cgen: allow dump(unsafe{nil}) and dump(voidptr(123)) in the same program

vlib/v/gen/c/dumpexpr.v

@@ -82,11 +82,13 @@ fn (mut g Gen) dump_expr(node ast.DumpExpr) {
 }
 
 fn (mut g Gen) dump_expr_definitions() {
+	mut dump_already_generated_fns := map[string]bool{}
 	mut dump_typedefs := map[string]bool{}
 	mut dump_fns := strings.new_builder(100)
 	mut dump_fn_defs := strings.new_builder(100)
 	for dump_type, cname in g.table.dumps {
 		dump_sym := g.table.sym(dump_type)
+		// eprintln('>>> dump_type: $dump_type | cname: $cname | dump_sym: $dump_sym.name')
 		mut name := cname
 		if dump_sym.language == .c {
 			name = name[3..]
@@ -147,6 +149,13 @@ fn (mut g Gen) dump_expr_definitions() {
 		}
 		dump_fn_name := '_v_dump_expr_${name}' +
 			(if is_ptr { '_ptr'.repeat(typ.nr_muls()) } else { '' })
+
+		// protect against duplicate declarations:
+		if dump_already_generated_fns[dump_fn_name] {
+			continue
+		}
+		dump_already_generated_fns[dump_fn_name] = true
+
 		dump_fn_defs.writeln('${str_dumparg_ret_type} ${dump_fn_name}(string fpath, int line, string sexpr, ${str_dumparg_type} dump_arg);')
 		if g.writeln_fn_header('${str_dumparg_ret_type} ${dump_fn_name}(string fpath, int line, string sexpr, ${str_dumparg_type} dump_arg)', mut
 			dump_fns)

vlib/v/tests/dump_fns_test.v

@@ -67,3 +67,13 @@ fn test_dump_of_type_that_has_no_custom_str_method() {
 	ps := &StructWithoutStrMethod{456}
 	assert dump(ps).x == 456
 }
+
+fn test_nil_values_and_voidptr_values_can_be_dumped_in_the_same_program() {
+	// Note, that nil is its own type in the main v repo,
+	// while dump() generates `_v_dump_expr_voidptr` for *both* `nil` and `voidptr` values.
+	a := unsafe { nil }
+	b := voidptr(456)
+	dump(a)
+	dump(b)
+	assert true
+}