fix async function #700

Job	Run time
rustfmt	13s
Test stable ubuntu-latest --no-default-features	2m 37s
Test stable ubuntu-latest	2m 49s
Test stable ubuntu-latest --all-features	3m 34s
Test stable macos-latest --no-default-features	7m 21s
Test stable macos-latest	4m 36s
Test stable macos-latest --all-features	8m 2s
Test stable windows-latest --no-default-features	6m 1s
Test stable windows-latest	6m 30s
Test stable windows-latest --all-features	6m 40s
Test beta ubuntu-latest --all-features	4m 3s
Test nightly ubuntu-latest --all-features	3m 16s
Release x86_64-unknown-linux-gnu	3m 27s
Release aarch64-unknown-linux-gnu	3m 29s
Release armv7-unknown-linux-gnueabihf	3m 27s
Release arm-unknown-linux-gnueabihf	4m 10s
Release x86_64-apple-darwin	3m 52s
Release aarch64-apple-darwin	3m 7s
Release x86_64-pc-windows-msvc	4m 39s
	1h 21m 53s

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