[rust] Skip unnecessary calls to find_among #116

Job	Run time
build (C distribution build, -O2 -Wall -W -std=c90 -Wdeclaration-after-statement -Werror)	28s
build (C distribution build (clang), -O2 -Wall -W -std=c90 -Wdeclaration-after-statement -Werror,...	1m 42s
build (C, y, -O2 -Wall -W -std=c99 -Werror)	28s
build (C (clang), y, -O2 -Wall -W -std=c99 -Werror, clang)	1m 18s
build (java, java, javac, -Xlint:all -Werror)	58s
build (go_old, ubuntu-22.04, golang-1.13, go)	1m 25s
build (go_new, ubuntu-24.04, golang-1.22, go)	1m 11s
build (javascript_node, node, nodejs)	1m 2s
build (rust, rust, rustc)	4m 45s
build (csharp, mcs, mono-devel)	53s
build (Pascal, fpc, fpc)	1m 33s
build (Python 3.8, python3.8, ubuntu-20.04, python3.8, 1)	19m 0s
build (Python 3.10, python3.10, ubuntu-22.04, python3.10, 1)	17m 8s
build (Python 3.12, python3.12, ubuntu-24.04, python3.12, 1)	14m 47s
build (Python (pypy3), pypy3, pypy3)	2m 47s
build (Ada, gprbuild, gnat gprbuild)	57s
build (Windows (C), windows-latest, y, sccache)	1m 38s
build (Windows (Go), windows-latest, go, mingw32-make, mingw-w64-ucrt-x86_64-go, sccache)	3m 45s
	1h 15m 45s

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