Visual Studio minimum version and settings support

design/032-msvc_support.md

@@ -0,0 +1,85 @@
+# Proposal: Visual Studio minimum version and settings support
+
+
+| **Status**        |                                                   |
+|:------------------|:--------------------------------------------------|
+| **RFC #**         | [031](https://github.com/conan-io/tribe/pull/32)  |
+| **Submitted**     | 2022-02-11                                        |
+| **Tribe votes**   |                                                   |
+
+
+## Summary
+
+Conan 2.0 will implement Visual Studio compiler support via the ``msvc`` compiler setting, that
+will use the compiler version instead of the IDE version. The legacy ``Visual Studio`` compiler
+setting will be dropped.
+
+The supported versions will be:
+- Visual Studio 2017 will have tested (in Conan CI) support, both for the ``MSBuild`` and ``CMake`` integrations.
+- Visual Studio 2015 will have community-tested support both for ``MSBuild`` and ``CMake`` integrations.
+- Visual Studio 2012 will have community-tested support for ``CMake`` integrations.
+This support will be stable and not dropped. The necessary features to support toolsets like ``v141_xp`` will be added to ``msvc`` settings.
+
+
+## Motivation
+
+One of the first tribe proposals, the one proposing [Visual Studio 2017](https://github.com/conan-io/tribe/pull/5) as the minimum supported version in Conan 2.0 was dropped because of the
+requests to model the Conan settings after the compiler version and not the IDE version, as a more
+important action than defining the minimum supported version.
+
+The new ``msvc`` compiler setting has already been introduced as experimental in Conan 1.X and after
+a few iterations, it seems it is a valid approach, and it could be moved forward. Now it seems based
+on the accumulated experience, we could define some base Visual Studio support in Conan 2.0.
+
+This initial proposal was limited to 2017 tested support and 2015 community support via CMake, but has been expanded after some
+extra testing in https://github.com/conan-io/conan/pull/10808, and validation by some users.
+
+
+## Detailed Design
+
+The Conan built-in settings define the new compiler ``msvc`` as:
+
+```yaml
+    msvc:
+        version: [170, 180, 190, 191, 192, 193]
+        update: [None, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+        runtime: [static, dynamic]
+        runtime_type: [Debug, Release]
+        cppstd: [98, 14, 17, 20, 23]
+        toolset: [None, v110_xp, v120_xp, v140_xp, v141_xp]
+```
+
+The ``version`` fields matches the first 3 digits of the [msvc compiler version](https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B):
+- IDE versions 14.0 (VS 2015) map to compiler version 1900
+- IDE versions 14.1-14.16 (VS 2017) map to compiler version 1910-1916
+- IDE versions 14.20-14.29 (VS 2019) map to compiler version 1920-1929
+
+So as the ``version`` field, the matching digits that are stable for an IDE version are used. 
+This defines the "base" binary compatibility for Conan, the one used by default in ConanCenter:
+the binaries compiled with the same IDE version are compatible by default.
+
+For users wanting to opt-in into more granular binary compatibility, they can use the ``update``
+version number, which correspond to the last digit in the ``msvc`` compiler version.
+
+Also note that the runtime is now defined as "static" or "dynamic", and not as the raw MT/MD flags, 
+accordingly the ``runtime_type`` specifies the Debug/Release variant of this same runtime. Also, the
+``cppstd`` field is mandatory, even if it is the compiler default ``14``, it has to be specified in profiles.
+
+## Implementation details
+
+- The latest Conan 1.X release already implements ``msvc`` support for the new build system integrations.
+- The previous ``Visual Studio`` setting will be removed in Conan 2.0
+
+The integration of LLVM/Clang compilers via VS toolset might be provided by the ``compiler=clang`` setting and adding more information to that
+compiler, because the actual compiler is clang, not Microsoft cl, following the same rationale applied in the compiler version for ``msvc``, the
+important bit is the compiler not the IDE and not the build system.
+
+
+## Migration plan
+
+Conan 1.X already implements a path forward. There is default binary compatibility that maps betweeen the ``Visual Studio`` and ``msvc`` compiler settings. Users moving to ``msvc`` settings
+shouldn't need to rebuild immediately all binaries, but it can be incremental.
+
+Only the new build system integrations that will exist in Conan 2.0 fully implement ``msvc`` support, so this assumes moving to the new integrations is necessary. That means that the path to
+upgrade should be first, move to the new build system integrations, second move to the ``msvc``
+settings.