Rust: pin integration test toolchain to 1.94.1

Write a `rust-toolchain.toml` into each integration test's working
directory so that tests use a consistent Rust version regardless of
what is pre-installed on the runner image.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

rust/ql/integration-tests/conftest.py

@@ -1,9 +1,34 @@
+import textwrap
+
 import pytest
 import json
 import commands
 import pathlib
 import tomllib
 
+# Last known Rust version that doesn't cause integration test failures.
+# TODO: remove this pinning once we figure out what's going wrong with newer versions.
+_PINNED_TOOLCHAIN = "1.94.1"
+
+
+@pytest.fixture(autouse=True)
+def _pin_rust_toolchain(cwd):
+    """Pin the Rust toolchain for integration tests.
+
+    Integration tests run from temporary directories, so they don't pick up
+    rust-toolchain.toml via rustup's file-based resolution. Writing the file
+    into the test's working directory ensures a consistent toolchain version
+    regardless of what is pre-installed on the runner image.
+
+    The toolchain must be pre-installed by the CI prepare action to avoid
+    concurrent rustup installs from parallel pytest-xdist workers.
+    """
+    (cwd / "rust-toolchain.toml").write_text(textwrap.dedent(f"""\
+        [toolchain]
+        channel = "{_PINNED_TOOLCHAIN}"
+        components = ["rust-src"]
+    """))
+
 
 @pytest.fixture(params=[2018, 2021, 2024])
 def rust_edition(request):
@@ -33,7 +58,7 @@ def cargo(cwd, rust_edition):
 
 @pytest.fixture(scope="session")
 def rust_sysroot_src() -> str:
-    rust_sysroot = pathlib.Path(commands.run("rustc --print sysroot", _capture=True))
+    rust_sysroot = pathlib.Path(commands.run(f"rustc +{_PINNED_TOOLCHAIN} --print sysroot", _capture=True))
     ret = rust_sysroot.joinpath("lib", "rustlib", "src", "rust", "library")
     assert ret.exists()
     return str(ret)