To make this more concrete, you're suggesting something like

   [dependencies]
   hyper = { metapackage = { rust-platform = "2.7" } }

I like the general theme of having the platform be just a set of known-compatible versions, but on the other hand this feels like it loses out on many of the ease-of-use advantages of just specifying a platform version and knowing that you have all the crates inside of it.