为什么从 u64 到 usize 的类型转换允许使用 `as` 而不是 `From`?

问题描述

使用 'as' 的第一个转换可以编译，但使用 'From' 特征的第二个转换不会:

The first conversion using 'as' compiles, but the second one using the 'From' trait does not:

fn main() {
    let a: u64 = 5;
    let b = a as usize;
    let b = usize::from(a);
}

使用 Rust 1.34.0，我收到以下错误:

Using Rust 1.34.0, I get the following error:

error[E0277]: the trait bound `usize: std::convert::From<u64>` is not satisfied
 --> src/main.rs:4:13
  |
4 |     let b = usize::from(a);
  |             ^^^^^^^^^^^ the trait `std::convert::From<u64>` is not implemented for `usize`
  |
  = help: the following implementations were found:
            <usize as std::convert::From<bool>>
            <usize as std::convert::From<std::num::NonZeroUsize>>
            <usize as std::convert::From<u16>>
            <usize as std::convert::From<u8>>
  = note: required by `std::convert::From::from`

当我将 u64 替换为 u8 时，不再出现错误.从错误消息中，我了解到 From 特征仅适用于 u8，而不适用于其他整数类型.

When I replace u64 with u8, there is no more error. From the error message, I understand that the From trait is implemented only for u8, but not for the other integer types.

如果有充分的理由，那么为什么使用 'as' 的转换也不应该编译失败?

If there is a good reason for that, then why shouldn't the conversion using 'as' should also fail to compile?

推荐答案

as 转换与 From 转换有着根本的不同.From 转换是简单 且安全的"，而 as 转换纯粹是安全的".考虑数字类型时，From 转换仅在保证输出相同 时存在，即没有信息丢失(没有截断或地板或精度损失).但是，as 类型转换没有这个限制.

as casts are fundamentally different from From conversions. From conversions are "simple and safe" whereas as casts are purely "safe". When considering numeric types, From conversions exist only when the output is guaranteed to be the same, i.e. there is no loss of information (no truncation or flooring or loss of precision). as casts, however, do not have this limitation.

引用文档，

[usize] 的大小是引用内存中任何位置需要多少字节.例如，在 32 位目标上，这是 4 个字节，而在 64 位目标上，这是 8 个字节."

由于大小取决于目标架构，并且无法在编译前确定，因此无法保证数字类型和 usize 之间的 From 转换是可能的.但是，as 转换将始终按照列出的规则运行 这里.

Since the size depends on the target architecture and cannot be determined before compilation, there is no guarantee that a From conversion between a numeric type and usize is possible. An as cast, however, will always operate by the rules listed here.

例如，在 32 位系统上，usize 等价于 u32.由于 usize 小于 u64，因此在将 u64 转换为 usize，因此 From 转换不存在.但是，usize 的大小始终保证为 8 位或更大，并且 u8 到 usize From转换永远存在.

For instance, on a 32-bit system, usize is equivalent to u32. Since a usize is smaller than a u64, there can be loss of information (truncation) when converting a u64 into a usize and hence a From conversion cannot exist. However, the size of a usize is always guaranteed to be 8 bits or greater and a u8 to usize From conversion will always exist.

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