Cargo versioning

[joshtriplett]

Rendered

RFC co-authored by myself and @wycats, and pre-reviewed by @alexcrichton.

[withoutboats]

To summarize the RFC as I understand it: the idea is that cargo can figure out your minimum version requirements based on what features of cargo are necessary to compile it, and that it will then record it in the index entry for your crate. This will then influence error messages and version resolution. The user interface doesn't need to change to support this (that is, you don't specify a minimum version in your Cargo.toml).

Is that right?

[bryteise]

Would it be possible to allow an optional cargo version number if crate authors want to be able to enforce they will be using other newer features of cargo but currently do not?

[joshtriplett]

@withoutboats Correct. This is best-effort, and if the schemas turn out to be incorrect we can fix bugs in them.

@bryteise Perhaps, though the default Cargo.toml should not include a version number. But I think the reverse seems more important: "don't let me use any features from Cargo newer than this". Again, best-effort, but useful as a local development complement to a good CI system.

[Ericson2314]

Sorry to take so long to get to this. This is generally quite good, but I am also a bit wary of no manual recourse in Cargo.toml files themselves. In particular, the tentative ideas about non-registry dependencies:

For crates obtained via alternate registries or directly from version control or filesystem paths, Cargo may attempt to use the schema provided by crates.io-index to parse the Cargo.toml file and distinguish types of errors, but may have to continue and warn rather than stopping with an error.

seem sub-par. crates.io access for non-crates.io deps seems likely to cause confusion, non-determinism, and annoyance when there is no internet access. An optional schema version in Cargo.toml circumnavigates the any need for such heuristics.

[Ericson2314]

N.B. haskell/cabal#4899 is the plan for schema versions for Cabal. Their situation is more awkward because Cabal files are a non-standard syntax, in addition to schema within that syntax, but I'd thought i'd still link for reference.