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Return `Result` from all `generate_*` methods #2037

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	14s
build (1.56.0, wasm32-unknown-unknown)	1s
build (stable, thumbv7em-none-eabi)	1s
build (stable, wasm32-unknown-unknown)	1s
test (1.56.0)	14s
test (stable)	13s
minimal-versions / minimal-versions	43s
	1m 27s

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