cgen: fix empty struct init using macro (#9787)

vlib/v/gen/c/cgen.v

@@ -5122,9 +5122,10 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 	}
 	// The rest of the fields are zeroed.
 	// `inited_fields` is a list of fields that have been init'ed, they are skipped
-	// mut nr_fields := 0
+	mut nr_fields := 1
 	if sym.kind == .struct_ {
 		info := sym.info as ast.Struct
+		nr_fields = info.fields.len
 		if info.is_union && struct_init.fields.len > 1 {
 			verror('union must not have more than 1 initializer')
 		}
@@ -5205,7 +5206,10 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 				}
 				g.write(field.name)
 			} else {
-				g.zero_struct_field(field)
+				if !g.zero_struct_field(field) {
+					nr_fields--
+					continue
+				}
 			}
 			if is_multiline {
 				g.writeln(',')
@@ -5220,7 +5224,11 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 	}
 
 	if !initialized {
-		g.write('0')
+		if nr_fields > 0 {
+			g.write('0')
+		} else {
+			g.write('EMPTY_STRUCT_INITIALIZATION')
+		}
 	}
 
 	g.write('}')
@@ -5231,19 +5239,26 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 	}
 }
 
-fn (mut g Gen) zero_struct_field(field ast.StructField) {
+fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
+	sym := g.table.get_type_symbol(field.typ)
+	if sym.kind == .struct_ {
+		info := sym.info as ast.Struct
+		if info.fields.len == 0 {
+			return false
+		}
+	}
 	field_name := c_name(field.name)
 	g.write('.$field_name = ')
-	sym := g.table.get_type_symbol(field.typ)
 	if field.has_default_expr {
 		if sym.kind in [.sum_type, .interface_] {
 			g.expr_with_cast(field.default_expr, field.default_expr_typ, field.typ)
-			return
+			return true
 		}
 		g.expr(field.default_expr)
 	} else {
 		g.write(g.type_default(field.typ))
 	}
+	return true
 }
 
 // fn (mut g Gen) zero_struct_fields(info ast.Struct, inited_fields map[string]int) {

vlib/v/gen/c/cheaders.v

@@ -179,7 +179,7 @@ string _STR_TMP(const char *fmt, ...) {
 	c_common_macros                 = '
 #define EMPTY_VARG_INITIALIZATION 0
 #define EMPTY_STRUCT_DECLARATION
-#define EMPTY_STRUCT_INITIALIZATION 0
+#define EMPTY_STRUCT_INITIALIZATION
 // Due to a tcc bug, the length of an array needs to be specified, but GCC crashes if it is...
 #define EMPTY_ARRAY_OF_ELEMS(x,n) (x[])
 #define TCCSKIP(x) x
@@ -209,13 +209,13 @@ string _STR_TMP(const char *fmt, ...) {
 #endif
 #ifdef _MSC_VER
 	#undef __V_GCC__
+	#undef EMPTY_STRUCT_INITIALIZATION
+	#define EMPTY_STRUCT_INITIALIZATION 0
 #endif
 
 #ifdef __TINYC__
 	#undef EMPTY_STRUCT_DECLARATION
-	#undef EMPTY_STRUCT_INITIALIZATION
 	#define EMPTY_STRUCT_DECLARATION char _dummy
-	#define EMPTY_STRUCT_INITIALIZATION 0
 	#undef EMPTY_ARRAY_OF_ELEMS
 	#define EMPTY_ARRAY_OF_ELEMS(x,n) (x[n])
 	#undef __NOINLINE

vlib/v/tests/generics_with_nested_generics_fn_test.v

@@ -1,9 +1,7 @@
 struct NestedGeneric {
-	a int
 }
 
 struct Context {
-	b int
 }
 
 struct App {