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Simplify the code in optimized extend #267

Job	Run time
tests (1.51.0, serde, false, --skip spare_capacity_mut)	7m 29s
tests (stable, true, false)	4m 34s
tests (beta, serde, false)	1m 26s
tests (1.70.0, serde, false)	52s
tests (nightly, serde, borsh, zeroize, false)	2m 0s
nostd/thumbv6m-none-eabi/stable	55s
miri	2m 12s
	19m 28s