parser,ast,cgen: support nested unions with field union {, to impro…

…ve interoperability with C (similar to `field struct {`) (#23539)

vlib/v/ast/table.v

@@ -94,9 +94,11 @@ pub mut:
 	pointer_size      int
 	// cache for type_to_str_using_aliases
 	cached_type_to_str map[u64]string
-	anon_struct_names  map[string]int // anon struct name -> struct sym idx
-	// counter for anon struct, avoid name conflicts.
+	// counters and maps for anon structs and unions, to avoid name conflicts.
+	anon_struct_names   map[string]int // anon struct name -> struct sym idx
 	anon_struct_counter int
+	anon_union_names    map[string]int // anon union name -> union sym idx
+	anon_union_counter  int
 }
 
 // used by vls to avoid leaks
@@ -912,6 +914,11 @@ pub fn (mut t Table) register_anon_struct(name string, sym_idx int) {
 	t.anon_struct_names[name] = sym_idx
 }
 
+@[inline]
+pub fn (mut t Table) register_anon_union(name string, sym_idx int) {
+	t.anon_union_names[name] = sym_idx
+}
+
 pub fn (t &Table) known_type(name string) bool {
 	return t.type_idxs[name] != 0 || t.parsing_type == name || name in ['i32', 'byte']
 }

vlib/v/fmt/tests/union_with_nested_union_keep.vv

@@ -0,0 +1,8 @@
+union MyNested1 {
+	a            i32
+	b            i32
+	nested_union union {
+		c f32
+		d char
+	}
+}

vlib/v/gen/c/cgen.v

@@ -1413,6 +1413,10 @@ fn (mut g Gen) write_results() {
 		ck := c_name(k)
 		g.typedefs.writeln('typedef struct ${ck} ${ck};')
 	}
+	for k, _ in g.table.anon_union_names {
+		ck := c_name(k)
+		g.typedefs.writeln('typedef union ${ck} ${ck};')
+	}
 }
 
 fn (mut g Gen) find_or_register_shared(t ast.Type, base string) string {

vlib/v/parser/struct.v

@@ -37,8 +37,13 @@ fn (mut p Parser) struct_decl(is_anon bool) ast.StructDecl {
 		return ast.StructDecl{}
 	}
 	mut name := if is_anon {
-		p.table.anon_struct_counter++
-		'_VAnonStruct${p.table.anon_struct_counter}'
+		if is_union {
+			p.table.anon_union_counter++
+			'_VAnonUnion${p.table.anon_union_counter}'
+		} else {
+			p.table.anon_struct_counter++
+			'_VAnonStruct${p.table.anon_struct_counter}'
+		}
 	} else {
 		p.check_name()
 	}
@@ -241,8 +246,11 @@ fn (mut p Parser) struct_decl(is_anon bool) ast.StructDecl {
 				// struct field
 				field_name = p.check_name()
 				p.inside_struct_field_decl = true
-				if p.tok.kind == .key_struct
-					|| (p.tok.kind == .key_shared && p.peek_tok.kind == .key_struct) {
+				is_anon_struct := p.tok.kind == .key_struct
+					|| (p.tok.kind == .key_shared && p.peek_tok.kind == .key_struct)
+				is_anon_union := p.tok.kind == .key_union
+					|| (p.tok.kind == .key_shared && p.peek_tok.kind == .key_union)
+				if is_anon_struct || is_anon_union {
 					// Anon structs
 					field_is_shared := p.tok.kind == .key_shared
 					p.anon_struct_decl = p.struct_decl(true)
@@ -402,7 +410,11 @@ fn (mut p Parser) struct_decl(is_anon bool) ast.StructDecl {
 	}
 	mut ret := p.table.register_sym(sym)
 	if is_anon {
-		p.table.register_anon_struct(name, ret)
+		if is_union {
+			p.table.register_anon_union(name, ret)
+		} else {
+			p.table.register_anon_struct(name, ret)
+		}
 	}
 	// allow duplicate c struct declarations
 	if ret == -1 && language != .c {

vlib/v/tests/unions/nested_union_test.v

@@ -0,0 +1,28 @@
+union MyNested1 {
+mut:
+	a            i32
+	b            i32
+	nested_union union {
+	mut:
+		c f32
+		d char
+	}
+}
+
+fn test_nested_unions() {
+	mut m := MyNested1{}
+	unsafe {
+		m.a = 12
+		assert m.b == 12
+		println(m.a)
+		m.b = -99
+		assert m.b == -99
+		println(m.b)
+		m.nested_union.c = 3.14
+		assert m.nested_union.c == 3.14
+		println(m.nested_union.c)
+		m.nested_union.d = 88
+		println(int(m.nested_union.d))
+		assert int(m.nested_union.d) == 88
+	}
+}