Rust: pin integration test toolchain to 1.94.1

Pin `RUSTUP_TOOLCHAIN` in rust integration tests to the version from
`rust-toolchain.toml`, so that tests are not affected by the Rust version
pre-installed on runner images.

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

Author: redsun82

rust/ql/integration-tests/conftest.py

@@ -1,9 +1,24 @@
+import os
 import pytest
 import json
 import commands
 import pathlib
 import tomllib
 
+_toolchain_file = pathlib.Path(__file__).resolve().parent.parent / "test" / "rust-toolchain.toml"
+_toolchain_channel = tomllib.loads(_toolchain_file.read_text())["toolchain"]["channel"]
+
+
+@pytest.fixture(autouse=True)
+def _pin_rust_toolchain(environment):
+    """Pin the Rust toolchain to the version specified in ql/test/rust-toolchain.toml.
+
+    Integration tests run from temporary directories, so they don't pick up the
+    rust-toolchain.toml file via rustup's file-based resolution. Setting
+    RUSTUP_TOOLCHAIN ensures a consistent version regardless of the runner image.
+    """
+    os.environ["RUSTUP_TOOLCHAIN"] = _toolchain_channel
+
 
 @pytest.fixture(params=[2018, 2021, 2024])
 def rust_edition(request):
@@ -33,7 +48,7 @@ def update(file):
 
 @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 +{_toolchain_channel} --print sysroot", _capture=True))
     ret = rust_sysroot.joinpath("lib", "rustlib", "src", "rust", "library")
     assert ret.exists()
     return str(ret)

rust/ql/test/rust-toolchain.toml

@@ -2,6 +2,6 @@
 # IMPORTANT: this can also have an impact on QL test results
 
 [toolchain]
-channel = "1.90"
+channel = "1.94.1"
 profile = "minimal"
 components = [ "rust-src" ]