rust-pyo3: conversions to GraphError Python exception

The nice thing is that with PyO3, exceptions can be instantiated
without holding the GIL. Hence the only thing that prevents us to
implement `Into<PyErr>` for `hg::GraphError` is that neither is
defined by the current crate.

We could use a wrapping "newtype", but the compiler is not so clever yet that
it could chain automatically to two needed `into()`, so we'll end up with
some type conversion anyway, involving something like `GraphErrorWrapper`.
At this point, explicitly named methods are just simpler.

rust/Cargo.lock

@@ -722,6 +722,7 dependencies = [
  "pyo3",
  "python3-sys",
  "stable_deref_trait",
+ "vcsgraph",
 ]
 
 [[package]]

rust/hg-pyo3/Cargo.toml

@@ -18,4 +18,5 log = "0.4.17"
 derive_more = "0.99.17"
 env_logger = "0.9.3"
 lazy_static = "*"
+vcsgraph = "0.2.0"
 

rust/hg-pyo3/src/exceptions.rs

@@ -1,4 +1,34
-use pyo3::create_exception;
 use pyo3::exceptions::PyValueError;
+use pyo3::import_exception;
+use pyo3::{create_exception, PyErr};
+
+use crate::revision::PyRevision;
 
 create_exception!(pyo3_rustext, GraphError, PyValueError);
+import_exception!(mercurial.error, WdirUnsupported);
+
+impl GraphError {
+    pub fn from_hg(inner: hg::GraphError) -> PyErr {
+        match inner {
+            hg::GraphError::ParentOutOfRange(r) => {
+                GraphError::new_err(("ParentOutOfRange", PyRevision(r.0)))
+            }
+        }
+    }
+    pub fn from_vcsgraph(inner: vcsgraph::graph::GraphReadError) -> PyErr {
+        match inner {
+            vcsgraph::graph::GraphReadError::InconsistentGraphData => {
+                GraphError::new_err("InconsistentGraphData")
+            }
+            vcsgraph::graph::GraphReadError::InvalidKey => {
+                GraphError::new_err("ParentOutOfRange")
+            }
+            vcsgraph::graph::GraphReadError::KeyedInvalidKey(r) => {
+                GraphError::new_err(("ParentOutOfRange", r))
+            }
+            vcsgraph::graph::GraphReadError::WorkingDirectoryUnsupported => {
+                WdirUnsupported::new_err(())
+            }
+        }
+    }
+}