CPS: Refactor getting file set destination for install

Modify `cmExportInstallPackageInfoGenerator::GetFileSetDirectory` to not
have side effects, moving that logic to the call site instead. This will
allow the method to be used in other contexts.

Also, tweak a bunch of methods called by this method to not
unnecessarily require mutable objects.

Source/cmExportInstallPackageInfoGenerator.cxx

@@ -220,21 +220,19 @@ std::string cmExportInstallPackageInfoGenerator::GetCxxModulesDirectory() const
 
 cm::optional<std::string>
 cmExportInstallPackageInfoGenerator::GetFileSetDirectory(
-  cmGeneratorTarget* gte, cmTargetExport const* te,
+  cmGeneratorTarget const* target, cmTargetExport const* targetExport,
   cmGeneratorFileSet const* fileSet, cm::optional<std::string> const& config)
 {
-  cmInstallFileSetGenerator::DestinationContext result =
-    te->FileSetGenerators.at(fileSet->GetName())
-      ->GetDestination(gte, config.value_or(""));
+  cmInstallFileSetGenerator const* const fsg =
+    targetExport->FileSetGenerators.at(fileSet->GetName());
+  cmInstallFileSetGenerator::DestinationContext const result =
+    fsg->GetDestination(target, config.value_or(""));
 
-  if (config && !result.HadContextSensitiveCondition) {
-    return {};
-  }
-  if (!config && result.HadContextSensitiveCondition) {
-    this->RequiresConfigFiles = true;
+  if (!config == result.HadContextSensitiveCondition) {
     return {};
   }
 
+  // Use cm::optional here to enable NRVO.
   cm::optional<std::string> dest = cmOutputConverter::EscapeForCMake(
     result.UnescapedDestination, cmOutputConverter::WrapQuotes::NoWrap);
 
@@ -267,8 +265,11 @@ bool cmExportInstallPackageInfoGenerator::GenerateFileSetProperties(
       this->GetFileSetDirectory(gte, te, fileSet, config);
 
     if (fileSet->GetType() == cm::FileSetMetadata::HEADERS) {
-      if (fileSetDirectory &&
-          !cm::contains(seenIncludeDirectories, *fileSetDirectory)) {
+      if (!fileSetDirectory) {
+        if (!config) {
+          this->RequiresConfigFiles = true;
+        }
+      } else if (!cm::contains(seenIncludeDirectories, *fileSetDirectory)) {
         component["includes"].append(*fileSetDirectory);
         seenIncludeDirectories.insert(*fileSetDirectory);
       }

Source/cmExportInstallPackageInfoGenerator.h

@@ -70,8 +70,8 @@ protected:
 
   std::string GetCxxModulesDirectory() const override;
 
-  cm::optional<std::string> GetFileSetDirectory(
-    cmGeneratorTarget* gte, cmTargetExport const* te,
+  static cm::optional<std::string> GetFileSetDirectory(
+    cmGeneratorTarget const* target, cmTargetExport const* targetExport,
     cmGeneratorFileSet const* fileSet,
     cm::optional<std::string> const& config = {});
 

Source/cmInstallFileSetGenerator.cxx

@@ -106,7 +106,7 @@ std::string cmInstallFileSetGenerator::GetDestination(
 }
 
 cmInstallFileSetGenerator::DestinationContext
-cmInstallFileSetGenerator::GetDestination(cmGeneratorTarget* gte,
+cmInstallFileSetGenerator::GetDestination(cmGeneratorTarget const* gte,
                                           std::string const& config) const
 {
   cmGeneratorExpression ge(*gte->Makefile->GetCMakeInstance());

Source/cmInstallFileSetGenerator.h

@@ -35,7 +35,7 @@ public:
     bool HadContextSensitiveCondition;
   };
   std::string GetDestination(std::string const& config) const;
-  DestinationContext GetDestination(cmGeneratorTarget* gt,
+  DestinationContext GetDestination(cmGeneratorTarget const* gt,
                                     std::string const& config) const;
   bool GetOptional() const { return this->Optional; }
   std::string GetFileSetName() const { return this->FileSetName; }

Source/cmInstallGenerator.cxx

@@ -230,8 +230,8 @@ std::string cmInstallGenerator::ConvertToAbsoluteDestination(
   return result;
 }
 
-void cmInstallGenerator::CheckAbsoluteDestination(std::string const& dest,
-                                                  cmLocalGenerator* lg) const
+void cmInstallGenerator::CheckAbsoluteDestination(
+  std::string const& dest, cmLocalGenerator const* lg) const
 {
   if (!cmSystemTools::FileIsFullPath(dest)) {
     return;

Source/cmInstallGenerator.h

@@ -58,7 +58,7 @@ public:
       installation script.  */
   static std::string ConvertToAbsoluteDestination(std::string const& dest);
   void CheckAbsoluteDestination(std::string const& dest,
-                                cmLocalGenerator* lg) const;
+                                cmLocalGenerator const* lg) const;
 
   /** Test if this generator installs something for a given configuration.  */
   bool InstallsForConfig(std::string const& config);