parser: support `module:` for immutable private struct fields (#8140)

vlib/v/ast/ast.v

@@ -194,9 +194,11 @@ pub:
 	name      string
 	gen_types []table.Type
 	is_pub    bool
+	// _pos fields for vfmt
 	mut_pos      int // mut:
 	pub_pos      int // pub:
 	pub_mut_pos  int // pub mut:
+	module_pos   int // module:
 	language     table.Language
 	is_union     bool
 	attrs        []table.Attr

vlib/v/fmt/fmt.v

@@ -695,6 +695,8 @@ pub fn (mut f Fmt) struct_decl(node ast.StructDecl) {
 			f.writeln('pub:')
 		} else if i == node.pub_mut_pos {
 			f.writeln('pub mut:')
+		} else if i == node.module_pos {
+			f.writeln('module:')
 		}
 		end_pos := field.pos.pos + field.pos.len
 		comments := field.comments

vlib/v/parser/struct.v

@@ -92,6 +92,7 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 	mut pub_pos := -1
 	mut pub_mut_pos := -1
 	mut global_pos := -1
+	mut module_pos := -1
 	mut is_field_mut := false
 	mut is_field_pub := false
 	mut is_field_global := false
@@ -156,6 +157,17 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 				is_field_pub = true
 				is_field_mut = true
 				is_field_global = true
+			} else if p.tok.kind == .key_module {
+				if module_pos != -1 {
+					p.error('redefinition of `module` section')
+					return {}
+				}
+				p.next()
+				p.check(.colon)
+				module_pos = fields.len
+				is_field_pub = false
+				is_field_mut = false
+				is_field_global = false
 			}
 			for p.tok.kind == .comment {
 				comments << p.comment()
@@ -319,6 +331,7 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 		mut_pos: mut_pos - embeds.len
 		pub_pos: pub_pos - embeds.len
 		pub_mut_pos: pub_mut_pos - embeds.len
+		module_pos: module_pos - embeds.len
 		language: language
 		is_union: is_union
 		attrs: attrs

vlib/v/parser/tests/struct_module_section.out

@@ -0,0 +1,12 @@
+vlib/v/parser/tests/struct_module_section.vv:16:3: error: field `i` of struct `S1` is immutable
+   14 | 
+   15 | mut s := S1{}
+   16 | s.i++
+      |   ^
+   17 | mut s2 := S2{}
+   18 | s2.j++
+vlib/v/parser/tests/struct_module_section.vv:18:4: error: field `j` of struct `S2` is immutable
+   16 | s.i++
+   17 | mut s2 := S2{}
+   18 | s2.j++
+      |    ^

vlib/v/parser/tests/struct_module_section.vv

@@ -0,0 +1,18 @@
+struct S1 {
+pub mut:
+	v byte
+module:
+	i int
+}
+
+struct S2 {
+module:
+	j int
+mut:
+	v byte
+}
+
+mut s := S1{}
+s.i++
+mut s2 := S2{}
+s2.j++