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Return `Result` from all `generate_*` methods #2036

Job	Run time
build (1.56.0, thumbv7em-none-eabi)	3m 40s
build (1.56.0, wasm32-unknown-unknown)	3m 56s
build (stable, thumbv7em-none-eabi)	12s
build (stable, wasm32-unknown-unknown)	19s
test (1.56.0)	5m 16s
test (stable)	23s
minimal-versions / minimal-versions	58s
	14m 44s

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