参数解析

可以使用模式匹配来解析简单的参数：

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如果你将程序命名为 match_args.rs 并使用 rustc match_args.rs 编译它，你可以按以下方式执行：

$ ./match_args Rust
This is not the answer.
$ ./match_args 42
This is the answer!
$ ./match_args do something
error: second argument not an integer
usage:
match_args <string>
    Check whether given string is the answer.
match_args {increase|decrease} <integer>
    Increase or decrease given integer by one.
$ ./match_args do 42
error: invalid command
usage:
match_args <string>
    Check whether given string is the answer.
match_args {increase|decrease} <integer>
    Increase or decrease given integer by one.
$ ./match_args increase 42
43