fix code running problem #222

Job	Run time
Intra-doc links	4m 8s
Bitrot check	1m 45s
Build target wasm32-unknown-unknown	5m 5s
Book tests	3m 55s
Build target wasm32-wasi	4m 4s
Rustfmt	16s
Test on ubuntu-latest	9m 35s
Test on i686-unknown-linux-gnu	16m 23s
Test on windows-latest	12m 58s
Test on i686-unknown-linux-gnu with beta features	16m 7s
Test on macOS-latest	20m 17s
Test on i686-unknown-linux-gnu with nightly features	18m 54s
Test on ubuntu-latest with beta features	9m 49s
Test on windows-latest with beta features	12m 36s
Test on macOS-latest with beta features	11m 17s
Test on ubuntu-latest with nightly features	10m 51s
Test on windows-latest with nightly features	13m 56s
Test on macOS-latest with nightly features	12m 5s
	3h 4m 1s

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