autofree: free strings on re-assignments

vlib/v/gen/cgen.v

@@ -1325,6 +1325,13 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 		ast.MatchExpr { return_type = it.return_type }
 		else {}
 	}
+	// Free the old value assigned to this string var (only if it's `str = [new value]`)
+	if g.pref.autofree && assign_stmt.op == .assign && assign_stmt.left_types.len == 1 &&
+		assign_stmt.left_types[0] == table.string_type && assign_stmt.left[0] is ast.Ident {
+		g.write('string_free(&')
+		g.expr(assign_stmt.left[0])
+		g.writeln('); // free str on re-assignment')
+	}
 	// json_test failed w/o this check
 	if return_type != table.void_type && return_type != 0 {
 		sym := g.table.get_type_symbol(return_type)

vlib/v/gen/fn.v

@@ -308,6 +308,7 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 		g.or_block(tmp_opt, node.or_block, node.return_type)
 		if is_gen_or_and_assign_rhs {
 			g.write('\n$cur_line$tmp_opt')
+			// g.insert_before_stmt('\n /* VVV */ $tmp_opt')
 		}
 	}
 }
@@ -544,10 +545,12 @@ fn (mut g Gen) fn_call(node ast.CallExpr) {
 			t := '_tt${g.tmp_count2}_arg_expr_${fn_name}_$i'
 			g.called_fn_name = name
 			str_expr := g.write_expr_to_string(arg.expr)
-			// g.insert_before_stmt('string $t = $str_expr; // new. to free $i ')
+			g.insert_before_stmt('string $t = $str_expr; // new3. to free $i ')
+			/*
 			cur_line = g.go_before_stmt(0)
 			// println('cur line ="$cur_line"')
 			g.writeln('string $t = $str_expr; // new. to free $i ')
+			*/
 		}
 	}
 	// Handle `print(x)`

vlib/v/tests/valgrind/1.strings_and_arrays.v

@@ -49,6 +49,11 @@ fn str_replace() {
 	println(r)
 }
 
+fn reassign_str() {
+	mut s := 'a' + 'b'
+	s = 'x' + 'y' // 'a' + 'b' must be freed before the re-assignment
+}
+
 fn match_expr() string {
 	x := 2
 	res := match x {
@@ -76,6 +81,7 @@ fn main() {
 	str_tmp_expr()
 	str_inter()
 	match_expr()
+	reassign_str()
 	// str_replace()
 	println('end')
 }