rust-cpython: bindings for MissingAncestors
The exposition is rather straightforward, except for the
remove_ancestors_from() method, which forces us to an inefficient
conversion between Python sets and Rust HashSets.
Two alternatives are proposed in comments:
- changing the inner API to "emit" the revision numbers to discard
this would be a substantial change, and it would be better only in the
cases where there are more to retain than to discard
- mutating the Python set directly: this would force us to define an abstract
`RevisionSet` trait, and implement it both for plain `HashSet` and for
a struct enclosing a Python set with the GIL marker `Python<'p>`, also a non
trivial effort.
The main (and seemingly only) caller of this method being
`mercurial.setdiscovery`, which is currently undergoing serious refactoring,
it's not clear whether these improvements would be worth the effort right now,
so we're leaving it as-is.
Also, in `get_bases()` (will also be used by `setdiscovery`), we'd prefer
to build a Python set directly, but we resort to returning a tuple, waiting
to hear back from our PR onto rust-cpython about that
Differential Revision: https://phab.mercurial-scm.org/D5550
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