Fix class field gc #488

Job	Run time
rust_ci (macos-latest, stable)	58s
rust_ci (macos-latest, beta)	55s
rust_ci (macos-latest, nightly)	1m 33s
rust_ci (ubuntu-latest, stable)	46s
rust_ci (ubuntu-latest, beta)	52s
rust_ci (ubuntu-latest, nightly)	1m 34s
rust_ci (windows-latest, stable)	2m 40s
rust_ci (windows-latest, beta)	1m 33s
rust_ci (windows-latest, nightly)	2m 46s
	13m 37s

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