MinGW: Search for packages in standard MSYSTEM environment prefixes

MSYS2 and similar MinGW/MSYS distributions define development
environments with a `MSYSTEM` environment variable.  Each such
environment has a documented installation prefix for its packages,
often provided by a `MSYSTEM_PREFIX` environment variable.
Since commit 84a25fc263 (cmake_host_system_information: Add
MSYSTEM_PREFIX query, 2023-09-08) we can look up this prefix.

Add `$MSYSTEM_PREFIX/local` and `$MSYSTEM_PREFIX` to our system search
prefixes when targeting MinGW under `MSYSTEM` environments.  This is
their equivalent to `/usr/local` and `/usr`, which we search by default
on UNIX systems.

Issue: #24216

Help/release/dev/mingw-search-prefixes.rst

@@ -0,0 +1,6 @@
+mingw-search-prefixes
+---------------------
+
+* When using MinGW tools in a ``MSYSTEM`` environment on Windows,
+  the ``$MSYSTEM_PREFIX/local`` and ``$MSYSTEM_PREFIX`` prefixes are
+  now added to :variable:`CMAKE_SYSTEM_PREFIX_PATH`.

Help/variable/CMAKE_SYSTEM_PREFIX_PATH.rst

@@ -38,6 +38,12 @@ OpenBSD:
 Unix:
   * ``ENV{CONDA_PREFIX}`` when using a conda compiler
 
+MSYSTEM environment with MinGW toolchain:
+  .. versionadded:: 3.28
+
+  * ``ENV{MSYSTEM_PREFIX}/local``
+  * ``ENV{MSYSTEM_PREFIX}``
+
 Windows:
   * ``ENV{ProgramW6432}``
   * ``ENV{ProgramFiles}``

Modules/Platform/Windows-GNU.cmake

@@ -10,6 +10,25 @@ set(__WINDOWS_GNU 1)
 
 set(MINGW 1)
 
+# On Windows hosts, in MSYSTEM environments, search standard prefixes.
+if(CMAKE_HOST_WIN32)
+  # Bootstrap CMake does not have cmake_host_system_information.
+  if(COMMAND cmake_host_system_information)
+    cmake_host_system_information(RESULT _MSYSTEM_PREFIX QUERY MSYSTEM_PREFIX)
+  elseif(IS_DIRECTORY "$ENV{MSYSTEM_PREFIX}")
+    set(_MSYSTEM_PREFIX "$ENV{MSYSTEM_PREFIX}")
+  endif()
+
+  # Search this MSYSTEM environment's equivalent to /usr/local and /usr.
+  if(_MSYSTEM_PREFIX)
+    list(PREPEND CMAKE_SYSTEM_PREFIX_PATH "${_MSYSTEM_PREFIX}")
+    if(IS_DIRECTORY "${_MSYSTEM_PREFIX}/local")
+      list(PREPEND CMAKE_SYSTEM_PREFIX_PATH "${_MSYSTEM_PREFIX}/local")
+    endif()
+  endif()
+  unset(_MSYSTEM_PREFIX)
+endif()
+
 set(CMAKE_IMPORT_LIBRARY_PREFIX "lib")
 set(CMAKE_SHARED_LIBRARY_PREFIX "lib")
 set(CMAKE_SHARED_MODULE_PREFIX  "lib")

Tests/RunCMake/CMakeLists.txt

@@ -518,10 +518,10 @@ foreach(var
 endforeach()
 add_RunCMake_test(file-DOWNLOAD)
 add_RunCMake_test(file-RPATH -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME})
-add_RunCMake_test(find_file)
-add_RunCMake_test(find_library -DCYGWIN=${CYGWIN} -DMSYS=${MSYS})
-add_RunCMake_test(find_package -DMSYS=${MSYS})
-add_RunCMake_test(find_path)
+add_RunCMake_test(find_file -DMINGW=${MINGW})
+add_RunCMake_test(find_library -DMINGW=${MINGW} -DCYGWIN=${CYGWIN} -DMSYS=${MSYS})
+add_RunCMake_test(find_package -DMINGW=${MINGW} -DMSYS=${MSYS})
+add_RunCMake_test(find_path -DMINGW=${MINGW})
 add_RunCMake_test(find_program -DCMAKE_SYSTEM_NAME=${CMAKE_SYSTEM_NAME})
 add_RunCMake_test(foreach)
 add_RunCMake_test(function)

Tests/RunCMake/find_file/MSYSTEM_PREFIX-stdout.txt

@@ -0,0 +1 @@
+-- MSYSTEM_PREFIX_H='[^']*/Tests/RunCMake/find_file/MSYSTEM_PREFIX/include/msystem_prefix.h'

Tests/RunCMake/find_file/MSYSTEM_PREFIX.cmake

@@ -0,0 +1,5 @@
+set(ENV{MSYSTEM} "FAKEMSYS")
+set(ENV{MSYSTEM_PREFIX} "${CMAKE_CURRENT_LIST_DIR}/MSYSTEM_PREFIX")
+enable_language(C)
+find_file(MSYSTEM_PREFIX_H NAMES msystem_prefix.h)
+message(STATUS "MSYSTEM_PREFIX_H='${MSYSTEM_PREFIX_H}'")

