This is yet another time where you should be giving concrete examples. You say that you are looking at code where the optimization you want will reduce memory consumption by 50%, that this makes the difference between fitting into RAM and not, and that it involves composing structs where you don't control all of the type definitions. OK. I'm actually willing to believe that if you show us the actual code with the problem. You might even be able to convince me that your suggested change really is the only change that will solve the problem!
But if you just keep saying "hey,
(((u32, u8), u16), u8) could take less space therefore Rust must make it take less space, compatibility and type system constraints be damned," then I and everyone else here are going to write this thread off as yet another time Soni came in with an impractical proposal and wouldn't work with us to turn it into a practical one.