diff --git a/contrib/auth_delay/auth_delay.c b/contrib/auth_delay/auth_delay.c
index d11dd1e416..ca1e483d61 100644
--- a/contrib/auth_delay/auth_delay.c
+++ b/contrib/auth_delay/auth_delay.c
@@ -68,7 +68,7 @@ _PG_init(void)
 							NULL,
 							NULL);
 
-	EmitWarningsOnPlaceholders("auth_delay");
+	MarkGUCPrefixReserved("auth_delay");
 
 	/* Install Hooks */
 	original_client_auth_hook = ClientAuthentication_hook;
diff --git a/contrib/auto_explain/auto_explain.c b/contrib/auto_explain/auto_explain.c
index 59ba63455f..a695fba0c5 100644
--- a/contrib/auto_explain/auto_explain.c
+++ b/contrib/auto_explain/auto_explain.c
@@ -231,7 +231,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	EmitWarningsOnPlaceholders("auto_explain");
+	MarkGUCPrefixReserved("auto_explain");
 
 	/* Install hooks. */
 	prev_ExecutorStart = ExecutorStart_hook;
diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
index 0289ea657c..8a2a90ad97 100644
--- a/contrib/pg_prewarm/autoprewarm.c
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -136,7 +136,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	EmitWarningsOnPlaceholders("pg_prewarm");
+	MarkGUCPrefixReserved("pg_prewarm");
 
 	RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState)));
 
diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
index 726ba59e2b..8b2a8bbfe6 100644
--- a/contrib/pg_stat_statements/pg_stat_statements.c
+++ b/contrib/pg_stat_statements/pg_stat_statements.c
@@ -437,7 +437,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	EmitWarningsOnPlaceholders("pg_stat_statements");
+	MarkGUCPrefixReserved("pg_stat_statements");
 
 	/*
 	 * Request additional shared resources.  (These are no-ops if we're not in
diff --git a/contrib/pg_trgm/trgm_op.c b/contrib/pg_trgm/trgm_op.c
index 0407c7dd64..e9b7981619 100644
--- a/contrib/pg_trgm/trgm_op.c
+++ b/contrib/pg_trgm/trgm_op.c
@@ -101,7 +101,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	EmitWarningsOnPlaceholders("pg_trgm");
+	MarkGUCPrefixReserved("pg_trgm");
 }
 
 /*
diff --git a/contrib/postgres_fdw/option.c b/contrib/postgres_fdw/option.c
index c2c4e36802..194c81ef35 100644
--- a/contrib/postgres_fdw/option.c
+++ b/contrib/postgres_fdw/option.c
@@ -532,5 +532,5 @@ _PG_init(void)
 							   NULL,
 							   NULL);
 
-	EmitWarningsOnPlaceholders("postgres_fdw");
+	MarkGUCPrefixReserved("postgres_fdw");
 }
diff --git a/contrib/sepgsql/hooks.c b/contrib/sepgsql/hooks.c
index 44a09c35a7..c0d977d4fa 100644
--- a/contrib/sepgsql/hooks.c
+++ b/contrib/sepgsql/hooks.c
@@ -455,7 +455,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 
-	EmitWarningsOnPlaceholders("sepgsql");
+	MarkGUCPrefixReserved("sepgsql");
 
 	/* Initialize userspace access vector cache */
 	sepgsql_avc_init();
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index f345eceb5b..d239004347 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -9360,11 +9360,15 @@ DefineCustomEnumVariable(const char *name,
 }
 
 /*
+ * Mark the given GUC prefix as "reserved".
+ *
+ * This prints warnings if there are any existing placeholders matching
+ * the prefix, and then prevents new ones from being created.
  * Extensions should call this after they've defined all of their custom
- * GUCs, to help catch misspelled config-file entries,
+ * GUCs, to help catch misspelled config-file entries.
  */
 void
-EmitWarningsOnPlaceholders(const char *className)
+MarkGUCPrefixReserved(const char *className)
 {
 	int			classLen = strlen(className);
 	int			i;
diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
index aa18d304ac..5ba95ced2c 100644
--- a/src/include/utils/guc.h
+++ b/src/include/utils/guc.h
@@ -354,7 +354,10 @@ extern void DefineCustomEnumVariable(const char *name,
 									 GucEnumAssignHook assign_hook,
 									 GucShowHook show_hook);
 
