Summary:
This makes intersection set stop early. It's useful to stop iteration on some
lazy sets. For example, the below `ancestors(tip) & span` or
`descendants(1) & span` sets can take seconds to calculate without this
optimization.
```
In [1]: cl.dag.ancestors([cl.tip()]) & cl.tonodes(bindings.dag.spans.unsaferange(len(cl)-10,len(cl)))
Out[1]: <and <lazy-id> <dag [...]>>
In [3]: %time len(cl.dag.ancestors([cl.tip()]) & cl.tonodes(bindings.dag.spans.unsaferange(len(cl)-10,len(cl))))
CPU times: user 364 µs, sys: 0 ns, total: 364 µs
Wall time: 362 µs
In [7]: %time len(cl.dag.descendants([repo[1].node()]) & cl.tonodes(bindings.dag.spans.unsaferange(0,100)))
CPU times: user 0 ns, sys: 574 µs, total: 574 µs
Wall time: 583 µs
```
Reviewed By: sfilipco
Differential Revision: D22638458
fbshipit-source-id: b9064ce2ff1aecc2d7d00025928dfcb3c0d78e0c
Any native code (C/C++/Rust) that Mercurial (either core or extensions)
depends on should go here. Python code, or native code that depends on
Python code (e.g. #include <Python.h> or use cpython) is disallowed.
As we start to convert more of Mercurial into Rust, and write new paths
entrirely in native code, we'll want to limit our dependency on Python, which is
why this barrier exists.
See also hgext/extlib/README.md, mercurial/cext/README.mb.
How do I choose between lib and extlib (and cext)?
If your code is native and doesn't depend on Python (awesome!), it goes here.
Otherwise, put it in hgext/extlib (if it's only used by extensions) or
mercurial/cext (if it's used by extensions or core).