ast, fmt: fix formatting type decl with anon struct (#19356)

vlib/v/ast/ast.v

@@ -379,11 +379,11 @@ pub:
 	generic_types []Type
 	is_pub        bool
 	// _pos fields for vfmt
-	mut_pos      int // mut:
+	mut_pos      int = -1 // mut:
-	pub_pos      int // pub:
+	pub_pos      int = -1 // pub:
-	pub_mut_pos  int // pub mut:
+	pub_mut_pos  int = -1 // pub mut:
-	global_pos   int // __global:
+	global_pos   int = -1 // __global:
-	module_pos   int // module:
+	module_pos   int = -1 // module:
 	language     Language
 	is_union     bool
 	attrs        []Attr

vlib/v/fmt/fmt.v

@@ -1502,6 +1502,17 @@ pub fn (mut f Fmt) alias_type_decl(node ast.AliasTypeDecl) {
 	if node.is_pub {
 		f.write('pub ')
 	}
+	// aliases of anon struct: `type foo = struct {}`
+	sym := f.table.sym(node.parent_type)
+	if sym.info is ast.Struct {
+		if sym.info.is_anon {
+			f.write('type ${node.name} = ')
+			f.write_anon_struct_field_decl(node.parent_type, ast.StructDecl{ fields: sym.info.fields })
+			f.comments(node.comments, has_nl: false)
+			f.mark_types_import_as_used(node.parent_type)
+			return
+		}
+	}
 	ptype := f.table.type_to_str_using_aliases(node.parent_type, f.mod2alias)
 	f.write('type ${node.name} = ${ptype}')
 

vlib/v/fmt/tests/type_decl_with_anon_struct_keep.vv

@@ -0,0 +1,15 @@
+pub type Unit1 = struct {}
+
+pub type Unit2 = struct {
+		name string
+		age  int
+	}
+
+pub const unit1 = Unit1{}
+
+pub const unit2 = Unit2{'bob', 22}
+
+fn main() {
+	println(unit1)
+	println(unit2)
+}