bug#53127: [PATCH RFC] Turning Rust/Cargo inputs into “regular” inputs?

[Liliana Marie Prikler <liliana.prikler@gmail.com>]

Am Samstag, dem 08.01.2022 um 18:57 +0100 schrieb Ludovic Courtès:
> 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.)
I suppose adding (package-source rust-cargo) to inputs to preserve the
old meaning would not make much sense?  If so, what about having a
source output and using (list `(,rust-cargo "source") ...)?

> 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.
What about making the change incrementally, so that outer layers can
start adopting the new style while inner layers are being
rebootstrapped.  I also think it'd make sense to see how we could
detect cycles through static analysis.

> 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.
Could we rewrite the toml file to tell Cargo it has no power over us? 
Could we define bootstrap mockups?

> PS: I guess you already knew all this Efraim but I’m kinda
>     (re)discovering it and now experiencing frustration firsthand. 
> :-)
Let's hope at least someone in our team has overcome Rust fatigue by
the time the GCC frontend for it lands.  Rust is an incredibly good
language, all it needs is a reasonable compiler and build system.

Cheers