checker: add a separate error msg for fail_if_immutable for anon fns (#18854)

vlib/v/checker/checker.v

@@ -685,8 +685,13 @@ fn (mut c Checker) fail_if_immutable(mut expr ast.Expr) (string, token.Pos) {
 			if mut expr.obj is ast.Var {
 				if !expr.obj.is_mut && !c.pref.translated && !c.file.is_translated
 					&& !c.inside_unsafe {
-					c.error('`${expr.name}` is immutable, declare it with `mut` to make it mutable',
+					if c.inside_anon_fn {
-						expr.pos)
+						c.error('the closure copy of `${expr.name}` is immutable, declare it with `mut` to make it mutable',
+							expr.pos)
+					} else {
+						c.error('`${expr.name}` is immutable, declare it with `mut` to make it mutable',
+							expr.pos)
+					}
 				}
 				expr.obj.is_changed = true
 				if expr.obj.typ.share() == .shared_t {

vlib/v/checker/tests/closure_copy_immutable_var_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/closure_copy_immutable_var_err.vv:5:3: error: the closure copy of `name` is immutable, declare it with `mut` to make it mutable
+    3 | 
+    4 |     fn [name] () {
+    5 |         name = 'Ivan'
+      |         ~~~~
+    6 |         println(name)
+    7 |     }()

vlib/v/checker/tests/closure_copy_immutable_var_err.vv

@@ -0,0 +1,8 @@
+fn main() {
+	mut name := 'John'
+
+	fn [name] () {
+		name = 'Ivan'
+		println(name)
+	}()
+}

vlib/v/checker/tests/closure_immutable.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/closure_immutable.vv:4:3: error: `a` is immutable, declare it with `mut` to make it mutable
+vlib/v/checker/tests/closure_immutable.vv:4:3: error: the closure copy of `a` is immutable, declare it with `mut` to make it mutable
     2 |     a := 1
     3 |     f1 := fn [a] () {
     4 |         a++
@@ -12,7 +12,7 @@ vlib/v/checker/tests/closure_immutable.vv:7:16: error: original `a` is immutable
       |                   ^
     8 |         a++
     9 |         println(a)
-vlib/v/checker/tests/closure_immutable.vv:13:3: error: `b` is immutable, declare it with `mut` to make it mutable
+vlib/v/checker/tests/closure_immutable.vv:13:3: error: the closure copy of `b` is immutable, declare it with `mut` to make it mutable
    11 |     mut b := 2
    12 |     f3 := fn [b] () {
    13 |         b++