add_custom_command: Target-dependent generator expression support

OUTPUT variant with a TARGET given to allow resolving target-based generator
expressions wouldn't work because OUTPUT is resolved before generator targets
are created, i.e. FindGeneratorTargetToUse() returns nullptr.
This is a known limitation, see #21364.

Implements #21336.

Source/cmCustomCommand.cxx

@@ -150,3 +150,13 @@ void cmCustomCommand::SetCMP0116Status(cmPolicies::PolicyStatus cmp0116)
 {
   this->CMP0116Status = cmp0116;
 }
+
+const std::string& cmCustomCommand::GetTarget() const
+{
+  return this->Target;
+}
+
+void cmCustomCommand::SetTarget(const std::string& target)
+{
+  this->Target = target;
+}

Source/cmCustomCommand.h

@@ -100,6 +100,10 @@ public:
   cmPolicies::PolicyStatus GetCMP0116Status() const;
   void SetCMP0116Status(cmPolicies::PolicyStatus cmp0116);
 
+  /** Set/Get the associated target */
+  const std::string& GetTarget() const;
+  void SetTarget(const std::string& target);
+
 private:
   std::vector<std::string> Outputs;
   std::vector<std::string> Byproducts;
@@ -107,6 +111,7 @@ private:
   cmCustomCommandLines CommandLines;
   cmListFileBacktrace Backtrace;
   cmImplicitDependsList ImplicitDepends;
+  std::string Target;
   std::string Comment;
   std::string WorkingDirectory;
   std::string Depfile;

Source/cmCustomCommandGenerator.cxx

@@ -29,7 +29,7 @@ namespace {
 std::string EvaluateSplitConfigGenex(
   cm::string_view input, cmGeneratorExpression const& ge, cmLocalGenerator* lg,
   bool useOutputConfig, std::string const& outputConfig,
-  std::string const& commandConfig,
+  std::string const& commandConfig, cmGeneratorTarget const* target,
   std::set<BT<std::pair<std::string, bool>>>* utils = nullptr)
 {
   std::string result;
@@ -87,7 +87,7 @@ std::string EvaluateSplitConfigGenex(
     // Evaluate this genex in the selected configuration.
     std::unique_ptr<cmCompiledGeneratorExpression> cge =
       ge.Parse(std::string(genex));
-    result += cge->Evaluate(lg, *config);
+    result += cge->Evaluate(lg, *config, target);
 
     // Record targets referenced by the genex.
     if (utils) {
@@ -114,7 +114,8 @@ std::vector<std::string> EvaluateDepends(std::vector<std::string> const& paths,
     std::string const& ep =
       EvaluateSplitConfigGenex(p, ge, lg, /*useOutputConfig=*/true,
                                /*outputConfig=*/outputConfig,
-                               /*commandConfig=*/commandConfig);
+                               /*commandConfig=*/commandConfig,
+                               /*target=*/nullptr);
     cm::append(depends, cmExpandedList(ep));
   }
   for (std::string& p : depends) {
@@ -157,6 +158,7 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(
   : CC(&cc)
   , OutputConfig(crossConfig ? *crossConfig : config)
   , CommandConfig(std::move(config))
+  , Target(cc.GetTarget())
   , LG(lg)
   , OldStyle(cc.GetEscapeOldStyle())
   , MakeVars(cc.GetEscapeAllowMakeVars())
@@ -171,6 +173,8 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(
   }
 
   cmGeneratorExpression ge(cc.GetBacktrace());
+  cmGeneratorTarget const* target{ lg->FindGeneratorTargetToUse(
+    this->Target) };
 
   const cmCustomCommandLines& cmdlines = this->CC->GetCommandLines();
   for (cmCustomCommandLine const& cmdline : cmdlines) {
@@ -180,7 +184,7 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(
     for (std::string const& clarg : cmdline) {
       std::string parsed_arg = EvaluateSplitConfigGenex(
         clarg, ge, this->LG, useOutputConfig, this->OutputConfig,
-        this->CommandConfig, &this->Utilities);
+        this->CommandConfig, target, &this->Utilities);
       if (this->CC->GetCommandExpandLists()) {
         cm::append(argv, cmExpandedList(parsed_arg));
       } else {
@@ -249,9 +253,9 @@ cmCustomCommandGenerator::cmCustomCommandGenerator(
 
   const std::string& workingdirectory = this->CC->GetWorkingDirectory();
   if (!workingdirectory.empty()) {
-    this->WorkingDirectory =
-      EvaluateSplitConfigGenex(workingdirectory, ge, this->LG, true,
-                               this->OutputConfig, this->CommandConfig);
+    this->WorkingDirectory = EvaluateSplitConfigGenex(
+      workingdirectory, ge, this->LG, true, this->OutputConfig,
+      this->CommandConfig, target);
     // Convert working directory to a full path.
     if (!this->WorkingDirectory.empty()) {
       std::string const& build_dir = this->LG->GetCurrentBinaryDirectory();

Source/cmCustomCommandGenerator.h

@@ -25,6 +25,7 @@ class cmCustomCommandGenerator
   cmCustomCommand const* CC;
   std::string OutputConfig;
   std::string CommandConfig;
+  std::string Target;
   cmLocalGenerator* LG;
   bool OldStyle;
   bool MakeVars;

Source/cmLocalGenerator.cxx

@@ -4096,6 +4096,7 @@ void AddCustomCommandToTarget(cmLocalGenerator& lg,
   cc.SetDepfile(depfile);
   cc.SetJobPool(job_pool);
   cc.SetCMP0116Status(cmp0116);
+  cc.SetTarget(target->GetName());
   switch (type) {
     case cmCustomCommandType::PRE_BUILD:
       target->AddPreBuildCommand(std::move(cc));