Msvc static runtime (#2355)

* Document static MSVC runtime builds and test them in CI (#1655)

Use CMake's standard CMAKE_MSVC_RUNTIME_LIBRARY (policy CMP0091) instead of a
new option. Add a /MT leg for static-library builds to the MSVC CI job.

* docs: note CMAKE_MSVC_RUNTIME_LIBRARY needs CMake 3.15+

* docs: drop CMAKE_CXX_FLAGS /MT note

---------

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>

.github/workflows/build-and-test.yml

diff --git a/.github/workflows/build-and-test.yml b/.github/workflows/build-and-test.yml
index 5ef8386..9ac59bf 100644
--- a/.github/workflows/build-and-test.yml
+++ b/.github/workflows/build-and-test.yml
@@ -56,7 +56,7 @@
         run: ctest -C ${{ matrix.build_type }} -VV
 
   msvc:
-    name: ${{ matrix.os }}.${{ matrix.build_type }}.${{ matrix.lib }}.${{ matrix.msvc }}
+    name: ${{ matrix.os }}.${{ matrix.build_type }}.${{ matrix.lib }}.${{ matrix.crt }}.${{ matrix.msvc }}
     runs-on: ${{ matrix.os }}
     defaults:
         run:
@@ -73,7 +73,18 @@
         lib:
           - shared
           - static
+        crt:
+          - MD
+          - MT
+        exclude:
+          # a DLL with its own static CRT can't safely share CRT objects with the exe
+          - lib: shared
+            crt: MT
         include:
+          - crt: MD
+            runtime: 'MultiThreaded$<$<CONFIG:Debug>:Debug>DLL'
+          - crt: MT
+            runtime: 'MultiThreaded$<$<CONFIG:Debug>:Debug>'
           - msvc: VS-18-2025
             os: windows-2025
             generator: 'Visual Studio 18 2026'
@@ -94,6 +105,7 @@
           -G "${{ matrix.generator }}"
           -DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON
           -DBUILD_SHARED_LIBS=${{ matrix.lib == 'shared' }}
+          '-DCMAKE_MSVC_RUNTIME_LIBRARY=${{ matrix.runtime }}'
         env:
           RUNNER_WORKSPACE: ${{ runner.workspace }}
 

docs/platform_specific_build_instructions.md

diff --git a/docs/platform_specific_build_instructions.md b/docs/platform_specific_build_instructions.md
index 5c1439d..c26d43d 100644
--- a/docs/platform_specific_build_instructions.md
+++ b/docs/platform_specific_build_instructions.md
@@ -35,6 +35,14 @@
 
 When using the static library, make sure to add `BENCHMARK_STATIC_DEFINE` under `[Configuration Properties > C/C++ > Preprocessor > Preprocessor Definitions]`
 
+By default the library links against the DLL C runtime (`/MD`, `/MDd`). To use the static C runtime (`/MT`, `/MTd`) instead, set CMake's [`CMAKE_MSVC_RUNTIME_LIBRARY`](https://cmake.org/cmake/help/latest/variable/CMAKE_MSVC_RUNTIME_LIBRARY.html) (CMake 3.15 or newer) when configuring. It applies to `benchmark`, `benchmark_main` and the downloaded GoogleTest alike:
+
+```
+cmake -S . -B build -DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON -DCMAKE_MSVC_RUNTIME_LIBRARY="MultiThreaded$<$<CONFIG:Debug>:Debug>"
+```
+
+Combine it with the static library (`BUILD_SHARED_LIBS=OFF`), since a DLL with its own static C runtime can't safely share C runtime objects with the executable.
+
 Can also use the graphical version of CMake:
 * Open `CMake GUI`.
 * Under `Where to build the binaries`, same path as source plus `build`.