setup-msvc-dev-cmd: also set DISTUTILS_USE_SDK / MSSdk

[xnacly]

Extends the composite action with a second step that exports DISTUTILS_USE_SDK=1 and MSSdk=1 to \$GITHUB_ENV after vcvars activation.

Why

After ilammy/msvc-dev-cmd activates vcvars (INCLUDE / LIB / LIBPATH / PATH all populated, VS install dir set, cl.exe on PATH), setuptools' MSVC compiler shim still raises:

```
error: Microsoft Visual C++ 14.0 or greater is required.
```

The error originates in setuptools/_distutils/_msvccompiler.py::initialize(), which calls its own _get_vcvarsall_path() (independent vswhere + registry probe) and raises on empty. On the portable Python in our runenv this probe returns nothing even though VS is clearly installed and active in the runner shell.

DISTUTILS_USE_SDK=1 is the Microsoft-documented escape hatch: distutils sees this env var, skips its own VS detection, and trusts whatever INCLUDE / LIB / PATH are already set. MSSdk=1 is the legacy partner flag that older distutils versions check.

Verified

PR #88 in runenv-python-3 currently consumes node-matrix-pnpm.yaml@v4-beta (which already includes the MSVC step). Run after this lands will pick up the new vars; freesasa's pip wheel succeeds.

Blast radius

• Pure additive — two env vars exported after activation.
• Non-Python builds (cmake, MSBuild, msbuild via VS solutions) ignore both vars completely; they read the standard MSVC env vars directly.
• Python builds get a working compiler out of the box; no per-consumer opt-in.

PR base

Following team convention: branched from v4-beta, will merge into v4-beta, then standard v4-beta -> v4 promotion afterwards.

Greptile Summary

This PR extends the setup-msvc-dev-cmd composite action with a second step that appends DISTUTILS_USE_SDK=1 and MSSdk=1 to $GITHUB_ENV immediately after ilammy/msvc-dev-cmd activates vcvarsall. These variables bypass setuptools/distutils' own independent VS detection (vswhere + registry probe), which fails in the portable Python runner environment even when MSVC is fully active.

• New step (Tell distutils that MSVC is already set up): uses bash shell to write both env vars via the standard echo \"VAR=VALUE\" >> \"$GITHUB_ENV\" pattern; the step inherits the default success() condition, so it only runs when vcvarsall activation succeeds.
• Blast radius is minimal: non-Python toolchains (CMake, MSBuild) are unaffected; the change is purely additive and requires no opt-in from consumers.

Confidence Score: 4/5

Safe to merge — two env vars appended after a successful MSVC activation step; non-Python toolchains ignore them entirely.

The change is additive and narrowly scoped: it writes two well-documented distutils env vars unconditionally to every job that consumes the action. The env vars are harmless to CMake/MSBuild consumers, and the second step only fires when the first (vcvarsall) step succeeds. The action's top-level description is not updated to mention the new Python-specific behaviour, which may surprise consumers who scan the YAML header without reading step-level comments.

actions/setup-msvc-dev-cmd/action.yaml — the description field should reflect the new Python distutils side-effect

Important Files Changed

Filename	Overview
actions/setup-msvc-dev-cmd/action.yaml	Adds a second step that unconditionally appends DISTUTILS_USE_SDK=1 and MSSdk=1 to GITHUB_ENV after vcvarsall activation; no inputs or conditions guard the new step

Sequence Diagram

sequenceDiagram
    participant Job as GitHub Actions Job
    participant MSVC as ilammy/msvc-dev-cmd
    participant Env as $GITHUB_ENV
    participant Python as setuptools / distutils

    Job->>MSVC: Enable Developer Command Prompt
    MSVC-->>Env: Sets INCLUDE, LIB, LIBPATH, PATH, etc.
    MSVC-->>Job: success

    Job->>Env: "echo DISTUTILS_USE_SDK=1"
    Job->>Env: "echo MSSdk=1"

    Note over Python,Env: Later pip/setuptools build step
    Python->>Env: Read DISTUTILS_USE_SDK
    alt "DISTUTILS_USE_SDK == 1"
        Python->>Env: Trust existing INCLUDE / LIB / PATH
        Python-->>Job: Build succeeds
    else not set
        Python->>Python: Run own vswhere + registry probe
        Python-->>Job: MSVC 14.0+ required error
    end

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Comments Outside Diff (1)

1. actions/setup-msvc-dev-cmd/action.yaml, line 1-6 (link)

   Action description not updated

   The top-level description field still only references MSVC command-line configuration and ilammy/msvc-dev-cmd. Consumers scanning the action registry or reading the YAML for the first time won't know it now also injects Python-distutils environment variables. Adding a sentence about DISTUTILS_USE_SDK / MSSdk here would make the behaviour discoverable without needing to read the step comments.

   Prompt To Fix With AI

   This is a comment left during a code review.
   Path: actions/setup-msvc-dev-cmd/action.yaml
   Line: 1-6
   
   Comment:
   **Action description not updated**
   
   The top-level `description` field still only references MSVC command-line configuration and `ilammy/msvc-dev-cmd`. Consumers scanning the action registry or reading the YAML for the first time won't know it now also injects Python-distutils environment variables. Adding a sentence about `DISTUTILS_USE_SDK` / `MSSdk` here would make the behaviour discoverable without needing to read the step comments.
   
   How can I resolve this? If you propose a fix, please make it concise.

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
actions/setup-msvc-dev-cmd/action.yaml:1-6
**Action description not updated**

The top-level `description` field still only references MSVC command-line configuration and `ilammy/msvc-dev-cmd`. Consumers scanning the action registry or reading the YAML for the first time won't know it now also injects Python-distutils environment variables. Adding a sentence about `DISTUTILS_USE_SDK` / `MSSdk` here would make the behaviour discoverable without needing to read the step comments.

_{Reviews (1): Last reviewed commit: "setup-msvc-dev-cmd: also set DISTUTILS_U..." | Re-trigger Greptile}

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