Packaging rust crates without compiling dependencies again for every dependent (wasting build time)

[Maxime Devos <maximedevos@telenet.be>]

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Hi various distributions packaging rust things (^),

Currently, most (all?) distributions seem to package ‘source-only
crates’, which when ‘compiled’, just copy some source code to a
location.  This works, but the downside is that if both a package X and
a package Y depends on rust-Z, then rust-Z will effectively be compiled
twice: when building X, but also when building Y.

This seems a waste of compilation time to me, so I've been writing a
build tool ‘antioxidant’ (*) for the distribution Guix that avoids
double compilation, at
<https://notabug.org/maximed/cargoless-rust-experiments>).

To avoid the double compilation things, antioxidant calls the
underlying rustc by itself instead of cargo, and it reads and
interprets Cargo.toml by itself instead of letting cargo do that.

Currently, it's rather Guix-specific, but in principle it should be
possible to turn it in some binary 'antioxidant' that could be used on
other distributions as well if someone is interested in that, e.g.
something like

$ # (not implemented yet!)
$ antioxidant configure --prefix=$HOME/extra-software -Lcrate=/usr/lib/crates -L/usr/lib -Lcrate=$HOME/extra-software/lib/crates --features=x,y,z
$ antioxidant make
$ antioxidant make check
$ antioxidant make install DESTDIR=... # this does not install source code, only the compiled crates

(*) A ‘build system’, in Guix terminology.

Currently, it can succesfully compile 'hexyl', 'sniffglue' and all
their dependencies.  I'm working on addressing limitations that
prevent antioxidant from compiling other binaries (currently trying
to make 'agate' and all its dependencies compile).

Greetings,
Maxime.

(^) If you are reading this from guix-devel@: some of the distributions I
intend to contact have a no-crossposting policy, so I'm first sending this to
guix-devel@, then sending some mails to individual distributions with a link
to this e-mail.

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