[Pkg-rust-maintainers] Bug#987324: rust-hashbrown: missing ahash feature makes building hashlink crate impossible

[Daniel Kahn Gillmor]

Package: rust-hashbrown
Version: 0.9.1-2
Control: forwarded -1 https://github.com/kyren/hashlink/issues/6

I'm trying to build the hashlink crate, which is needed by a transitive
dependency of future work on Sequoia (https://sequoia-pgp.org).

However, the build of hashlink 0.6.0 fails with an error:

```
error[E0599]: no variant or associated item named `default` found for enum `DefaultHashBuilder` in the current scope
  --> src/linked_hash_map.rs:50:57
   |
50 |             hash_builder: hash_map::DefaultHashBuilder::default(),
   |                                                         ^^^^^^^ variant or associated item not found in `DefaultHashBuilder`

error[E0599]: no variant or associated item named `default` found for enum `DefaultHashBuilder` in the current scope
  --> src/linked_hash_map.rs:60:57
   |
60 |             hash_builder: hash_map::DefaultHashBuilder::default(),
   |                                                         ^^^^^^^ variant or associated item not found in `DefaultHashBuilder`

error: aborting due to 2 previous errors
```

I reported this to the hashlink project upstream, and @nathaniel-daniel
identified the problem as being the stripping of the ahash feature from
the hashbrown crate in debian.

Presumably this means that we need the ahash crate in debian first; and
then we should enable that feature for the hashbrown crate.

     --dkg
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