Use script to run cargo test, simplify subprocess call.

.azure/test-linux.yml

@@ -96,20 +96,13 @@ jobs:
             sudo apt-get install -y graphviz
           displayName: 'Install optional non-Python dependencies'
 
-      # Note that we explicitly use the virtual env with Qiskit installed to run the Rust
-      # tests since some of them still depend on Qiskit's Python API via PyO3.
       - ${{ if eq(parameters.testRust, true) }}:
-          # We need to avoid linking our crates into full Python extension libraries during Rust-only
-          # testing because Rust/PyO3 can't handle finding a static CPython interpreter.
+          # Note that we use the virtual env with Qiskit installed to run the Rust
+          # tests since some of them still depend on Qiskit's Python API via PyO3.
           - bash: |
               source test-job/bin/activate
               python tools/report_numpy_state.py
-              PYTHONUSERBASE="$VIRTUAL_ENV" cargo test --no-default-features
-            env:
-              # On Linux we link against `libpython` dynamically, but it isn't written into the rpath
-              # of the test executable (I'm not 100% sure why ---Jake).  It's easiest just to forcibly
-              # include the correct place in the `dlopen` search path.
-              LD_LIBRARY_PATH: '$(usePython.pythonLocation)/lib:$LD_LIBRARY_PATH'
+              python tools/run_cargo_test.py
             displayName: "Run Rust tests"
 
       - bash: |

tools/run_cargo_test.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2025.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""
+Utility script to invoke cargo test within the current Python environment.
+
+Notably, this sets up the environment variables necessary for Qiskit to be
+found by PyO3 / our Rust test executable.
+"""
+
+import os
+import subprocess
+import sys
+import sysconfig
+
+# This allows the Python interpreter baked into our test executable to find the
+# Qiskit installed in the active environment.
+os.environ["PYTHONUSERBASE"] = sys.prefix
+
+# On Linux, the test executable's RPATH doesn't contain libpython, so we add it
+# to the dlopen search path here.
+os.environ["LD_LIBRARY_PATH"] = os.pathsep.join(
+    filter(None, [sysconfig.get_config_var("LIBDIR"), os.getenv("LD_LIBRARY_PATH")])
+)
+
+# The '--no-default-features' flag is used here to disable PyO3's
+# 'extension-module' when running the tests (which would otherwise cause link
+# errors).
+subprocess.run(["cargo", "test", "--no-default-features"], check=True)

tox.ini

@@ -34,14 +34,8 @@ commands =
 [testenv:rust]
 basepython = python3
 package_env = .pkg-rust
-setenv =
-  PYTHONUSERBASE={envdir}
 allowlist_externals = cargo
-commands =
-  python -c '\
-  import os, subprocess, sysconfig;\
-  os.environ["LD_LIBRARY_PATH"] = os.pathsep.join(filter(None, [sysconfig.get_config_var("LIBDIR"), os.getenv("LD_LIBRARY_PATH")]));\
-  subprocess.run(["cargo", "test", "--no-default-features"]).check_returncode()'
+commands = python tools/run_cargo_test.py
 
 # This is a special package_env used by the 'rust' env above
 # to force Qiskit's Rust code to build in debug mode. We do this