We would like to support all combinations of the following axes

• Src type sized or unsized
• Dst type sized or unsized
• Src larger than, same size as, or smaller than Dst type for the same fat pointer metadata
  • Includes when Src is sized and Dst is unsized and vice-versa
• Dst same size as Src given a metadata fix-up operation

Some specific operations we could support:

• Sizes same w/o metadata fix-up: permits shared and exclusive reference casts
• For same metadata, Dst smaller than Src: supports shared reference casts
  • When both types are sized: supports by-value transmute
• For same metadata, Dst larger than Src: supports by-value transmute if uninit bytes are valid at the end of Dst
• Sizes same given metadata fix-up: shared and exclusive casts (with metadata fix-up)