rust-cpython: bump cpython to 0.4 to switch to upstreamed PySharedRef

rust: update dependencies For no particular reason, but just because I'll bump the rust-cpython version.

contrib: a small script to nudge lingering diff After a discussion on IRC with various reviewers. It seems like a good idea to have some automatic cleanup of old, inactive diffs. Here is a small script able to do so. I am preparing to unleash it on our phabricator instance.

packaging: add support for PyOxidizer I've successfully built Mercurial on the development tip of PyOxidizer on Linux and Windows. It mostly "just works" on Linux. Windows is a bit more finicky. In-memory resource files are probably not all working correctly due to bugs in PyOxidizer's naming of modules. PyOxidizer now now supports installing files next to the produced binary. (We do this for templates in the added file.) So a workaround should be available. Also, since the last time I submitted support for PyOxidizer, PyOxidizer gained the ability to auto-generate Rust projects to build executables. So we don't need to worry about vendoring any Rust code to initially support PyOxidizer. However, at some point we will likely want to write our own command line driver that embeds a Python interpreter via PyOxidizer so we can run Rust code outside the confines of a Python interpreter. But that will be a follow-up. I would also like to add packaging.py CLI commands to build PyOxidizer distributions. This can come later, if ever. PyOxidizer's new "targets" feature makes it really easy to define packaging tasks in its Starlark configuration file. While not much is implemented yet, eventually we should be able to produce MSIs, etc using a `pyoxidizer build` one-liner. We'll get there... Differential Revision: https://phab.mercurial-scm.org/D7450

mergestate: add accessors for local and other nodeid, not just contexts The mergestate can contain invalid nodeids. In that case, `mergestate.localctx` or `mergestate.otherctx` will fail. This patch provides a way of accessing the nodeid without failing in such cases. Differential Revision: https://phab.mercurial-scm.org/D8040

rebase: define base in only place in defineparents() Just a little refactoring to prepare for the next patch. Differential Revision: https://phab.mercurial-scm.org/D7906

tests: use full `uncommit` command name in tests I'm about to add a `hg uncopy`, so the `hg unc` we used for `hg uncommit` would become ambiguous. Differential Revision: https://phab.mercurial-scm.org/D8028

graft: default `base` argument to common case of `ctx.p1()` I also updated the callers that wanted that, partly to simplify and partly to show that it works. Differential Revision: https://phab.mercurial-scm.org/D8027

graft: let caller pass in overlayworkingctx to merge.graft() Passing in a different `wctx` than `repo[None]` is useful because it allows the caller to decide to not touch the working directory. Differential Revision: https://phab.mercurial-scm.org/D8026

copies: fix crash when copy source is not in graft base Differential Revision: https://phab.mercurial-scm.org/D8046

tests: add test showing crash when shelving ghosted rename target When you `hg rename` a file and then delete the rename target, `hg shelve` will give you a traceback. Note that the shelve succeeds and the shelve is correct, it's just the update to the parent that fails (i.e. to the parent of the commit that was created for the shelve). This can be squashed into the next commit if the reviewer prefers. Differential Revision: https://phab.mercurial-scm.org/D8045

rust-cpython: mark all PyLeaked methods as unsafe Unfortunately, these methods can be abused to obtain the inner 'static reference. The simplest (pseudo-code) example is: let leaked: PyLeaked<&'static _> = shared.leak_immutable(); let static_ref: &'static _ = &*leaked.try_borrow(py)?; // PyLeakedRef::deref() tries to bound the lifetime to itself, but // the underlying data is a &'static reference, so the returned // reference can be &'static. This problem can be easily fixed by coercing the lifetime, but there are many other ways to achieve that, and there wouldn't be a generic solution: let leaked: PyLeaked<&'static [_]> = shared.leak_immutable(); let leaked_iter: PyLeaked<slice::Iter<'static, _>> = unsafe { leaked.map(|v| v.iter()) }; let static_slice: &'static [_] = leaked_iter.try_borrow(py)?.as_slice(); So basically I failed to design the safe borrowing interface. Maybe we'll instead have to add much more restricted interface on top of the unsafe PyLeaked methods? For instance, Iterator::next() could be implemented if its Item type is not &'a (where 'a may be cheated.) Anyway, this seems not an easy issue, so it's probably better to leave the current interface as unsafe, and get broader comments while upstreaming this feature.

rust-cpython: make PySharedRef::try_borrow_mut() return BorrowMutError As I said, it shouldn't be an error of Python layer, but is something like a coding error. Returning BorrowMutError makes more sense. There's a weird hack to propagate the borrow-by-leaked state to RefCell to obtain BorrowMutError. If we don't like it, maybe we can add our own BorrowMutError.

rust-cpython: inline PySharedState::leak_immutable() and PyLeaked::new() For the same reason as the previous patch. The unsafe stuff can be better documented if these functions are inlined.

rust-cpython: inline PySharedState::try_borrow_mut() Since the core borrowing/leaking logic has been moved to PySharedRef* and PyLeaked*, it doesn't make sense that PySharedState had a function named "try_borrow_mut". Let's turn it into a pure data struct.