Fix Rust compiler warnings

[iamyulong]

Summary

Fix Rust compiler warnings

[github-actions]

Benchmark for ce13d58

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Test	Base	PR	%
costing::bench_prepare_wasm	64.1±0.54ms	63.8±0.48ms	-0.47%
costing::decode_sbor	13.1±0.04µs	13.1±0.02µs	0.00%
costing::decode_sbor_bytes	36.8±0.25µs	36.8±0.03µs	0.00%
costing::deserialize_wasm	1238.2±1.65µs	1241.1±1.85µs	+0.23%
costing::instantiate_flash_loan	3.9±0.55ms	3.9±0.52ms	0.00%
costing::instantiate_radiswap	5.0±0.07ms	5.0±0.07ms	0.00%
costing::spin_loop	21.4±0.01ms	21.5±0.01ms	+0.47%
costing::validate_sbor_payload	26.6±0.02µs	26.8±0.06µs	+0.75%
costing::validate_sbor_payload_bytes	354.9±1.24ns	354.3±1.45ns	-0.17%
costing::validate_secp256k1	80.3±0.06µs	80.3±0.07µs	0.00%
costing::validate_wasm	34.7±0.08ms	34.7±0.05ms	0.00%
decimal::add/0	7.2±0.00ns	7.2±0.00ns	0.00%
decimal::add/rust-native	9.5±0.01ns	9.5±0.00ns	0.00%
decimal::add/wasmer	134.3±0.13ns	142.6±0.05ns	+6.18%
decimal::add/wasmer-call-native	521.7±0.11ns	519.5±0.21ns	-0.42%
decimal::add/wasmi	466.8±5.78ns	444.3±4.80ns	-4.82%
decimal::add/wasmi-call-native	3.3±0.01µs	3.3±0.02µs	0.00%
decimal::div/0	183.1±0.06ns	164.5±0.17ns	-10.16%
decimal::from_string/0	153.7±0.16ns	153.8±0.04ns	+0.07%
decimal::mul/0	148.0±0.11ns	127.9±0.09ns	-13.58%
decimal::mul/rust-native	134.2±0.04ns	133.8±0.11ns	-0.30%
decimal::mul/wasmer	1730.3±0.68ns	1730.7±0.37ns	+0.02%
decimal::mul/wasmer-call-native	660.4±0.10ns	659.5±0.86ns	-0.14%
decimal::mul/wasmi	26.8±0.04µs	26.9±0.02µs	+0.37%
decimal::mul/wasmi-call-native	3.4±0.02µs	3.5±0.02µs	+2.94%
decimal::pow/0	620.8±0.16ns	621.5±0.30ns	+0.11%
decimal::pow/rust-native	608.2±0.22ns	608.3±0.22ns	+0.02%
decimal::pow/wasmer	7.5±0.01µs	7.5±0.01µs	0.00%
decimal::pow/wasmer-call-native	1083.4±0.13ns	1080.0±0.42ns	-0.31%
decimal::pow/wasmi	127.9±0.10µs	128.0±0.07µs	+0.08%
decimal::pow/wasmi-call-native	3.4±0.01µs	3.3±0.01µs	-2.94%
decimal::root/0	9.3±0.00µs	9.5±0.00µs	+2.15%
decimal::sub/0	7.2±0.00ns	7.2±0.00ns	0.00%
decimal::to_string/0	489.0±0.29ns	489.3±0.25ns	+0.06%
precise_decimal::add/0	8.0±0.00ns	8.0±0.00ns	0.00%
precise_decimal::add/rust-native	10.4±0.01ns	10.4±0.02ns	0.00%
precise_decimal::add/wasmer	142.5±0.08ns	139.3±0.07ns	-2.25%
precise_decimal::add/wasmer-call-native	534.7±0.18ns	531.4±2.31ns	-0.62%
precise_decimal::add/wasmi	534.9±0.78ns	530.7±0.21ns	-0.79%
precise_decimal::add/wasmi-call-native	3.6±0.01µs	3.5±0.01µs	-2.78%
precise_decimal::div/0	268.1±0.05ns	261.4±0.25ns	-2.50%
precise_decimal::from_string/0	196.0±0.11ns	194.5±0.25ns	-0.77%
precise_decimal::mul/0	287.7±0.04ns	279.9±0.09ns	-2.71%
precise_decimal::mul/rust-native	262.0±0.10ns	262.4±0.07ns	+0.15%
precise_decimal::mul/wasmer	4.0±0.00µs	4.1±0.00µs	+2.50%
precise_decimal::mul/wasmer-call-native	833.6±0.17ns	831.5±0.56ns	-0.25%
precise_decimal::mul/wasmi	74.6±0.06µs	74.3±0.06µs	-0.40%
precise_decimal::mul/wasmi-call-native	3.8±0.01µs	3.9±0.02µs	+2.63%
precise_decimal::pow/0	1566.7±0.20ns	1567.5±0.37ns	+0.05%
precise_decimal::pow/rust-native	1270.4±0.35ns	1278.8±0.61ns	+0.66%
precise_decimal::pow/wasmer	19.2±0.01µs	19.2±0.01µs	0.00%
precise_decimal::pow/wasmer-call-native	2.0±0.00µs	2.0±0.00µs	0.00%
precise_decimal::pow/wasmi	359.0±0.17µs	358.6±0.08µs	-0.11%
precise_decimal::pow/wasmi-call-native	7.6±0.03µs	7.4±0.02µs	-2.63%
precise_decimal::root/0	62.2±0.01µs	61.0±0.02µs	-1.93%
precise_decimal::sub/0	8.3±0.00ns	8.3±0.01ns	0.00%
precise_decimal::to_string/0	742.5±0.20ns	743.8±0.51ns	+0.18%
schema::validate_payload	331.7±1.08µs	331.8±0.21µs	+0.03%
transaction::radiswap	5.2±0.04ms	5.2±0.04ms	0.00%
transaction::transfer	1677.4±3.20µs	1692.2±3.87µs	+0.88%
transaction_processing::prepare	2.4±0.00ms	2.4±0.00ms	0.00%
transaction_processing::prepare_and_decompile	6.1±0.01ms	6.2±0.01ms	+1.64%
transaction_processing::prepare_and_decompile_and_recompile	23.1±0.28ms	22.7±0.24ms	-1.73%
transaction_validation::validate_manifest	43.8±0.03µs	43.8±0.04µs	0.00%
transaction_validation::verify_bls_2KB	1010.6±28.12µs	1009.1±2.09µs	-0.15%
transaction_validation::verify_bls_32B	1016.0±8.22µs	1006.9±1.28µs	-0.90%
transaction_validation::verify_ecdsa	78.0±0.05µs	78.0±0.25µs	0.00%
transaction_validation::verify_ed25519	51.5±0.03µs	51.5±0.14µs	0.00%