if() command: add RULE operator

Help/command/if.rst

@@ -44,8 +44,8 @@ Compound conditions are evaluated in the following order of precedence:
 2. Unary tests such as:
 
   * The `Existence Checks`_ :cref:`COMMAND`, :cref:`DEFINED`,
-    :cref:`DIAGNOSTIC`, :cref:`EXISTS`, :cref:`POLICY`, :cref:`TARGET`, and
-    :cref:`TEST`.
+    :cref:`DIAGNOSTIC`, :cref:`EXISTS`, :cref:`POLICY`, :cref:`RULE`,
+    :cref:`TARGET`, and :cref:`TEST`.
   * The `File Operations`_ :cref:`IS_READABLE`, :cref:`IS_WRITABLE`,
     :cref:`IS_EXECUTABLE`, :cref:`IS_DIRECTORY`, :cref:`IS_SYMLINK`, and
     :cref:`IS_ABSOLUTE`.
@@ -142,6 +142,13 @@ Existence Checks
 
   True if the given name is an existing policy (of the form ``CMP<NNNN>``).
 
+.. signature:: if(RULE <rule-name>)
+
+  .. versionadded:: 4.5
+
+  True if the given name is an existing rule created by a call to the
+  :command:`add_custom_rule` command and is visible in the current directory.
+
 .. signature:: if(TARGET <target-name>)
 
   True if the given name is an existing logical target name created

Help/release/dev/if-RULE-operator.rst

@@ -0,0 +1,5 @@
+if-RULE-operator
+----------------
+
+* The :command:`if` command gained a ``RULE`` operator to check if a rule
+  exists.

Source/cmConditionEvaluator.cxx

@@ -53,6 +53,7 @@ auto const keyOR = "OR"_s;
 auto const keyParenL = "("_s;
 auto const keyParenR = ")"_s;
 auto const keyPOLICY = "POLICY"_s;
+auto const keyRULE = "RULE"_s;
 auto const keySTREQUAL = "STREQUAL"_s;
 auto const keySTRGREATER = "STRGREATER"_s;
 auto const keySTRGREATER_EQUAL = "STRGREATER_EQUAL"_s;
@@ -515,6 +516,12 @@ bool cmConditionEvaluator::HandleLevel1(cmArgumentList& newArgs, std::string&,
       newArgs.ReduceOneArg(
         cmPolicies::GetPolicyID(args.next->GetValue().c_str(), pid), args);
     }
+    // does a rule exist
+    else if (this->IsKeyword(keyRULE, *args.current)) {
+      newArgs.ReduceOneArg(
+        static_cast<bool>(this->Makefile.FindRuleToUse(args.next->GetValue())),
+        args);
+    }
     // does a target exist
     else if (this->IsKeyword(keyTARGET, *args.current)) {
       newArgs.ReduceOneArg(static_cast<bool>(this->Makefile.FindTargetToUse(

Tests/RunCMake/CustomRule/ExistenceCheck.cmake

@@ -0,0 +1,22 @@
+
+enable_language(C)
+
+if(RULE foo)
+  message(SEND_ERROR "RULE 'foo' unexpectedly found")
+endif()
+
+add_custom_rule(foo OUTPUT out COMMAND cmd)
+if(NOT RULE foo)
+  message(SEND_ERROR "RULE 'foo' unexpectedly not found")
+endif()
+
+add_library(foo)
+add_subdirectory(subdir2)
+if(RULE simple)
+  message(SEND_ERROR "RULE 'simple' from 'subdir2' unexpectedly found")
+endif()
+
+add_subdirectory(subdir1)
+if(NOT RULE simple)
+  message(SEND_ERROR "RULE 'simple' from 'subdir1' unexpectedly not found")
+endif()

Tests/RunCMake/CustomRule/RunCMakeTest.cmake

@@ -24,3 +24,5 @@ if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang|MSVC|SunPro|XL|HP")
   run_configure_and_build(Configurators)
   run_configure_and_build(DerivedRule)
 endif()
+
+run_cmake(ExistenceCheck)