TYPE_SIZE }> : IDataType,
  {
      const TYPE_SIZE: usize = 0;

      type DataType = <Condition<Left, Right, { Left::TYPE_SIZE > Right::TYPE_SIZE }> as IDataType>::DataType;
  }