checker: add check for mut ident but not if mut ident is for interfaces (#16214)

vlib/v/checker/if.v

@@ -353,6 +353,15 @@ fn (mut c Checker) smartcast_if_conds(node ast.Expr, mut scope ast.Scope) {
 							&& (node.left as ast.SelectorExpr).is_mut) {
 							c.fail_if_immutable(node.left)
 						}
+						// TODO: Add check for sum types in a way that it doesn't break a lot of compiler code
+						if node.left is ast.Ident
+							&& (left_sym.kind == .interface_ && right_sym.kind != .interface_) {
+							v := scope.find_var(node.left.name) or { &ast.Var{} }
+							if v.is_mut && !node.left.is_mut {
+								c.error('smart casting a mutable interface value requires `if mut $node.left.name is ...`',
+									node.left.pos)
+							}
+						}
 						if left_sym.kind in [.interface_, .sum_type] {
 							c.smartcast(node.left, node.left_type, right_type, mut scope)
 						}

vlib/v/checker/tests/if_smartcast_mut_var_interface_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/if_smartcast_mut_var_interface_err.vv:13:5: error: smart casting a mutable interface value requires `if mut foo_mut is ...`
+   11 | fn main() {
+   12 |     mut foo_mut := Foo(Bar{})
+   13 |     if foo_mut is Bar {
+      |        ~~~~~~~
+   14 |         println(json.encode(foo_mut))
+   15 |     }

vlib/v/checker/tests/if_smartcast_mut_var_interface_err.vv

@@ -0,0 +1,16 @@
+import json
+
+interface Foo {
+	a int
+}
+
+struct Bar {
+	a int
+}
+
+fn main() {
+	mut foo_mut := Foo(Bar{})
+	if foo_mut is Bar {
+		println(json.encode(foo_mut))
+	}
+}

vlib/v/checker/tests/incorrect_smartcast_err.out

@@ -5,6 +5,13 @@ vlib/v/checker/tests/incorrect_smartcast_err.vv:10:5: notice: smartcasting requi
       |        ~~~~~~~
    11 |         println(example.field)
    12 |     }
+vlib/v/checker/tests/incorrect_smartcast_err.vv:10:5: error: smart casting a mutable interface value requires `if mut example is ...`
+    8 | fn main(){
+    9 |     mut example := IExample(Example{field:"test"})
+   10 |     if example is Example{
+      |        ~~~~~~~
+   11 |         println(example.field)
+   12 |     }
 vlib/v/checker/tests/incorrect_smartcast_err.vv:11:19: error: type `IExample` has no field named `field`
     9 |     mut example := IExample(Example{field:"test"})
    10 |     if example is Example{

vlib/v/tests/interface_fields_test.v

@@ -26,12 +26,12 @@ fn use_interface(a Animal) {
 }
 
 fn mutate_interface(mut a Animal) {
-	if a is Cat {
+	if mut a is Cat {
 		a.breed = 'Siamese'
 	} else {
 		a.breed = 'Golden Retriever'
 	}
-	if a is Cat {
+	if mut a is Cat {
 		assert a.breed == 'Siamese'
 	} else {
 		assert a.breed == 'Golden Retriever'

vlib/v/tests/struct_embedding_with_interface_test.v

@@ -43,7 +43,7 @@ pub fn (mut ll LinearLayout) layout() string {
 	for mut l in ll.layoutables {
 		dump(l.type_name())
 		output += '$l.type_name()\n'
-		if l is Container {
+		if mut l is Container {
 			dump(l.type_name())
 			output += '$l.type_name()\n'
 		}