Fix noop_method_call warning in doctest #2301

Job	Run time
dependencies	27s
features (1.38)	1m 17s
features (nightly)	42s
test (1.38, ubuntu-latest)	2m 5s
test (1.38, windows-latest)	58s
test (stable, ubuntu-latest)	2m 0s
test (stable, windows-latest)	1m 58s
test (nightly, ubuntu-latest)	2m 5s
test (nightly, windows-latest)	1m 54s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	2m 5s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	2m 1s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	2m 1s
codegen	51s
clippy	32s
careful	2m 2s
docs	18s
loom	1m 26s
miri	2m 2s
rustfmt	17s
san	2m 3s
shellcheck	8s
ci	1s
	29m 13s

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