cmake: support setting the intermediate dir strategy from the environment

Help/envvar/CMAKE_INTERMEDIATE_DIR_STRATEGY.rst

@@ -0,0 +1,10 @@
+CMAKE_INTERMEDIATE_DIR_STRATEGY
+-------------------------------
+
+.. versionadded:: 4.2
+
+.. include:: include/ENV_VAR.rst
+
+``CMAKE_INTERMEDIATE_DIR_STRATEGY`` is a string specifying the strategy to use
+for target intermediate directories. It initializes the
+:variable:`CMAKE_INTERMEDIATE_DIR_STRATEGY` variable.

Help/manual/cmake-env-variables.7.rst

@@ -65,6 +65,7 @@ Environment Variables that Control the Build
    /envvar/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES_EXCLUDE
    /envvar/CMAKE_LANG_LINKER_LAUNCHER
    /envvar/CMAKE_MSVCIDE_RUN_PATH
+   /envvar/CMAKE_INTERMEDIATE_DIR_STRATEGY
    /envvar/CMAKE_NO_VERBOSE
    /envvar/CMAKE_OSX_ARCHITECTURES
    /envvar/CMAKE_TEST_LAUNCHER

Help/manual/cmake-variables.7.rst

@@ -261,6 +261,7 @@ Variables that Change Behavior
    /variable/CMAKE_MESSAGE_LOG_LEVEL
    /variable/CMAKE_MFC_FLAG
    /variable/CMAKE_MODULE_PATH
+   /variable/CMAKE_INTERMEDIATE_DIR_STRATEGY
    /variable/CMAKE_PKG_CONFIG_DISABLE_UNINSTALLED
    /variable/CMAKE_PKG_CONFIG_PC_LIB_DIRS
    /variable/CMAKE_PKG_CONFIG_PC_PATH

Help/variable/CMAKE_INTERMEDIATE_DIR_STRATEGY.rst

@@ -0,0 +1,28 @@
+CMAKE_INTERMEDIATE_DIR_STRATEGY
+-------------------------------
+
+.. versionadded:: 4.2
+
+``CMAKE_INTERMEDIATE_DIR_STRATEGY`` is a string cache variable specifying the
+strategy to use for target intermediate directories and their contents. The
+supported values are:
+
+- ``FULL``: Intermediate directories are named based on a
+  ``<TARGET_NAME>.dir`` pattern (with some slight deviations and sanitizations
+  applied in various places). Object file names are based on the filename of
+  the source file being compiled.
+- ``SHORT``: Intermediate directories are named from the hash of the target
+  name and the build directory location. Object file names are based on hashes
+  of the source file name to reduce path lengths. This may help with projects
+  that generate long paths in the build directory to support building in
+  directories other than those near a root path.
+
+When unset or the named strategy is not supported, the ``FULL`` strategy is
+used.
+
+.. note::
+  This only works as a cache variable, not a locally-scoped variable.
+
+.. note::
+  Not all generators support all strategies and paths may differ between
+  generators.

Source/cmake.cxx

@@ -933,6 +933,8 @@ void cmake::LoadEnvironmentPresets()
   readGeneratorVar("CMAKE_GENERATOR_INSTANCE", this->GeneratorInstance);
   readGeneratorVar("CMAKE_GENERATOR_PLATFORM", this->GeneratorPlatform);
   readGeneratorVar("CMAKE_GENERATOR_TOOLSET", this->GeneratorToolset);
+  this->IntermediateDirStrategy =
+    cmSystemTools::GetEnvVar("CMAKE_INTERMEDIATE_DIR_STRATEGY");
 }
 
 namespace {
@@ -2600,6 +2602,14 @@ int cmake::ActualConfigure()
                         "Name of generator toolset.", cmStateEnums::INTERNAL);
   }
 
+  if (!this->State->GetInitializedCacheValue(
+        "CMAKE_INTERMEDIATE_DIR_STRATEGY") &&
+      this->IntermediateDirStrategy) {
+    this->AddCacheEntry(
+      "CMAKE_INTERMEDIATE_DIR_STRATEGY", *this->IntermediateDirStrategy,
+      "Select the intermediate directory strategy", cmStateEnums::STRING);
+  }
+
   if (!this->State->GetInitializedCacheValue("CMAKE_TEST_LAUNCHER")) {
     cm::optional<std::string> testLauncher =
       cmSystemTools::GetEnvVar("CMAKE_TEST_LAUNCHER");

Source/cmake.h

@@ -768,6 +768,7 @@ protected:
   std::string GeneratorInstance;
   std::string GeneratorPlatform;
   std::string GeneratorToolset;
+  cm::optional<std::string> IntermediateDirStrategy;
   bool GeneratorInstanceSet = false;
   bool GeneratorPlatformSet = false;
   bool GeneratorToolsetSet = false;