`pip.parse` drops extras declared on the wheel's dependency (`package[extra]`)

Start in python/private/pypi/pep508_deps.bzl, then read pep508_requirement.bzl to trace how extras are parsed and resolved for transitive requirements. Reproduce with fastmcp==4.0.5 using the documented uv and Bazel commands. Done means the fastmcp_slim target preserves the client and server extras and its gated dependencies are available in the resulting importable environment.

🐞 bug report

Affected Rule

pip.parse

Is this a regression?

Not really, it's caused by upgrading fastmcp from v3 to v4

Description

fastmcp==4.0.5 declares fastmcp-slim[client,server]==4.0.5 unconditionally
in its Requires-Dist, and fastmcp-slim gates its client and server
dependencies (httpx2, authlib, mcp, starlette, and others) behind those
extras. When fastmcp is pulled in through pip.parse, rules_python does not
enable the client and server extras on the fastmcp_slim target, so those
gated dependencies are never wired into the dependency graph. Depending on
@pypi//fastmcp therefore produces a different importable environment than
pip install fastmcp: import fastmcp.client fails at runtime with
ModuleNotFoundError: No module named 'httpx2', even though httpx2 and the
other extra dependencies are pinned in the lock and exist as @pypi//… targets.

The cause is in python/private/pypi/pep508_deps.bzl. The
fastmcp-slim[client,server] edge is parsed into a requirement with
.name == "fastmcp-slim" and .extras == ["client", "server"]
(pep508_requirement.bzl), but _resolve_extras() only consults req.extras
for self-edges (if req.name != self_name: continue), and the dependency label
is built from req.name alone, so the [client,server] qualifier is discarded.
Whether a package's extra == '...' dependencies are wired is then decided
solely by the extras passed to that package's own whl_library, which is
populated from how the package appears as a requirement line in the lock.
fastmcp-slim appears only as a plain transitive pin with no extras, so its
whl_library receives extras = [] and the client and server dependencies are
dropped.

🔬 Minimal Reproduction

uv add fastmcp==4.0.5
uv export --format requirements.txt > ./requirements.txt
bazel build //...

🔥 Exception or Error

ImportError: FastMCP client support is not installed. Install `fastmcp` or `fastmcp-slim[client]`.

🌍 Your Environment

Operating System:

ubuntu 24.04

Output of bazel version:

Build label: 8.6.0
Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer
Build time: Thu Feb 26 19:55:20 2026 (1772135720)
Build timestamp: 1772135720
Build timestamp as int: 1772135720

Rules_python version:

1.9.2

Anything else relevant?