[Libguestfs] Cargo edition problem with Debian 11

[Richard W.M. Jones]

https://gitlab.com/nbdkit/nbdkit/-/jobs/3598390121

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cargo build --release --example ramdisk
 Downloading crates ...
  Downloaded float-cmp v0.9.0
  Downloaded downcast v0.11.0
  Downloaded mockall_derive v0.11.3
  Downloaded itertools v0.10.5
  Downloaded aho-corasick v0.7.20
  Downloaded predicates-tree v1.0.7
error: failed to parse manifest at `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/predicates-tree-1.0.7/Cargo.toml`
Caused by:
  failed to parse the `edition` key
Caused by:
  this version of Cargo is older than the `2021` edition, and only supports `2015` and `2018` editions.
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nbdkit itself uses edition = 2018, but this seems to affects one of
the dependencies.

I'm not sure how to solve this, but one ideas I had is in ./configure
to check if the cargo/rust we're trying to use doesn't support some
base edition (eg. latest edition supported < 2021) then we would
disable rust bindings.

Unfortunately actually getting the latest supported edition seems
hard.  The best I could find is parsing this which doesn't seem ideal:

$ rustc --help |& grep -- --edition
        --edition 2015|2018|2021|2024

What do you think?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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