checker: add mutability check for comptime assignments (#18354)

vlib/v/checker/checker.v

@@ -642,6 +642,13 @@ fn (mut c Checker) fail_if_immutable(expr_ ast.Expr) (string, token.Pos) {
 			return '', expr.pos
 		}
 		ast.ComptimeSelector {
+			if mut expr.left is ast.Ident {
+				if mut expr.left.obj is ast.Var {
+					if expr.left.obj.ct_type_var != .generic_param {
+						c.fail_if_immutable(expr.left)
+					}
+				}
+			}
 			return '', expr.pos
 		}
 		ast.Ident {

vlib/v/checker/tests/comptime_assign_missing_mut_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/comptime_assign_missing_mut_err.vv:13:4: error: `res` is immutable, declare it with `mut` to make it mutable
+   11 |         val := doc.value(field.name)
+   12 |         $if field.typ is string {
+   13 |             res.$(field.name) = val.string()
+      |             ~~~
+   14 |         }
+   15 |     }

vlib/v/checker/tests/comptime_assign_missing_mut_err.vv

@@ -0,0 +1,21 @@
+import toml
+
+struct Person {
+	name string
+}
+
+fn decode[T](toml_str string) !T {
+	res := T{}
+	doc := toml.parse_text(toml_str)!.to_any()
+	$for field in T.fields {
+		val := doc.value(field.name)
+		$if field.typ is string {
+			res.$(field.name) = val.string()
+		}
+	}
+	return res
+}
+
+p_str := 'name = "John"'
+
+decode[Person](p_str)!