Merge pull request #48 from rmja/upgrade-toolchain

Allow compilation on earlier rust versions

src/lib.rs

@@ -1,5 +1,8 @@
 #![cfg_attr(not(test), no_std)]
-#![allow(async_fn_in_trait)]
+#![allow(unknown_lints, async_fn_in_trait)]
+#![allow(stable_features)]
+#![feature(async_fn_in_trait)]
+#![feature(impl_trait_projections)]
 #![doc = include_str!("../README.md")]
 use core::{num::ParseIntError, str::Utf8Error};