-extern void EmitWarningsOnPlaceholders(const char *className);
+extern void MarkGUCPrefixReserved(const char *className);
+
+/* old name for MarkGUCPrefixReserved, for backwards compatibility: */
+#define EmitWarningsOnPlaceholders(className) MarkGUCPrefixReserved(className)
 
 extern const char *GetConfigOption(const char *name, bool missing_ok,
 								   bool restrict_privileged);
diff --git a/src/pl/plperl/plperl.c b/src/pl/plperl/plperl.c
index 1c77211ac4..2497315e3b 100644
--- a/src/pl/plperl/plperl.c
+++ b/src/pl/plperl/plperl.c
@@ -453,7 +453,7 @@ _PG_init(void)
 							   PGC_SUSET, 0,
 							   NULL, NULL, NULL);
 
-	EmitWarningsOnPlaceholders("plperl");
+	MarkGUCPrefixReserved("plperl");
 
 	/*
 	 * Create hash tables.
diff --git a/src/pl/plpgsql/src/pl_handler.c b/src/pl/plpgsql/src/pl_handler.c
index 00aace2f39..55c39d208d 100644
--- a/src/pl/plpgsql/src/pl_handler.c
+++ b/src/pl/plpgsql/src/pl_handler.c
@@ -197,7 +197,7 @@ _PG_init(void)
 							   plpgsql_extra_errors_assign_hook,
 							   NULL);
 
-	EmitWarningsOnPlaceholders("plpgsql");
+	MarkGUCPrefixReserved("plpgsql");
 
 	plpgsql_HashTableInit();
 	RegisterXactCallback(plpgsql_xact_cb, NULL);
diff --git a/src/pl/tcl/pltcl.c b/src/pl/tcl/pltcl.c
index 7c045f4560..ab759833db 100644
--- a/src/pl/tcl/pltcl.c
+++ b/src/pl/tcl/pltcl.c
@@ -474,8 +474,8 @@ _PG_init(void)
 							   PGC_SUSET, 0,
 							   NULL, NULL, NULL);
 
-	EmitWarningsOnPlaceholders("pltcl");
-	EmitWarningsOnPlaceholders("pltclu");
+	MarkGUCPrefixReserved("pltcl");
+	MarkGUCPrefixReserved("pltclu");
 
 	pltcl_pm_init_done = true;
 }
diff --git a/src/test/modules/delay_execution/delay_execution.c b/src/test/modules/delay_execution/delay_execution.c
index 8ec623ac52..25722d87cc 100644
--- a/src/test/modules/delay_execution/delay_execution.c
+++ b/src/test/modules/delay_execution/delay_execution.c
@@ -91,7 +91,7 @@ _PG_init(void)
 							NULL,
 							NULL);
 
-	EmitWarningsOnPlaceholders("delay_execution");
+	MarkGUCPrefixReserved("delay_execution");
 
 	/* Install our hook */
 	prev_planner_hook = planner_hook;
diff --git a/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c b/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c
index 3ba33e501c..7c469fd57e 100644
--- a/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c
+++ b/src/test/modules/ssl_passphrase_callback/ssl_passphrase_func.c
@@ -49,7 +49,7 @@ _PG_init(void)
 							   NULL,
 							   NULL);
 
-	EmitWarningsOnPlaceholders("ssl_passphrase");
+	MarkGUCPrefixReserved("ssl_passphrase");
 
 	if (ssl_passphrase)
 		openssl_tls_init_hook = set_rot13;
diff --git a/src/test/modules/worker_spi/worker_spi.c b/src/test/modules/worker_spi/worker_spi.c
index adb02d8cb8..3e9227aa8a 100644
--- a/src/test/modules/worker_spi/worker_spi.c
+++ b/src/test/modules/worker_spi/worker_spi.c
@@ -322,7 +322,7 @@ _PG_init(void)
 							   0,
 							   NULL, NULL, NULL);
 
-	EmitWarningsOnPlaceholders("worker_spi");
+	MarkGUCPrefixReserved("worker_spi");
 
 	/* set up common data for all our workers */
 	memset(&worker, 0, sizeof(worker));