Subject: [PATCH RFC] Turning Rust/Cargo inputs into “regular” inputs?

From: Ludovic Courtès <[email-masked]>
Date: Sat, 08 Jan 2022 18:57:34 +0100

Hello!

I’m opening this issue to discuss the possibility of changing
#:cargo-inputs and #:cargo-development-inputs to regular inputs, as a
followup to:

  https://issues.guix.gnu.org/51845#10

I have a preliminary patch for ‘guix style’ and (guix build-system
cargo), but there’s a couple of stumbling blocks.

First, after the hacky patch in the discussion above, I attempted to
turn #:cargo-inputs into ‘propagated-inputs’ (instead of ‘inputs’),
because that seemed to be somewhat more logical.  That cannot work
though, because then those packages would propagate to non-Rust
packages; for example, librsvg would depend on the “build output” of
rust-* instead of just depending on its source.  Anyway, I’m back to
‘inputs’.

Second, until now, these two things would have a different meaning:

  #:cargo-inputs (list rust-cargo)

vs.

  (inputs (list rust-cargo))

In the latter case, the package depends on the build result of
‘rust-cargo’; in the former case, the package depends on the source of
‘rust-cargo’.  (See ‘rav1e’ for an example where this happens.)

If we turn all #:cargo-inputs into ‘inputs’, how can we distinguish
these two cases?  A package like ‘rust-cargo’ is sometimes depended on
for its source, sometimes for its build result; thus, we cannot just
annotate the ‘rust-cargo’ package itself.

Last, the change to ‘inputs’ would introduce a few cycles at the
<package> level.  Those cycles vanish when we lower to bags and
derivations.  However, because of these cycles, things like ‘guix
refresh -l’ may not work; there might be other unexpected and undesired
side effects.

Some of these cycles could in theory be removed.  For instance,
‘rust-cfg-if’ has an optional dependency on ‘rust-compiler-builtins’,
which leads to a cycle, but Cargo won’t let us actually remove that
dependency, even though it’s optional.

In short: it’s complicated!

Thoughts?  Is status quo a lesser evil, after all?…

Ludo’.

PS: I guess you already knew all this Efraim but I’m kinda
    (re)discovering it and now experiencing frustration firsthand.  :-)