prelude/rust: from_any_dir.py needs Python ≥ 3.12 (Path.relative_to(walk_up=True)) but the bootstrap toolchain runs whatever host python3 is

[yesme]

Summary

prelude/rust/tools/from_any_dir.py (the CC/CXX/LD/AR shim used by cargo_buildscript) calls Path.relative_to(..., walk_up=True), which only exists on Python ≥ 3.12. Nothing in the prelude declares or checks that minimum: system_python_bootstrap_toolchain() runs the shim with whatever bare python3 the host provides. On macOS the Command Line Tools still ship /usr/bin/python3 = 3.9.6 (verified on macOS 26.6.2), so every Rust build script that compiles C through the shim fails on a stock Mac.

Where

• prelude/rust/tools/from_any_dir.py line 40 (unchanged between the 2026-08-22 tag and main as of 2026-09-04):

  if original_cwd.is_relative_to(interim_cwd):
      interim_cwd = interim_cwd.relative_to(original_cwd, walk_up=True)

  walk_up was added in Python 3.12 (https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.relative_to).
• prelude/toolchains/python.bzl: system_python_bootstrap_toolchain defaults the interpreter to the bare name python3 (_interpreter_for_os), so the shim runs under the host's default Python.
• prelude/rust/cargo_buildscript.bzl _make_cc_shim: wraps CC/CXX/LD/AR in the shim for every buildscript_run.

Reproduce

1. A host whose default python3 is < 3.12 (e.g. macOS with only the Command Line Tools: /usr/bin/python3 --version → Python 3.9.6).
2. Default toolchains: system_python_bootstrap_toolchain() and system_cxx_toolchain().
3. Any Rust crate whose build.rs compiles C via the cc crate, run as a cargo_buildscript (e.g. Reindeer-generated libsqlite3-sys 0.38.2 with buildscript.run = true, bundled feature).

Result: the build script's first cc invocation goes through __cc_shim.sh → python3 from_any_dir.py … and dies with

TypeError: relative_to() got an unexpected keyword argument 'walk_up'

Real-world case: yesme/hctl2#157 (fixed downstream in yesme/hctl2#165 by pinning a python-build-standalone 3.12 interpreter and injecting its absolute path into the bootstrap toolchain).

Suggested fix

Any of these would do; the first is a one-line change and drops the 3.12 requirement entirely:

1. Use os.path.relpath, which has walked up since forever:

   interim_cwd = Path(os.path.relpath(interim_cwd, original_cwd))

2. Or keep walk_up but fail early with a clear message (sys.version_info < (3, 12)) instead of a TypeError from inside a build-script shim.
3. And/or document the minimum interpreter version for system_python_bootstrap_toolchain in prelude/toolchains/python.bzl.

Happy to send a PR for (1) if that is the preferred direction.

Environment

• buck2 prelude 2026-08-22 (file identical on main at 2026-09-04)
• macOS 26.6.2 arm64, /usr/bin/python3 3.9.6 (Apple Command Line Tools)
• Reindeer v2026.08.24.00, libsqlite3-sys 0.38.2

Companion issue about the same shim: #1491