Specifying synchronized dependencies

[mikedilger]

It would be helpful in some situations to be able to specify synchronization between dependencies, e.g.:

exactly_one_of { 
  {  serde = 0.6;  serde_codegen = 0.6 },
  { serde = 0.7; serde_codegen = 0.7 }
}

In some downstream projects, cargo occasionally mixes-and-matches these versions for textnonce and the compile error then appears in textnonce, for which a bug is logged that doesn't really exist. It's very confusing to the downstream user. The real bug is that one of their dependencies requires serde 0.6, another requires serde 0.7, and it was never going to work.... but the blame lands on textnonce because that's what fails to compile first: mikedilger/textnonce#7

[mikedilger]

Alternatively conditional dependencies, e.g.:

serde = ">= 0.6, < 0.8"
serde_codegen = { if serde = "0.6" then "0.6";  if serde = "0.7" then "0.7" }

(ignore my atrocious syntax)

[alexcrichton]

This is somewhat along the lines of #2064 I think?

[mikedilger]

Similar

[carols10cents]

I believe this will be solved by public/private dependencies, the RFC for which has been accepted, and now has a tracking issue: rust-lang/rust#44663