I find the Motivation of the RFC not ‘motivational’ enough. It seems the main motivation is interop with C bitfield structs, which is fair, however bitfields are somewhat niche even in C and it is not clear if the RFC is intended to solve just that problem or more and if it is the right solution.
Additionally, the C
MipsInstruction example itself is questionable, since C struct bitfield layout is implementation-dependent, ie. in standard portable C you should not use bitfields to decode/encode a mips instruction.
Finally, as @comex pointed out, with const generics custom-width integers would be best implemented in a library.
Sorry for so much criticism.