cgen: fix codegen to emit callexpr one time for in expr optimization

vlib/v/gen/c/infix.v

@@ -657,6 +657,7 @@ fn (mut g Gen) infix_expr_in_op(node ast.InfixExpr) {
 // and transform them in a series of equality comparison
 // i.e. `a in [1,2,3]` => `a == 1 || a == 2 || a == 3`
 fn (mut g Gen) infix_expr_in_optimization(left ast.Expr, left_type ast.Type, right ast.ArrayInit) {
+	tmp_var := if left is ast.CallExpr { g.new_tmp_var() } else { '' }
 	mut elem_sym := g.table.sym(right.elem_type)
 	left_parent_idx := g.table.sym(left_type).parent_idx
 	for i, array_expr in right.exprs {
@@ -719,13 +720,39 @@ fn (mut g Gen) infix_expr_in_optimization(left ast.Expr, left_type ast.Type, rig
 						g.write('${ptr_typ}_struct_eq(')
 					}
 				}
-				g.expr(left)
+				if left is ast.CallExpr {
+					if i == 0 {
+						line := g.go_before_last_stmt().trim_space()
+						g.empty_line = true
+						g.write('${g.styp(left.return_type)} ${tmp_var} = ')
+						g.expr(left)
+						g.writeln(';')
+						g.write2(line, tmp_var)
+					} else {
+						g.write(tmp_var)
+					}
+				} else {
+					g.expr(left)
+				}
 				g.write(', ')
 				g.expr(array_expr)
 				g.write(')')
 			}
 			else { // works in function kind
-				g.expr(left)
+				if left is ast.CallExpr {
+					if i == 0 {
+						line := g.go_before_last_stmt().trim_space()
+						g.empty_line = true
+						g.write('${g.styp(left.return_type)} ${tmp_var} = ')
+						g.expr(left)
+						g.writeln(';')
+						g.write2(line, tmp_var)
+					} else {
+						g.write(tmp_var)
+					}
+				} else {
+					g.expr(left)
+				}
 				g.write(' == ')
 				g.expr(array_expr)
 			}

vlib/v/slow_tests/inout/in_expr_callexpr.out

@@ -0,0 +1,4 @@
+t
+true
+t2
+true

vlib/v/slow_tests/inout/in_expr_callexpr.vv

@@ -0,0 +1,14 @@
+fn t() bool {
+	println(@FN)
+	return true
+}
+
+fn t2() int {
+	println(@FN)
+	return 2
+}
+
+fn main() {
+	println(t() in [false, true])
+	println(t2() in [0, 1, 2, 3, 4, 5])
+}