VS10: Avoid creating .rule files next to outputs (#13141)

Hide custom command .rule files inside the CMakeFiles directory.  Ensure
a short, deterministic, and unique name by using a hash of the directory
path containing the output file.  Preserve the file name so the entry in
the IDE is human-readable.  Clarify the comment that explains why the
rule file must be created on disk.

Source/cmGlobalVisualStudio10Generator.cxx

@@ -212,3 +212,21 @@ bool cmGlobalVisualStudio10Generator::Find64BitTools(cmMakefile* mf)
     return false;
     }
 }
+
+//----------------------------------------------------------------------------
+std::string
+cmGlobalVisualStudio10Generator
+::GenerateRuleFile(std::string const& output) const
+{
+  // The VS 10 generator needs to create the .rule files on disk.
+  // Hide them away under the CMakeFiles directory.
+  std::string ruleDir = this->GetCMakeInstance()->GetHomeOutputDirectory();
+  ruleDir += cmake::GetCMakeFilesDirectory();
+  ruleDir += "/";
+  ruleDir += cmSystemTools::ComputeStringMD5(
+    cmSystemTools::GetFilenamePath(output).c_str());
+  std::string ruleFile = ruleDir + "/";
+  ruleFile += cmSystemTools::GetFilenameName(output);
+  ruleFile += ".rule";
+  return ruleFile;
+}

Source/cmGlobalVisualStudio10Generator.h

@@ -75,6 +75,10 @@ public:
   virtual const char* GetCMakeCFGIntDir() const
     { return "$(Configuration)";}
   bool Find64BitTools(cmMakefile* mf);
+
+  /** Generate an <output>.rule file path for a given command output.  */
+  virtual std::string GenerateRuleFile(std::string const& output) const;
+
 protected:
   virtual const char* GetIDEVersion() { return "10.0"; }
 

Source/cmVisualStudio10TargetGenerator.cxx

@@ -459,7 +459,8 @@ cmVisualStudio10TargetGenerator::WriteCustomRule(cmSourceFile* source,
                                                  command)
 {
   std::string sourcePath = source->GetFullPath();
-  // the rule file seems to need to exist for vs10
+  // VS 10 will always rebuild a custom command attached to a .rule
+  // file that doesn't exist so create the file explicitly.
   if (source->GetPropertyAsBool("__CMAKE_RULE"))
     {
     if(!cmSystemTools::FileExists(sourcePath.c_str()))