checker: make undefined ident error for closures more friendly (#18100)

vlib/v/checker/checker.v

@@ -3440,6 +3440,18 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 				c.error(util.new_suggestion(node.name, const_names_in_mod).say('undefined ident: `${node.name}`'),
 					node.pos)
 			} else {
+				// If a variable is not found in the scope of an anonymous function
+				// but is in an external scope, then we can suggest the user add it to the capturing list.
+				if c.inside_anon_fn {
+					found_var := c.fn_scope.find_var(node.name)
+
+					if found_var != none {
+						c.error('`${node.name}` must be added to the capture list for the closure to be used inside',
+							node.pos)
+						return ast.void_type
+					}
+				}
+
 				c.error('undefined ident: `${node.name}`', node.pos)
 			}
 		}

vlib/v/checker/tests/anon_fn_arg_type_err.out

@@ -1,12 +1,12 @@
 vlib/v/checker/tests/anon_fn_arg_type_err.vv:6:14: error: use `_` to name an unused parameter
     4 |     mut i := 1
-    5 |
+    5 | 
     6 |     func := fn (i) int {
       |                 ^
     7 |         return i
     8 |     }
-vlib/v/checker/tests/anon_fn_arg_type_err.vv:7:10: error: undefined ident: `i`
-    5 |
+vlib/v/checker/tests/anon_fn_arg_type_err.vv:7:10: error: `i` must be added to the capture list for the closure to be used inside
+    5 | 
     6 |     func := fn (i) int {
     7 |         return i
       |                ^
@@ -14,7 +14,7 @@ vlib/v/checker/tests/anon_fn_arg_type_err.vv:7:10: error: undefined ident: `i`
     9 |
 vlib/v/checker/tests/anon_fn_arg_type_err.vv:10:15: error: cannot use `int` as `i` in argument 1 to `func`
     8 |     }
-    9 |
+    9 | 
    10 |     println(func(i) == 1)
       |                  ^
    11 | }

vlib/v/parser/tests/closure_not_declared.out

@@ -1,4 +1,4 @@
-vlib/v/parser/tests/closure_not_declared.vv:4:9: error: undefined ident: `a`
+vlib/v/parser/tests/closure_not_declared.vv:4:9: error: `a` must be added to the capture list for the closure to be used inside
     2 |     a := 1
     3 |     f := fn () {
     4 |         print(a)