FetchContent: Set policies for most commands

Previously, FetchContent was relying on the policies of the
includer being compatible with the module. That made the
module vulnerable to subtle problems where policy settings
might lead to unexpected behavior.

With this change, we now specify the policies for as much of
the module as we can without breaking backward compatibility.
Only a few specific implementation details of the
FetchContent_MakeAvailable() macro have to remain uncontrolled
due to the way those parts can invoke user or project code which
may be relying on the includer's policy settings (intentionally or not).

Modules/FetchContent.cmake

@@ -1148,6 +1148,12 @@ current working directory.
 
 #]=======================================================================]
 
+# Control policies for most of the things defined by this module. Only a few
+# FetchContent_MakeAvailable() implementation details are excluded for
+# backward compatibility reasons (see just after the endblock()).
+block(SCOPE_FOR POLICIES)
+cmake_policy(VERSION 3.29)
+
 include(${CMAKE_CURRENT_LIST_DIR}/ExternalProject/shared_internal_commands.cmake)
 
 #=======================================================================
@@ -2210,7 +2216,13 @@ macro(FetchContent_MakeAvailable)
         )
 
         set(__cmake_fcProvider_${__cmake_contentNameLower} YES)
-        cmake_language(EVAL CODE "${__cmake_providerCommand}(${__cmake_providerArgs})")
+
+        # The provider needs to see policies from our caller, so we need a
+        # helper macro defined outside our policy block. We pass through a
+        # variable name rather than variable contents to avoid any potential
+        # problems with parsing macro arguments.
+        set(__cmake_fcCode "${__cmake_providerCommand}(${__cmake_providerArgs})")
+        __FetchContent_MakeAvailable_eval_code(__cmake_fcCode)
 
         list(POP_BACK __cmake_fcCurrentVarsStack
           __cmake_contentNameLower
@@ -2221,6 +2233,7 @@ macro(FetchContent_MakeAvailable)
           unset(CMAKE_EXPORT_FIND_PACKAGE_NAME)
         endif()
 
+        unset(__cmake_fcCode)
         unset(__cmake_fcProvider_${__cmake_contentNameLower})
         unset(__cmake_providerArgs)
         unset(__cmake_addfpargs)
@@ -2255,7 +2268,9 @@ macro(FetchContent_MakeAvailable)
           ${__cmake_contentName}
           ${__cmake_contentNameLower}
         )
-        find_package(${__cmake_contentName} ${__cmake_fpArgs})
+        # We pass variable names rather than their contents so as to avoid any
+        # potential problems with macro argument parsing
+        __FetchContent_MakeAvailable_find_package(__cmake_contentName __cmake_fpArgs)
         list(POP_BACK __cmake_fcCurrentNameStack
           __cmake_contentNameLower
           __cmake_contentName
@@ -2312,7 +2327,12 @@ macro(FetchContent_MakeAvailable)
         if(__cmake_arg_SYSTEM)
           list(APPEND __cmake_add_subdirectory_args SYSTEM)
         endif()
-        add_subdirectory(${__cmake_add_subdirectory_args})
+
+        # We pass a variable name rather than its contents so as to avoid any
+        # potential problems with macro argument parsing. It's highly unlikely
+        # in this case, but still theoretically possible someone might try to
+        # use a directory name that looks like a CMake variable evaluation.
+        __FetchContent_MakeAvailable_add_subdirectory(__cmake_add_subdirectory_args)
 
         list(POP_BACK __cmake_fcCurrentVarsStack CMAKE_EXPORT_FIND_PACKAGE_NAME)
         if(CMAKE_EXPORT_FIND_PACKAGE_NAME STREQUAL "<<::VAR_NOT_SET::>>")
@@ -2344,3 +2364,21 @@ macro(FetchContent_MakeAvailable)
   unset(__cmake_original_verify_setting)
 
 endmacro()
+
+endblock()   # End of FetchContent module's policy scope
+
+# These are factored out here outside our policies block to preserve policy
+# settings of the scope from which FetchContent was included. Any project or
+# user code that actually relies on this is fragile and should enforce its own
+# policies instead, but we keep these here to preserve backward compatibility.
+macro(__FetchContent_MakeAvailable_eval_code code_var)
+  cmake_language(EVAL CODE "${${code_var}}")
+endmacro()
+
+macro(__FetchContent_MakeAvailable_find_package first_arg_var remaining_args_var)
+  find_package(${${first_arg_var}} ${${remaining_args_var}})
+endmacro()
+
+macro(__FetchContent_MakeAvailable_add_subdirectory args_var)
+  add_subdirectory(${${args_var}})
+endmacro()