Xcode: Avoid unnecessary duplication of custom commands across targets

Do not attach a custom command to a target if it is already attached to one of
the target's dependencies.  The command's output will be available by the time
the target needs it because the dependency containing the command will have
already been built.

Since commit fb45559e09 (Xcode: Process targets in depth-first order during
generation, 2018-07-19, v3.13.0-rc1~293^2) we generate a target only after
generating its dependencies.  Therefore when visiting the custom commands in a
target, we can assume that custom commands in its dependencies have already
been visited.

Source/cmGlobalXCodeGenerator.cxx

@@ -679,6 +679,7 @@ void cmGlobalXCodeGenerator::ClearXCodeObjects()
   this->FileRefs.clear();
   this->ExternalLibRefs.clear();
   this->FileRefToBuildFileMap.clear();
+  this->CommandsVisited.clear();
 }
 
 void cmGlobalXCodeGenerator::addObject(std::unique_ptr<cmXCodeObject> obj)
@@ -1271,6 +1272,16 @@ bool cmGlobalXCodeGenerator::CreateXCodeTarget(
     return true;
   }
 
+  auto& gtgt_visited = this->CommandsVisited[gtgt];
+  auto& deps = this->GetTargetDirectDepends(gtgt);
+  for (auto& d : deps) {
+    // Take the union of visited source files of custom commands so far.
+    // ComputeTargetOrder ensures our dependencies already visited their
+    // custom commands and updated CommandsVisited.
+    auto& dep_visited = this->CommandsVisited[d];
+    gtgt_visited.insert(dep_visited.begin(), dep_visited.end());
+  }
+
   if (gtgt->GetType() == cmStateEnums::UTILITY ||
       gtgt->GetType() == cmStateEnums::INTERFACE_LIBRARY ||
       gtgt->GetType() == cmStateEnums::GLOBAL_TARGET) {
@@ -1628,8 +1639,9 @@ void cmGlobalXCodeGenerator::CreateCustomCommands(
   }
   // add all the sources
   std::vector<cmCustomCommand> commands;
+  auto& visited = this->CommandsVisited[gtgt];
   for (auto sourceFile : classes) {
-    if (sourceFile->GetCustomCommand()) {
+    if (sourceFile->GetCustomCommand() && visited.insert(sourceFile).second) {
       commands.push_back(*sourceFile->GetCustomCommand());
     }
   }
@@ -1893,6 +1905,17 @@ void cmGlobalXCodeGenerator::CreateCustomRulesMakefile(
         ccg.AppendArguments(c, cmd);
         makefileStream << "\t" << cmd << "\n";
       }
+
+      // Symbolic inputs are not expected to exist, so add dummy rules.
+      for (auto const& dep : realDepends) {
+        if (cmSourceFile* dsf =
+              target->GetLocalGenerator()->GetMakefile()->GetSource(
+                dep, cmSourceFileLocationKind::Known)) {
+          if (dsf->GetPropertyAsBool("SYMBOLIC")) {
+            makefileStream << this->ConvertToRelativeForMake(dep) << ":\n";
+          }
+        }
+      }
     }
   }
 }

Source/cmGlobalXCodeGenerator.h

@@ -306,6 +306,8 @@ private:
   std::string GeneratorToolset;
   std::map<cmGeneratorTarget const*, size_t> TargetOrderIndex;
   std::vector<std::string> EnabledLangs;
+  std::map<cmGeneratorTarget const*, std::set<cmSourceFile const*>>
+    CommandsVisited;
 };
 
 #endif