parser: improve error for fixed array, when it has len and cap attributes in the initialisation list (#16120)

vlib/v/checker/tests/with_check_option/v_tictactoe.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/with_check_option/v_tictactoe.vv:4:31: error: expected `init:`, not `len`
+vlib/v/checker/tests/with_check_option/v_tictactoe.vv:4:31: error: `len` and `cap` are invalid attributes for fixed array dimension
     2 | 
     3 | fn new_board() [][]string {
     4 |     mut board := [3][]string{ len: 3, init: []string{ len: 3, init: '' } }

vlib/v/parser/containers.v

@@ -80,7 +80,12 @@ fn (mut p Parser) array_init() ast.ArrayInit {
 					pos := p.tok.pos()
 					n := p.check_name()
 					if n != 'init' {
-						p.error_with_pos('expected `init:`, not `$n`', pos)
+						if is_fixed {
+							p.error_with_pos('`len` and `cap` are invalid attributes for fixed array dimension',
+								pos)
+						} else {
+							p.error_with_pos('expected `init:`, not `$n`', pos)
+						}
 						return ast.ArrayInit{}
 					}
 					p.check(.colon)

vlib/v/parser/tests/fixed_arr_len_cap_attr_err.out

@@ -0,0 +1,6 @@
+vlib/v/parser/tests/fixed_arr_len_cap_attr_err.vv:2:31: error: `len` and `cap` are invalid attributes for fixed array dimension
+    1 | fn new_board() [][]string {
+    2 |     mut board := [3][]string{ len: 3, init: []string{ len: 3, init: '' } }
+      |                               ~~~
+    3 |     for i in 0..9 {
+    4 |         board[i / 3][i % 3] = (i + 1).str()

vlib/v/parser/tests/fixed_arr_len_cap_attr_err.vv

@@ -0,0 +1,7 @@
+fn new_board() [][]string {
+    mut board := [3][]string{ len: 3, init: []string{ len: 3, init: '' } }
+    for i in 0..9 {
+        board[i / 3][i % 3] = (i + 1).str()
+    }
+    return board
+}