[Pkg-rust-maintainers] debcargo update handling alternative build depends

[Vasudev Kamath]

Hi Ximin,

After I introduced the alternative for version range semantics like
">=0.3.14, << 0.5.0" to resolve in the form
librust-rand-0.3+default-dev (>= 0.3.14) | librust-rand-0.4+default-dev

I observed a build failure which was actually suggesting it can't find
dh-cargo:amd64 >= 3. A closer investigation revealed following.

1.  cargo-tree which is used by our testing script resolves rand version
    range above to a single version 0.4.12 which satisfies upstream
    predicate.
2. Since we rely on cargo-tree to get the version information we build
    librust-rand-0.4+default-dev and librust-rand-0.3+default-dev is not
    built.
3. Since sbuild only checks for first option in case of alternatives in
    build dependency the build fails.

For the time being I've added --resolve-alternatives to sbuild command
in tests/sh/integrate.sh so that we can complete the build and see the
output. Now while writing this mail I'm trying to build debcargo and
hoping it will succeed. This fixed the case of build failure for num-bigint.

Now coming to the point on how do we really handle build dependency, I
think if we consider to build all crates during packaging we should have
some way to avoid having alternative build depends. What I currently
think of is have logic separately for generating build-depends which
will use cargo API to resolve above like version ranges to a single
suitable version and generate build depends on that single version. What
do you think of this approach?. If you have a better approach I would be
happy to know about it.

Best Regards,

--
Vasudev