diff --git a/gc.c b/gc.c
index f4c8a948c9..1f161b81f4 100644
--- a/gc.c
+++ b/gc.c
@@ -9436,8 +9436,6 @@ gc_update_references(rb_objspace_t * objspace, rb_heap_t *heap)
     gc_update_table_refs(objspace, finalizer_table);
 }
 
-static VALUE type_sym(size_t type);
-
 static VALUE
 gc_compact_stats(rb_execution_context_t *ec, VALUE self)
 {
diff --git a/io.c b/io.c
index b4d477c038..5151114ba7 100644
--- a/io.c
+++ b/io.c
@@ -215,7 +215,6 @@ static VALUE sym_DATA;
 static VALUE sym_HOLE;
 #endif
 
-static VALUE rb_io_initialize(int argc, VALUE *argv, VALUE io);
 static VALUE prep_io(int fd, int fmode, VALUE klass, const char *path);
 
 struct argf {
diff --git a/marshal.c b/marshal.c
index d629a11046..ea43c62b94 100644
--- a/marshal.c
+++ b/marshal.c
@@ -1186,10 +1186,8 @@ static const rb_data_type_t load_arg_data = {
 };
 
 #define r_entry(v, arg) r_entry0((v), (arg)->data->num_entries, (arg))
-static VALUE r_entry0(VALUE v, st_index_t num, struct load_arg *arg);
 static VALUE r_object(struct load_arg *arg);
 static VALUE r_symbol(struct load_arg *arg);
-static VALUE path2class(VALUE path);
 
 NORETURN(static void too_short(void));
 static void