Fix noop_method_call warning in doctest #2300

Job	Run time
features (1.38)	1m 25s
features (nightly)	47s
test (1.38, ubuntu-latest)	1m 26s
test (1.38, windows-latest)	1m 17s
test (stable, ubuntu-latest)	1m 23s
test (stable, windows-latest)	1m 18s
test (nightly, ubuntu-latest)	1m 24s
test (nightly, windows-latest)	1m 3s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	1m 22s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	1m 24s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	1m 24s
careful	1m 24s
loom	1m 22s
dependencies	25s
codegen	42s
miri	1m 23s
rustfmt	20s
san	1m 21s
docs	15s
shellcheck	8s
clippy	35s
ci	1s
	22m 9s

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