vfmt: fix the formatting of comments after const declarations (#20005)

vlib/v/fmt/fmt.v

@@ -913,7 +913,7 @@ pub fn (mut f Fmt) const_decl(node ast.ConstDecl) {
 	} else {
 		ast.Node(ast.NodeError{})
 	}
-	for _, field in node.fields {
+	for fidx, field in node.fields {
 		if field.comments.len > 0 {
 			if f.should_insert_newline_before_node(ast.Expr(field.comments[0]), prev_field) {
 				f.writeln('')
@@ -936,7 +936,8 @@ pub fn (mut f Fmt) const_decl(node ast.ConstDecl) {
 		f.write('= ')
 		f.expr(field.expr)
 		f.comments(field.end_comments, same_line: true)
-		if node.is_block && field.end_comments.len == 0 {
+		if node.is_block && fidx < node.fields.len - 1 && node.fields.len > 1 {
+			// old style grouped consts, converted to the new style ungrouped const
 			f.writeln('')
 		} else {
 			// Write out single line comments after const expr if present

vlib/v/fmt/tests/consts_expected.vv

@@ -25,6 +25,12 @@ const i_am_a_very_long_constant_name_so_i_stand_alone_and_my_length_is_over_90_c
 
 pub const i_am_pub_const = true
 
+const abc = 1 // abc
+
+const def = 2 // def
+
+const ghi = 3 // ghi
+
 fn main() {
 	a := [
 		[3, 5, 6],

vlib/v/fmt/tests/consts_input.vv

@@ -17,7 +17,7 @@ another_const = ['a', 'b'
 		]
 multiline_const = [
 		'first line',  'second line','third line',
-  'fourth line']		
+  'fourth line']
 )
 
 const (
@@ -28,6 +28,12 @@ pub const (
 i_am_pub_const=true
 )
 
+const (
+	abc = 1 // abc
+	def = 2 // def
+	ghi = 3 // ghi
+)
+
 fn main() {
 	a := [
 		[3,

vlib/v/parser/parser.v

@@ -3838,6 +3838,9 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 			p.vet_notice('use a fixed array, instead of a dynamic one', pos.line_nr, vet.FixKind.unknown,
 				.default)
 		}
+		if is_block {
+			end_comments << p.eat_comments(same_line: true)
+		}
 		field := ast.ConstField{
 			name: full_name
 			mod: p.mod
@@ -3851,6 +3854,9 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 		fields << field
 		p.table.global_scope.register(field)
 		comments = []
+		if is_block {
+			end_comments = []
+		}
 		if !is_block {
 			break
 		}