[ImBinarry]

Pyrefly now warns when the resolved Python version is older than the oldest version typeshed supports (currently 3.10). Older than that, the standard library isn't checked accurately, and that's what the warning is for.

It's emitted from ConfigFile::configure_at, prints to the terminal, and leaves the exit code alone.

Fixes #4357

• I am a human, and I fully understand the code I am submitting.

Test Plan

• cargo test -p pyrefly_config: 337 passed, including 2 new tests for the warning (3.9 warns; 3.10, 3.11 and 3.13 don't).
• cargo fmt --all -- --check and cargo clippy -p pyrefly_config --all-targets -- -D warnings: both clean.
• Manual CLI check: with python-version = "3.9" in pyrefly.toml, the warning prints once, with the config path, and the exit code stays 0. With python-version = "3.13" nothing prints.
• Didn't run the full CI suite locally (release clippy, bench-profile tests, conformance/Scrut); CI covers that. mypy_primer doesn't cover crates/ anyway, and nothing here changes type-checking behavior.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D131218584. (Because this pull request was imported automatically, there will not be any future comments.)

[stroxler]

Thanks for the PR!

It looks pretty good, but I think there are a few issues we should clean up:

• Gate the bundled-stub warning on typeshed_path.is_none(). A custom typeshed replaces the bundled stdlib, but the head warns anyway. Add a negative regression test.
• Improve the message: do not instruct a project legitimately targeting 3.9 to merely set its target to 3.10. That changes the semantics being checked. Explain the limitation and, if offering remediation, distinguish upgrading the supported target from supplying compatible custom stubs. Also reword “Typeshed drops stubs” to describe removal of older-version branches/guards. These wording issues are secondary to the two functional findings.
• Look into whether we can deliver the warning in cases where Pyrefly synthesizes a config; it appears the current implementation can drop the warning in some situations.

Just to explain the third point: it looks like CLI arguments are used, the warning appears to never reach the user. For example:

# File mode
pyrefly check main.py --python-version 3.9 --skip-interpreter-query
# Project mode
pyrefly check --python-version 3.9 --skip-interpreter-query

[ImBinarry]

Thanks for the review. I pushed fixes for all three in the new commits.

1. The warning now only fires when typeshed-path is unset, so custom typesheds stay quiet. Added a regression test for that: custom typeshed + python-version = "3.9" produces no warning.

2. Reworked the message. It now explains that the bundled stubs no longer include the version-specific branches and guards for older versions, and gives two options instead of just telling users to change the target: keep the target and supply compatible custom stubs via typeshed-path, or raise python-version if the project actually targets a newer version.

3. Root cause for the synthesized-config gap: the file-mode and project-mode fallbacks only logged their config errors at debug level, so warnings never reached the terminal. The fallback callbacks now return the warnings and ConfigFinder accumulates them (deduped, since a single run can synthesize several configs), so both modes print them. Your two example commands now each print the warning once, and --typeshed-path suppresses it.

Tests: cargo test -p pyrefly_config (338 passed), plus a new config-finder test covering the synthesized-config path. cargo fmt --all -- --check and clippy on the touched crate are clean. I didn't run the full suite locally, CI will cover it.

One design note: the delivery fix surfaces config warnings from synthesized configs in general, not just this warning. If you'd prefer a narrower mechanism, I can special-case this one.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

============================================================
SUMMARY
============================================================
Total: +8 new errors, -0 fixed errors
By preset: +0/-0 (default), +0/-0 (strict)

Projects with changes (8):
  git-revise: +1 -0
  pybind11: +1 -0
  pyinstrument: +1 -0
  manticore: +1 -0
  antidote: +1 -0
  pywin32: +1 -0
  dragonchain: +1 -0
  discord.py: +1 -0
============================================================

FULL DIFF DETAILS
------------------------------------------------------------

git-revise (https://github.com/mystor/git-revise)
+  WARN .../projects/git-revise/pyrefly.toml: Python version 3.8.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.8.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

pybind11 (https://github.com/pybind/pybind11)
+  WARN .../projects/pybind11/pyrefly.toml: Python version 3.9.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.9.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

pyinstrument (https://github.com/joerick/pyinstrument)
+  WARN .../projects/pyinstrument/pyrefly.toml: Python version 3.8.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.8.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

manticore (https://github.com/trailofbits/manticore)
+  WARN .../projects/manticore/pyrefly.toml: Python version 3.7.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.7.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

antidote (https://github.com/Finistere/antidote)
+  WARN .../projects/antidote/pyrefly.toml: Python version 3.7.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.7.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

pywin32 (https://github.com/mhammond/pywin32)
+  WARN .../projects/pywin32/pyrefly.toml: Python version 3.9.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.9.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

dragonchain (https://github.com/dragonchain/dragonchain)
+  WARN .../projects/dragonchain/pyrefly.toml: Python version 3.8.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.8.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

discord.py (https://github.com/Rapptz/discord.py)
+  WARN .../projects/discord.py/pyrefly.toml: Python version 3.8.0 is older than the oldest version described by the bundled typeshed stubs (3.10.0). The bundled stubs no longer include the version-specific branches and guards for older versions, so the standard library may be modeled inaccurately for this target. If the project must keep targeting 3.8.0, provide compatible custom stubs with `typeshed-path`; otherwise raise `python-version` to 3.10.0 or newer.

[ImBinarry]

@stroxler
The windows-latest failure came from the stale merge base: it predates main's
glob separator fix (#5615). I merged current main into the branch (00c94e1), and
the config tests pass locally (338).

The new CI runs are action_required and need approval since this is my first
contribution. I've also noted the mypy_primer differential in the PR description
(+8 warnings, no type errors).

[stroxler]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

This pull request has been merged in fc2194d.