Tests/RunCMake/find_file/RunCMakeTest.cmake

@@ -12,6 +12,10 @@ run_cmake(VALIDATOR-undefined-function)
 run_cmake(VALIDATOR-specify-macro)
 run_cmake(VALIDATOR)
 
+if(CMAKE_HOST_WIN32 AND MINGW)
+  run_cmake(MSYSTEM_PREFIX)
+endif()
+
 run_cmake_with_options(FromPATHEnvDebugVar --debug-find-var=PrefixInPATH_File)
 
 if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")

Tests/RunCMake/find_library/MSYSTEM_PREFIX-stdout.txt

@@ -0,0 +1 @@
+-- MSYSTEM_PREFIX_LIB='[^']*/Tests/RunCMake/find_library/MSYSTEM_PREFIX/lib/libmsystem_prefix.a'

Tests/RunCMake/find_library/MSYSTEM_PREFIX.cmake

@@ -0,0 +1,5 @@
+set(ENV{MSYSTEM} "FAKEMSYS")
+set(ENV{MSYSTEM_PREFIX} "${CMAKE_CURRENT_LIST_DIR}/MSYSTEM_PREFIX")
+enable_language(C)
+find_library(MSYSTEM_PREFIX_LIB NAMES msystem_prefix)
+message(STATUS "MSYSTEM_PREFIX_LIB='${MSYSTEM_PREFIX_LIB}'")

Tests/RunCMake/find_library/RunCMakeTest.cmake

@@ -20,6 +20,10 @@ run_cmake(VALIDATOR-undefined-function)
 run_cmake(VALIDATOR-specify-macro)
 run_cmake(VALIDATOR)
 
+if(CMAKE_HOST_WIN32 AND MINGW)
+  run_cmake(MSYSTEM_PREFIX)
+endif()
+
 run_cmake_script(FromScriptMode "-DTEMP_DIR=${RunCMake_BINARY_DIR}/FromScriptMode-temp")
 
 run_cmake_with_options(FromPATHEnvDebugVar --debug-find-var=CREATED_LIBRARY)

Tests/RunCMake/find_package/MSYSTEM_PREFIX-stdout.txt

@@ -0,0 +1 @@
+-- MsysPfx_DIR='[^']*/Tests/RunCMake/find_package/MSYSTEM_PREFIX'

Tests/RunCMake/find_package/MSYSTEM_PREFIX.cmake

@@ -0,0 +1,5 @@
+set(ENV{MSYSTEM} "FAKEMSYS")
+set(ENV{MSYSTEM_PREFIX} "${CMAKE_CURRENT_LIST_DIR}/MSYSTEM_PREFIX")
+enable_language(C)
+find_package(MsysPfx QUIET)
+message(STATUS "MsysPfx_DIR='${MsysPfx_DIR}'")

Tests/RunCMake/find_package/RunCMakeTest.cmake

@@ -72,6 +72,10 @@ run_cmake(REGISTRY_VIEW-no-view)
 run_cmake(REGISTRY_VIEW-wrong-view)
 run_cmake(REGISTRY_VIEW-propagated)
 
+if(CMAKE_HOST_WIN32 AND MINGW)
+  run_cmake(MSYSTEM_PREFIX)
+endif()
+
 if(CMAKE_HOST_WIN32)
   run_cmake(CMP0144-WARN-CaseInsensitive)
   run_cmake(CMP0144-OLD-CaseInsensitive)

Tests/RunCMake/find_path/MSYSTEM_PREFIX-stdout.txt

@@ -0,0 +1 @@
+-- MSYSTEM_PREFIX_INCLUDE_DIR='[^']*/Tests/RunCMake/find_path/MSYSTEM_PREFIX/include'

Tests/RunCMake/find_path/MSYSTEM_PREFIX.cmake

@@ -0,0 +1,5 @@
+set(ENV{MSYSTEM} "FAKEMSYS")
+set(ENV{MSYSTEM_PREFIX} "${CMAKE_CURRENT_LIST_DIR}/MSYSTEM_PREFIX")
+enable_language(C)
+find_path(MSYSTEM_PREFIX_INCLUDE_DIR NAMES msystem_prefix.h)
+message(STATUS "MSYSTEM_PREFIX_INCLUDE_DIR='${MSYSTEM_PREFIX_INCLUDE_DIR}'")

Tests/RunCMake/find_path/RunCMakeTest.cmake

@@ -12,6 +12,10 @@ run_cmake(VALIDATOR-undefined-function)
 run_cmake(VALIDATOR-specify-macro)
 run_cmake(VALIDATOR)
 
+if(CMAKE_HOST_WIN32 AND MINGW)
+  run_cmake(MSYSTEM_PREFIX)
+endif()
+
 if(APPLE)
   run_cmake(FrameworksWithSubdirs)
 endif()