ctest: Avoid unused-cli warning when propagating SITE/BUILDNAME

Commit 433721b7a7 (ctest: Pass CTEST_SITE/CTEST_BUILD_NAME to configure
step, 2026-06-15) updated ctest_configure() to pass `-DSITE=...` and
`-DBUILDNAME=...` to the configure command whenever CTEST_SITE or
CTEST_BUILD_NAME are set. Projects that don't include the CTest module never
reference these cache variables, causing CMake to report an unused-cli warning.

Use `-C <initial-cache>` instead of `-D` to seed SITE/BUILDNAME. Cache entries
set using `-C` are not tracked by CMake's unused-cli diagnostic, so projects
using the CTest module still get DartConfiguration.tcl populated correctly,
while projects that never reference SITE/BUILDNAME no longer see the warning.

Fixes: #27953

Source/CTest/cmCTestConfigureCommand.cxx

@@ -24,6 +24,7 @@
 #include "cmJSONState.h"
 #include "cmList.h"
 #include "cmMakefile.h"
+#include "cmOutputConverter.h"
 #include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
 #include "cmValue.h"
@@ -127,17 +128,29 @@ bool ConstructConfigureCommand(cmExecutionStatus& status, cmMakefile& mf,
 
   // Propagate CTEST_SITE / CTEST_BUILD_NAME into the SITE / BUILDNAME
   // cache variables so DartConfiguration.tcl gets generated with
-  // correct values.
+  // correct values. We use an initial cache script (-C) instead of
+  // defining them on the command-line (-D) to avoid unused-cli warnings
+  // for projects that don't include the CTest module.
   cmValue site = mf.GetDefinition("CTEST_SITE");
-  if (cmNonempty(site)) {
-    configureCommand += " \"-DSITE:STRING=";
-    configureCommand += *site;
-    configureCommand += "\"";
-  }
   cmValue buildName = mf.GetDefinition("CTEST_BUILD_NAME");
-  if (cmNonempty(buildName)) {
-    configureCommand += " \"-DBUILDNAME:STRING=";
-    configureCommand += *buildName;
+  if (cmNonempty(site) || cmNonempty(buildName)) {
+    std::string const initialCacheFile =
+      cmStrCat(cmSystemTools::CollapseFullPath(buildDirectory),
+               "/Testing/Temporary/CTestConfigureInitialCache.cmake");
+    cmGeneratedFileStream initialCache(initialCacheFile);
+    if (cmNonempty(site)) {
+      initialCache << "set(SITE " << cmOutputConverter::EscapeForCMake(*site)
+                   << " CACHE STRING \"\")\n";
+    }
+    if (cmNonempty(buildName)) {
+      initialCache << "set(BUILDNAME "
+                   << cmOutputConverter::EscapeForCMake(*buildName)
+                   << " CACHE STRING \"\")\n";
+    }
+    initialCache.Close();
+
+    configureCommand += " \"-C";
+    configureCommand += initialCacheFile;
     configureCommand += "\"";
   }
 

Tests/RunCMake/CTestCommandLine/RunCMakeTest.cmake

@@ -694,6 +694,33 @@ function(run_configure_empty_bindir)
 endfunction()
 run_configure_empty_bindir()
 
+# Verify that ctest_configure() does not trigger a spurious unused-cli
+# warning about BUILDNAME/SITE when CTEST_SITE/CTEST_BUILD_NAME are set
+# but the project does not include(CTest).  See issue #27953.
+function(run_configure_site_buildname_no_unused_cli)
+  set(src "${RunCMake_BINARY_DIR}/configure-site-buildname-no-unused-cli-src")
+  set(bin "${RunCMake_BINARY_DIR}/configure-site-buildname-no-unused-cli-bin")
+  file(REMOVE_RECURSE "${src}" "${bin}")
+  file(MAKE_DIRECTORY "${src}")
+  file(WRITE "${src}/CMakeLists.txt"
+    "cmake_minimum_required(VERSION 3.10)\nproject(Minimal LANGUAGES NONE)\n")
+  set(RunCMake_TEST_BINARY_DIR "${bin}")
+  set(RunCMake_TEST_NO_CLEAN 1)
+  ctest_source_dir_generator_args(generator_args)
+  set(RunCMake_TEST_NOT_EXPECT_stdout "CMake Warning \\(unused-cli\\)")
+  run_cmake_command(configure-site-buildname-no-unused-cli
+    ${CMAKE_CTEST_COMMAND}
+    --source-dir "${src}"
+    --build-dir  "${bin}"
+    ${generator_args}
+    -D "CTEST_SITE=Test Site"
+    -D "CTEST_BUILD_NAME=Test Build"
+    -T Configure
+    -V)
+  unset(RunCMake_TEST_NOT_EXPECT_stdout)
+endfunction()
+run_configure_site_buildname_no_unused_cli()
+
 # Verify expected error condition when --source-dir does not contain
 # a CMakeLists.txt file.
 function(run_configure_no_cmakelists)

Tests/RunCMake/CTestCommandLine/configure-site-buildname-no-unused-cli-stderr.txt

@@ -0,0 +1,2 @@
+^Cannot find file: [^
+]*/DartConfiguration\.tcl$