[Rust] updated SubGroup.generateDecoder() to resolve unnecessary cast warning in generated code #1020

Job	Run time
C++ Xcode (macOS)	1m 42s
C++ Clang 9 (Ubuntu 20.04)	1m 24s
C++ Clang 13 (Ubuntu 22.04)	1m 14s
C++ Clang 10 (Ubuntu 20.04)	1m 22s
C++ GCC 6 (Ubuntu 20.04)	1m 25s
C++ Clang 14 (Ubuntu 22.04)	1m 28s
C++ Clang 11 (Ubuntu 20.04)	1m 19s
C++ GCC 7 (Ubuntu 20.04)	1m 34s
C++ GCC 11 (Ubuntu 22.04)	1m 24s
C++ Clang 15 (Ubuntu 22.04)	1m 16s
C++ Clang 12 (Ubuntu 20.04)	1m 30s
C++ GCC 8 (Ubuntu 20.04)	1m 39s
C++ GCC 12 (Ubuntu 22.04)	1m 33s
C++ GCC 9 (Ubuntu 20.04)	1m 34s
C++ GCC 13 (Ubuntu 22.04)	1m 32s
C++ GCC 10 (Ubuntu 20.04)	1m 12s
C++ MSVC (Windows) (windows-latest)	5m 15s
C# 8.0.x	2m 31s
C++ Clang 16 (Ubuntu 22.04)	1m 21s
Golang 1.22.x	1m 9s
C++ Clang 17 (Ubuntu 22.04)	1m 23s
Java 8 (ubuntu-22.04)	1m 9s
Rust stable	48s
Java 8 (windows-latest)	3m 21s
Rust beta	54s
Java 17 (ubuntu-22.04)	1m 31s
Rust nightly	51s
Java 17 (windows-latest)	2m 20s
Java 21 (ubuntu-22.04)	1m 28s
Java 21 (windows-latest)	3m 27s
	50m 36s

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