gen: fix evaluating the receiver of array methods (.map(), .filter(), etc) more than once (#7130)

vlib/v/gen/cgen.v

@@ -4767,16 +4767,14 @@ fn (mut g Gen) gen_array_map(node ast.CallExpr) {
 	if inp_sym.kind != .array {
 		verror('map() requires an array')
 	}
-	g.writeln('')
+	g.write('${g.typ(node.left_type)} ${tmp}_orig = ')
-	g.write('int ${tmp}_len = ')
 	g.expr(node.left)
-	g.writeln('.len;')
+	g.writeln(';')
+	g.write('int ${tmp}_len = ${tmp}_orig.len;')
 	g.writeln('$ret_typ $tmp = __new_array(0, ${tmp}_len, sizeof($ret_elem_type));')
 	i := g.new_tmp_var()
 	g.writeln('for (int $i = 0; $i < ${tmp}_len; ++$i) {')
-	g.write('\t$inp_elem_type it = (($inp_elem_type*) ')
+	g.write('\t$inp_elem_type it = (($inp_elem_type*) ${tmp}_orig.data)[$i];')
-	g.expr(node.left)
-	g.writeln('.data)[$i];')
 	g.write('\t$ret_elem_type ti = ')
 	expr := node.args[0].expr
 	match expr {
@@ -4919,14 +4917,13 @@ fn (mut g Gen) gen_array_filter(node ast.CallExpr) {
 	info := sym.info as table.Array
 	styp := g.typ(node.return_type)
 	elem_type_str := g.typ(info.elem_type)
-	g.write('\nint ${tmp}_len = ')
+	g.write('${g.typ(node.left_type)} ${tmp}_orig = ')
 	g.expr(node.left)
-	g.writeln('.len;')
+	g.writeln(';')
+	g.write('int ${tmp}_len = ${tmp}_orig.len;')
 	g.writeln('$styp $tmp = __new_array(0, ${tmp}_len, sizeof($elem_type_str));')
 	g.writeln('for (int i = 0; i < ${tmp}_len; ++i) {')
-	g.write('  $elem_type_str it = (($elem_type_str*) ')
+	g.writeln('  $elem_type_str it = (($elem_type_str*) ${tmp}_orig.data)[i];')
-	g.expr(node.left)
-	g.writeln('.data)[i];')
 	g.write('if (')
 	expr := node.args[0].expr
 	match expr {

vlib/v/tests/array_methods_test.v

@@ -0,0 +1,20 @@
+struct Counter {
+mut:
+	val int
+}
+
+// if this is called more than once, the test'll fail
+fn (mut c Counter) new_arr(msg string) []int {
+	if c.val > 0 { panic(msg) }
+	c.val++
+	return [1, 3, 2]
+}
+
+fn test_array_eval_count() {
+	// `new_arr()` should only be evaluated once, not on every iteration
+	mut a1 := Counter{}
+	assert a1.new_arr('map() failed').map(it * 2) == [2, 6, 4]
+
+	mut a2 := Counter{}
+	assert a2.new_arr('filter() failed').filter(it < 3) == [1, 2]
+}