Add experimental uv resolver for Python lockfile generation

[Affanmir]

Summary

• Adds [python].lockfile_resolver = "uv" option (default: pex) for faster lockfile generation
• Uses uv pip compile to pre-resolve pinned requirements, then pex lock create --no-transitive to materialize the PEX lockfile
• Supports all lock styles including universal (via uv pip compile --universal)
• Adds [uv].args_for_lockfile_resolve for passthrough flags (e.g., --index-strategy unsafe-first-match)
• ~3-4x faster lockfile generation on cold cache (based on PR Add experimental uv resolver for Python lockfiles #22949 benchmarks)

Motivation

Issue: #20679 — faster, more ergonomic lockfile generation by leveraging uv's PubGrub resolver.

This builds on the uv PEX builder from #23197 (merged) and is inspired by the approach in #22949, with key improvements:

Aspect	PR #22949	This PR
Universal lock style	Blocked (raises error)	Supported via --universal
Conflicts with main	Creates uv.py (already exists)	Works with existing uv.py from #23197
Unrelated changes	Includes Rust file edits	Clean diff, Python only
Args passthrough	[uv].args (general)	[uv].args_for_lockfile_resolve (specific)
register.py	Modifies	No change needed

How it works

requirements (from targets)
        │
        │  [python].lockfile_resolver = "uv"
        ▼
  uv pip compile [--universal] [--python-version X.Y]
        │
        ▼
  pinned requirements.txt
        │
        │  pex lock create --no-transitive --style={universal,strict,sources}
        ▼
  PEX lockfile (same format as today)

Usage

[python]
enable_resolves = true
lockfile_resolver = "uv"  # opt-in

[uv]
args_for_lockfile_resolve = ["--index-strategy", "unsafe-first-match"]

Current limitations

Limitation	Reason
No complete_platforms	Not yet wired through uv step
No per-resolve overrides/sources/excludes	Avoids silently changing semantics
Non-universal lock styles require single Python major.minor	uv resolves for exactly one target interpreter

Code changes

File	Change
subsystems/setup.py	LockfileResolver enum + lockfile_resolver option
subsystems/uv.py	args_for_lockfile_resolve passthrough + updated DownloadedUv
goals/lockfile.py	uv pre-resolve pipeline in generate_lockfile()
goals/lockfile_test.py	6 new tests (validation + integration)
lockfiles.mdx	Documentation section
2.32.x.md	Release note

Test plan

• test_strip_named_repo — unit test for index URL stripping helper
• test_uv_resolver_rejects_complete_platforms — validates error on unsupported complete_platforms
• test_uv_resolver_strict_requires_single_python — validates error on multi-version strict
• test_uv_resolver_rejects_overrides — validates error on unsupported overrides
• test_uv_resolver_strict_generates_valid_lockfile — E2E: generates valid PEX lockfile with uv + strict
• test_uv_resolver_universal_generates_valid_lockfile — E2E: generates valid PEX lockfile with uv + universal
• All existing lockfile tests pass unchanged

A.I usage disclaimer
🤖 Generated with Claude Code

Would love a review from maintainers of the lockfile generation. Very willing to contribute and make any requested changed @benjyw

[benjyw]

Hi @Affanmir, thanks for this contribution.

I have some questions about the background, motivation, and creation of this change. It is unusual to receive a PR that directly competes with an existing, pending PR by another contributor that is already going through code review. We like to keep things collegial and friendly here, and so in this case it might have been appropriate to first discuss your change with the author of that PR, and with the maintainers of the project.

It would be helpful if you could jump on Slack, introduce yourself there, and give us a bit of background about your use of Pants, your motivation for this change, and how you created it using Claude.

Thanks!