checker: check unsafe map index operation (#17000)

vlib/v/checker/assign.v

@@ -109,9 +109,11 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 			if left in [ast.Ident, ast.SelectorExpr] {
 				c.prevent_sum_type_unwrapping_once = true
 			}
-			c.inside_left_assign = true
+			if left is ast.IndexExpr {
+				c.is_index_assign = true
+			}
 			left_type = c.expr(left)
-			c.inside_left_assign = false
+			c.is_index_assign = false
 			c.expected_type = c.unwrap_generic(left_type)
 		}
 		if node.right_types.len < node.left.len { // first type or multi return types added above

vlib/v/checker/checker.v

@@ -111,7 +111,7 @@ mut:
 	inside_println_arg               bool
 	inside_decl_rhs                  bool
 	inside_if_guard                  bool // true inside the guard condition of `if x := opt() {}`
-	inside_left_assign               bool
+	is_index_assign                  bool
 	comptime_call_pos                int // needed for correctly checking use before decl for templates
 	goto_labels                      map[string]ast.GotoLabel // to check for unused goto labels
 }
@@ -3816,7 +3816,7 @@ fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 			node.pos)
 	}
 
-	if !c.inside_unsafe && !c.is_builtin_mod && !c.inside_if_guard && !c.inside_left_assign
+	if !c.inside_unsafe && !c.is_builtin_mod && !c.inside_if_guard && !c.is_index_assign
 		&& typ_sym.kind == .map && node.or_expr.stmts.len == 0 {
 		elem_type := c.table.value_type(typ)
 		if elem_type.is_real_pointer() {

vlib/v/checker/tests/map_index_reference_value.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/map_index_reference_value.vv:7:3: notice: accessing a pointer map value requires an `or{}` block outside `unsafe`
+    5 | fn main() {
+    6 |     mut m := map[string]&Foo{}
+    7 |     m['bar'].bar = 'bar'
+      |      ~~~~~~~
+    8 |     // m['bar'] << 'baz' // etc
+    9 | }
+vlib/v/checker/tests/map_index_reference_value.vv:7:11: error: field `bar` of struct `&Foo` is immutable
+    5 | fn main() {
+    6 |     mut m := map[string]&Foo{}
+    7 |     m['bar'].bar = 'bar'
+      |              ~~~
+    8 |     // m['bar'] << 'baz' // etc
+    9 | }

vlib/v/checker/tests/map_index_reference_value.vv

@@ -0,0 +1,9 @@
+struct Foo {
+	bar string
+}
+
+fn main() {
+	mut m := map[string]&Foo{}
+	m['bar'].bar = 'bar'
+	// m['bar'] << 'baz' // etc
+}