Re: #:cargo-inputs don't honor --with-input

[Hartmut Goebel <h.goebel@crazy-compilers.com>]

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Hi Ludo,

Am 30.04.21 um 12:45 schrieb Ludovic Courtès:

> 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 |


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