[bug#35318] [PATCH] Update cargo-build-system to expand package inputs

[Danny Milosavljevic]

Hi Ludo,
Hi Ivan,

@Ludo:

Could you take a look at patch 1?  It's allowing the lookup of transitive
dependencies in Guix to be more flexible.  Is it OK?

It's used in patch 2 in order to consider both inputs and propagated inputs
rather than just propagated inputs.

@Ivan:

Thanks!  I've tested it and it works.

But I don't understand yet why you change the role of "inputs" compared
to how it is in the rest of Guix.

You have this:

+(define-public rust-proc-macro2
+  (package
[...]
+    (build-system cargo-build-system)
+    (native-inputs
+      `(("rust-quote" ,rust-quote "src")))
+    (inputs
+      `(("rust-unicode-xid" ,rust-unicode-xid "src")))
[...]

Here, inputs refer to SOURCE parts of packages which are definitely not
referred to at runtime.  Does "guix gc --references ...rust-proc-macro2..."
really refer to the source of rust-unicode-xid ?  I checked, it doesn't,
neither for the "src" derivation nor for the "out" derivation.

I think the general approach is good but I'm not certain that this won't
break other parts of Guix.  If it doesn't, fine.  @Ludo: WDYT?

Details:

A Rust crate has dependencies and dev-dependencies.

The crate needs the dev-dependencies only when building, not at runtime.

Let "transitives of X" mean "X and transitives of immediate dependencies of X 
and
transitives of immediate dev-dependencies of X", recursively.

The crate needs the source code of all its transitives to be available when
building, but needs none of the source code at runtime.

A crate at run time only requires the immediate, if even that (probably not!),
dependencies and none of the dev-dependencies, and not as source code.

So it's really not a propagated-input, although it kinda seems like a weird
version of a propagated-input while building (something like a
native-propagated-input).

If this can't be generalized (and I'm not sure of that--Go has a similar
static library-y view), we could also do those as (arguments ...) for the
rust build system only--although not sure how to do the resolving of
transitives then.

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