MSVC: cxx_runtime does not reach the std module build — /MD project against a /MT std.ifc fails with C2375

[Sunrisepeak]

What happens

With cxx_runtime = "host-coupled" in [build], the project's TUs are compiled
/MD but the std module interface they import was built /MT. cl warns and
then fails for real:

warning C5050: Possible incompatible environment while importing module 'std':
               _MSVC_MT is defined in module command line and not in current command line
warning C5050: Possible incompatible environment while importing module 'std':
               _MSVC_MD is defined in current command line and not in module command line

C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\corecrt_malloc.h(89):
    error C2375: 'free': redefinition; different linkage

Why it is not a stale artifact

I first assumed the CI cache. It is not:

• the cache key changed and there are no restore-keys, so ~/.mcpp came in cold
• the build log contains no command that builds std.ifc at all — it is
  served from mcpp's own global std cache
• the project TU command line does carry /MD, so cxx_runtime reached the
  project; it just did not reach std

So the std module is built with mcpp's default runtime regardless of what the
manifest asks for, and a host-coupled project can never agree with it on MSVC.

Expected

cxx_runtime is described as a contract for the produced artifact, and on MSVC
the runtime selection is an ABI-level property that the std module has to share
with everything importing it. Either the std module build should honour
cxx_runtime, or the std cache should be keyed by it so the two cannot diverge
silently.

Environment

• mcpp 2026.8.11.3 (installed via xlings install mcpp)
• windows-2025-vs2026 runner, MSVC 14.51.36231 (msvc@system)
• Repro: https://github.com/Sunrisepeak/xrgui/actions/runs/31593119218
  ([build] cxx_runtime = "host-coupled", cold cache)

Workaround in use

Setting cxx_runtime = "self-contained" — mcpp's own default, /MT on MSVC —
puts the project on the same runtime as the std module it is handed. That is
what this project does today, at the cost of differing from its xmake build,
which uses set_runtimes("MD").

[Sunrisepeak]

Sharper than the original report, and mcpp says it itself. Setting
cxx_runtime = "self-contained" produces:

warning: cxx_runtime: distributable target: cxx_runtime = "self-contained" is not
implemented for the MSVC runtime yet (it would need the /MT runtime);
using host-coupled — the artifact needs the VC++ redistributable

So on MSVC both settings end at the same place. Verified on cold caches:

cxx_runtime	project TUs	std module
host-coupled	/MD	/MT
self-contained	/MD (warned, falls back)	/MT

The project side is pinned to /MD by design-in-progress, and the std module is
handed over built /MT. Nothing in the manifest can bring them together, so any
MSVC project that does import std; and reaches a CRT declaration fails:

corecrt_malloc.h(89): error C2375: 'free': redefinition; different linkage

The inconsistency is internal to mcpp: the same build produces /MD TUs and a
/MT std.ifc. Until self-contained lands on MSVC, the std module presumably
wants to be built /MD to match where everything else already is.

Run: https://github.com/Sunrisepeak/xrgui/actions/runs/31594701447