Uh.  More generally, Rust packages kinda create a “shadow dependency
graph” via #:cargo-inputs & co., which breaks all the tools that are
unaware of it.  It was discussed several times on this list, and
apparently it’s unfortunately unavoidable at this time.  :-/

Maybe we can get rid of #:cargo-inputs at least:

guix/build-system/cargo.scm says: "Although cargo does not permit cyclic dependencies between crates,
however, it permits cycles to occur via dev-dependencies"

So we could change #:cargo-inputs into normal inputs and get at least part of the dependencies right.

I'm aware of the "special treatment" of cargo-inputs. Anyhow we could apply the following changes to the cargo build-system:

• The cargo build-system copies the "pre-built crate" (more on this below) into a new output called "rlib" or "crate". There already is a phase "packaging" which only needs to be changed to use the other output.

• All of today's #:cargo-inputs will be changed into normal inputs using the "rlib/crate" output. (To avoid duplicate assoc-rec keys we might need to change the name/keys, but this should be a minor issue.)
  

• If required, the cargo build-system can easily identify former #:cargo-inputs  by being inputs from a "rlib/crate" output.

Benefits up to here:

• The dependency graph would be much more complete - although "#:cargo-development-inputs" would still be missing.
• Package transformation options would work -again except for "#:cargo-development-inputs".
• If(!) we actually manage to make cargo pick "pre-built" crates, package definition will already be adjusted to use them.

Drawbacks up to here:

• Since the "packaging" phase copies the source, there is not much benefit in having a "rlib/crate" output yet. Actually, when a "rlib/crate" output needs to be build, the user will end up with two copies of the source (one from the git-checkout, one from packaging)

About "pre-built" crate: Given the many possible ways to build crates (e.g. switching on and off "features", different crate types), we might never be able to provide pre-built packages for all cases. Thus we might end up always providing the source, even if we manage to make cargo pick of pre-built artifacts.

About the output name: Rust has a notion of "rlib" (a specialized .a file), which seems to be the pre-built artifacts we are seeking. Thus the proposed name.

WDYT?

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |