add_custom_{command,target}: add genex support for COMMENT

Evaluate and expand generator expressions in the `COMMENT` argument of
the `add_custom_command()` and `add_custom_target()` commands.
This allows to include generator expressions, e.g. a targets location
$<TARGET_...> or the current configuration $<CONFIG>, in the build-time
messages.

Fixes #22507

Help/command/add_custom_command.rst

@@ -140,6 +140,10 @@ The options are:
   Display the given message before the commands are executed at
   build time.
 
+  .. versionadded:: 3.26
+    Arguments to ``COMMENT`` may use
+    :manual:`generator expressions <cmake-generator-expressions(7)>`.
+
 ``DEPENDS``
   Specify files on which the command depends.  Each argument is converted
   to a dependency as follows:

Help/command/add_custom_target.rst

@@ -109,6 +109,10 @@ The options are:
   Display the given message before the commands are executed at
   build time.
 
+  .. versionadded:: 3.26
+    Arguments to ``COMMENT`` may use
+    :manual:`generator expressions <cmake-generator-expressions(7)>`.
+
 ``DEPENDS``
   Reference files and outputs of custom commands created with
   :command:`add_custom_command` command calls in the same directory

Help/release/dev/custom-command-comment-genex.rst

@@ -0,0 +1,6 @@
+custom-command-comment-genex
+----------------------------
+
+* :command:`add_custom_command` and :command:`add_custom_target` now
+  support :manual:`generator expressions <cmake-generator-expressions(7)>`
+  in their ``COMMENT`` option.

Source/cmCustomCommandGenerator.cxx

@@ -148,6 +148,14 @@ std::string EvaluateDepfile(std::string const& path,
   std::unique_ptr<cmCompiledGeneratorExpression> cge = ge.Parse(path);
   return cge->Evaluate(lg, config);
 }
+
+std::string EvaluateComment(const char* comment,
+                            cmGeneratorExpression const& ge,
+                            cmLocalGenerator* lg, std::string const& config)
+{
+  std::unique_ptr<cmCompiledGeneratorExpression> cge = ge.Parse(comment);
+  return cge->Evaluate(lg, config);
+}
 }
 
 cmCustomCommandGenerator::cmCustomCommandGenerator(
@@ -465,10 +473,17 @@ std::string cmCustomCommandGenerator::GetInternalDepfile() const
 
 cm::optional<std::string> cmCustomCommandGenerator::GetComment() const
 {
-  if (const char* comment = this->CC->GetComment()) {
+  const char* comment = this->CC->GetComment();
-    return comment;
+  if (!comment) {
+    return cm::nullopt;
   }
-  return cm::nullopt;
+  if (!*comment) {
+    return std::string();
+  }
+
+  cmGeneratorExpression ge(*this->LG->GetCMakeInstance(),
+                           this->CC->GetBacktrace());
+  return EvaluateComment(comment, ge, this->LG, this->OutputConfig);
 }
 
 std::string cmCustomCommandGenerator::GetWorkingDirectory() const

Tests/RunCMake/add_custom_command/CommentGenex-build-stdout.txt

@@ -0,0 +1 @@
+lorem ipsum, 01

Tests/RunCMake/add_custom_command/CommentGenex.cmake

@@ -0,0 +1,9 @@
+add_custom_target(helper)
+set_property(TARGET helper PROPERTY MY_TEXT "lorem ipsum")
+add_custom_command(
+  OUTPUT out.txt
+  COMMAND ${CMAKE_COMMAND} -E echo true
+  COMMENT "$<TARGET_PROPERTY:helper,MY_TEXT>$<COMMA> $<STREQUAL:foo,bar>$<EQUAL:42,42>"
+)
+set_property(SOURCE out.txt PROPERTY SYMBOLIC 1)
+add_custom_target(main ALL DEPENDS out.txt)

Tests/RunCMake/add_custom_command/RunCMakeTest.cmake

@@ -60,3 +60,12 @@ function(test_genex name)
 endfunction()
 
 test_genex(TargetGenexEvent)
+
+if(NOT RunCMake_GENERATOR STREQUAL "Xcode")
+  block()
+    run_cmake(CommentGenex)
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CommentGenex-build)
+    set(RunCMake_TEST_NO_CLEAN 1)
+    run_cmake_command(CommentGenex-build ${CMAKE_COMMAND} --build .)
+  endblock()
+endif()

Tests/RunCMake/add_custom_target/CommentGenex-build-stdout.txt

@@ -0,0 +1 @@
+lorem ipsum, 01

Tests/RunCMake/add_custom_target/CommentGenex.cmake

@@ -0,0 +1,6 @@
+add_custom_target(helper)
+set_property(TARGET helper PROPERTY MY_TEXT "lorem ipsum")
+add_custom_target(main ALL
+  COMMAND ${CMAKE_COMMAND} -E true
+  COMMENT "$<TARGET_PROPERTY:helper,MY_TEXT>$<COMMA> $<STREQUAL:foo,bar>$<EQUAL:42,42>"
+)

Tests/RunCMake/add_custom_target/RunCMakeTest.cmake

@@ -23,3 +23,12 @@ function(run_TargetOrder)
   run_cmake_command(TargetOrder-build ${CMAKE_COMMAND} --build . -- ${build_flags})
 endfunction()
 run_TargetOrder()
+
+if(NOT RunCMake_GENERATOR STREQUAL "Xcode")
+  block()
+    run_cmake(CommentGenex)
+    set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CommentGenex-build)
+    set(RunCMake_TEST_NO_CLEAN 1)
+    run_cmake_command(CommentGenex-build ${CMAKE_COMMAND} --build .)
+  endblock()
+endif()