checker: disallow indexing mut struct, passed as a fn parameter (#19992)

vlib/v/checker/checker.v

@@ -4352,18 +4352,26 @@ fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 		}
 	}
 
-	if (typ.is_ptr() && !typ.has_flag(.shared_f) && !node.left.is_auto_deref_var())
+	if (typ.is_ptr() && !typ.has_flag(.shared_f) && (!node.left.is_auto_deref_var()
+		|| (typ_sym.kind == .struct_ && typ_sym.name != 'array')))
 		|| typ.is_pointer() {
 		mut is_ok := false
+		mut is_mut_struct := false
 		if mut node.left is ast.Ident {
 			if mut node.left.obj is ast.Var {
 				// `mut param []T` function parameter
 				is_ok = node.left.obj.is_mut && node.left.obj.is_arg && !typ.deref().is_ptr()
+					&& typ_sym.kind != .struct_
+				// `mut param Struct`
+				is_mut_struct = node.left.obj.is_mut && node.left.obj.is_arg
+					&& typ_sym.kind == .struct_
 			}
 		}
 		if !is_ok && node.index is ast.RangeExpr {
 			s := c.table.type_to_str(typ)
 			c.error('type `${s}` does not support slicing', node.pos)
+		} else if is_mut_struct {
+			c.error('type `mut ${typ_sym.name}` does not support slicing', node.pos)
 		} else if !c.inside_unsafe && !is_ok && !c.pref.translated && !c.file.is_translated {
 			c.warn('pointer indexing is only allowed in `unsafe` blocks', node.pos)
 		}

vlib/v/checker/tests/index_sumtype_interface_struct_params_err.out

@@ -0,0 +1,34 @@
+vlib/v/checker/tests/index_sumtype_interface_struct_params_err.vv:12:9: warning: pointer indexing is only allowed in `unsafe` blocks
+   10 | 
+   11 |     _ = st1[0]
+   12 |     _ = st2[0]
+      |            ~~~
+   13 |     _ = st3[0]
+   14 | }
+vlib/v/checker/tests/index_sumtype_interface_struct_params_err.vv:8:9: error: type `Foo` does not support indexing
+    6 | 
+    7 | fn sum_type_or_interface_as_parameters(mut foo Foo, mut bar Bar, mut st1 Struct, st2 &Struct, st3 Struct) {
+    8 |     _ = foo[0]
+      |            ~~~
+    9 |     _ = bar[0]
+   10 |
+vlib/v/checker/tests/index_sumtype_interface_struct_params_err.vv:9:9: error: type `Bar` does not support indexing
+    7 | fn sum_type_or_interface_as_parameters(mut foo Foo, mut bar Bar, mut st1 Struct, st2 &Struct, st3 Struct) {
+    8 |     _ = foo[0]
+    9 |     _ = bar[0]
+      |            ~~~
+   10 | 
+   11 |     _ = st1[0]
+vlib/v/checker/tests/index_sumtype_interface_struct_params_err.vv:11:9: error: type `mut Struct` does not support slicing
+    9 |     _ = bar[0]
+   10 | 
+   11 |     _ = st1[0]
+      |            ~~~
+   12 |     _ = st2[0]
+   13 |     _ = st3[0]
+vlib/v/checker/tests/index_sumtype_interface_struct_params_err.vv:13:9: error: type `Struct` does not support indexing
+   11 |     _ = st1[0]
+   12 |     _ = st2[0]
+   13 |     _ = st3[0]
+      |            ~~~
+   14 | }

vlib/v/checker/tests/index_sumtype_interface_struct_params_err.vv

@@ -0,0 +1,14 @@
+type Foo = int | map[int]int
+
+interface Bar {}
+
+struct Struct {}
+
+fn sum_type_or_interface_as_parameters(mut foo Foo, mut bar Bar, mut st1 Struct, st2 &Struct, st3 Struct) {
+	_ = foo[0]
+	_ = bar[0]
+
+	_ = st1[0]
+	_ = st2[0]
+	_ = st3[0]
+}