rust-cpython: start cpython crate bindings This changeset introduces the hg-cpython crate, that compiles as a shared library holding a whole Python package (mercurial.rustext), with only the empty 'ancestor' submodule for now. Such bindings will be easier and safer to develop and maintain that those of `hg-direct-ffi`. They don't involve C code, only unsafe Rust that's mostly isolated within the cpython crate. The long-term goal would be to import the provided modules, such as rustext.ancestor with mercurial.policy.importmod, same as we already do with cext modules. Differential Revision: https://phab.mercurial-scm.org/D5434

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use cpython::exc::ValueError;

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use cpython::{PyErr, Python};

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use hg;

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py_exception!(rustext, GraphError, ValueError);

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impl GraphError {

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    pub fn pynew(py: Python, inner: hg::GraphError) -> PyErr {

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        match inner {

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            hg::GraphError::ParentOutOfRange(r) => {

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                GraphError::new(py, ("ParentOutOfRange", r))

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            }

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        }

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    }

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}