rust - 错误警告 "value assigned is never read"

Question

我收到这个警告:

 Compiling substrate-transaction-graph v2.0.0 (/home/niko/sub/substrate/core/transaction-pool/graph)
warning: value assigned to `altered_priority` is never read
   --> core/transaction-pool/graph/src/pool.rs:137:15
    |
137 |                         let mut altered_priority=priority;
    |                                 ^^^^^^^^^^^^^^^^
    |
    = note: #[warn(unused_assignments)] on by default
    = help: maybe it is overwritten before being read?

关于这段代码的编译:

match self.api.validate_transaction(at, xt.clone())? {
    TransactionValidity::Valid { priority, requires, provides, longevity } => {
        info!(target: "txpool","priority before alteration: priority={:?}",priority);
        let mut altered_priority=priority;
        altered_priority=1;
        Ok(base::Transaction {
            data: xt,
            bytes,
            hash,
            priority: altered_priority,
            requires,
            provides,
            valid_till: block_number.as_().saturating_add(longevity),
         })
     },
     TransactionValidity::Invalid(e) => {
         bail!(error::Error::from(error::ErrorKind::InvalidTransaction(e)))
     },
     TransactionValidity::Unknown(e) => {
         self.listener.write().invalid(&hash);
         bail!(error::Error::from(error::ErrorKind::UnknownTransactionValidity(e)))
    },
}

我已经添加了 log 消息，并且在转储变量之后我可以确认它们具有它们应该具有的值(在代码执行之后)，即 priority Transaction 结构中的字段确实发生了变化:

2019-05-13 21:41:17 priority before alteration: priority=107
2019-05-13 21:41:17 Map TX begins
2019-05-13 21:41:17 TX IS OK: Transaction { hash: 0x79832c9790aee4b199a046cce27e46bb7e941f38e41d25629c922c318cf7c3a2, priority: 1, valid_till: 18446744073709551615, bytes: 107, requires: [], provides: [d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d0200000000000000], data: 81ffd43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27df063b12602c33fa92df4895ab3ecd9f2ad72544bd4b55f1c6c91c8c107dba3654fd13ca5e81612a7fe011414ca604e8f99feb1ed35ce471361ee2c14defdc503080003000ca10f}
2019-05-13 21:41:17 TX priority: 1
2019-05-13 21:41:17 Ok(Transaction { hash: 0x79832c9790aee4b199a046cce27e46bb7e941f38e41d25629c922c318cf7c3a2, priority: 1, valid_till: 18446744073709551615, bytes: 107, requires: [], provides: [d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d0200000000000000], data: 81ffd43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27df063b12602c33fa92df4895ab3ecd9f2ad72544bd4b55f1c6c91c8c107dba3654fd13ca5e81612a7fe011414ca604e8f99feb1ed35ce471361ee2c14defdc503080003000ca10f})
2019-05-13 21:41:17 Map TX ends

实际上，即使不转储值，从代码本身也可以明显看出该值用于创建结构 Transaction。那么，这是 Rust 中的错误吗？

Answer 1

您写了 let mut altered_priority = priority;，紧接着是 altered_priority = 1;。编译器警告您，您在这两个语句的第一个语句中分配给 altered_priority 的初始值 priority 永远不会被读取。这是正确的:从 altered_priority 变量中读取的唯一值是 1，而不是 priority。

没有理由写一个值然后立即无条件地用另一个值替换它。您可以简单地用 let altered_priority = 1; 替换这两个语句。 (或者您可以去掉该变量并编写 priority: 1 而不是下面的 priority: altered_priority。)