checker: add a more helpful suggestion for fixed_array = [1,2,3]

Author: spytheman

vlib/v/checker/assign.v

@@ -775,7 +775,15 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 					} else {
 						if right_type_unwrapped != ast.void_type {
 							if !var_option || (var_option && right_type_unwrapped != ast.none_type) {
-								c.error('cannot assign to `${left}`: ${err.msg()}', right.pos())
+								if left_sym.kind == .array_fixed && right_sym.kind == .array
+									&& right is ast.ArrayInit {
+									c.add_error_detail('try `${left} = ${right}!` instead (with `!` after the array literal)')
+									c.error('cannot assign to `${left}`: ${err.msg()}',
+										right.pos())
+								} else {
+									c.error('cannot assign to `${left}`: ${err.msg()}',
+										right.pos())
+								}
 							}
 						}
 					}

vlib/v/checker/tests/assign_array_init_to_fixed_array_var.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/assign_array_init_to_fixed_array_var.vv:2:5: error: cannot assign to `a`: expected `[3]int`, not `[]int`
+    1 | mut a := [1, 2, 10]!
+    2 | a = [1, 2, 3]
+      |     ~~~~~~~~~
+    3 | dump(a)
+Details: try `a = [1, 2, 3]!` instead (with `!` after the array literal)

vlib/v/checker/tests/assign_array_init_to_fixed_array_var.vv

@@ -0,0 +1,3 @@
+mut a := [1, 2, 10]!
+a = [1, 2, 3]
+dump(a)