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rust const fn what does it exactly mean(Rust Const Fn它的确切意思是什么)

翻译 作者:bug小助手 更新时间:2023-10-26 21:53:32 41 4
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Rust const fn


In the following article I saw the following:

在下面的文章中,我看到了以下几点:



A const fn allows you to execute code in a "const context." For
example:



const fn five() -> i32 {
5
}

const FIVE: i32 = five();

What does const fn exactly do in rust? Does it just say that the function can be completely be calculated at compile time (my guess based on reading the article)

const fn在rust中具体做什么?它是否只是说函数可以在编译时完全计算(我的猜测基于阅读文章)


更多回答

That's necessary, but not quite sufficient. See rust-lang.github.io/rfcs/0911-const-fn.html and reddit.com/r/rust/comments/iksmgk/psa_what_is_const_fn for more.

这是必要的,但还不够。有关更多信息,请参见rust-lang.githeb.io/rfcs/0911-const-fn.html和reddit.com/r/rust/comments/iksmgk/psa_what_is_const_fn。

优秀答案推荐

You are correct in your understanding. const fn is a feature in Rust that allows functions to be evaluated at compile time. This capability is essential for enabling users to use functions in const or static variables, which are guaranteed to have their values computed at compile time.

你的理解是正确的。常量FN是Rust中的一个功能,它允许在编译时计算函数。此功能对于使用户能够在常量或静态变量中使用函数至关重要,这些变量保证在编译时计算它们的值。


const fn functions have several key benefits and limitations:

常量FN函数有几个关键的优点和限制:


Benefits:

福利:



  • Compile-Time Evaluation: As you rightly pointed out, the primary benefit of const fn is that it enables compile-time evaluation. This means that the function's code is executed during compilation, and the result is embedded directly into the generated binary. This can lead to more efficient and optimized code.

  • Immutable Data: The values computed by const fn are immutable, making them suitable for use in various contexts where immutability is required, such as defining constant values or initializing static variables.

  • Type Safety: const fn functions are subject to Rust's strong type system, ensuring that the computed values are type-safe and adhere to the language's safety guarantees.


Limitations:

限制:



  • No I/O or Dynamic Computation: const fn functions cannot perform I/O operations, such as reading from files or interacting with external resources. Additionally, they cannot perform dynamic computations that depend on runtime inputs, as their inputs must also be known at compile time.

  • Limited Functionality: While const fn supports a wide range of operations, it has some limitations. For example, it cannot contain for loops, certain panicking operations, or calls to non-const fn functions. These restrictions ensure that the computations are deterministic and can be performed at compile time.

  • Recursive Complexity: Recursive const fn functions can be tricky to write and might lead to significant compile-time overhead, as the compiler must evaluate them at each level of recursion during compilation.


In summary, const fn in Rust empowers developers to perform computations at compile time, providing benefits like efficiency, immutability, and type safety. However, it comes with constraints to maintain predictability and reliability in compile-time execution. Understanding these limitations is crucial when using const fn in your Rust code

总之,Rust中的Const FN使开发人员能够在编译时执行计算,从而提供了诸如效率、不变性和类型安全等好处。但是,它附带了在编译时执行中维护可预测性和可靠性的约束。在Rust代码中使用Const Fn时,了解这些限制至关重要


更多回答

"function's code is executed during compilation, and the result is embedded directly into the generated binary" - not always. "The values computed by const fn are immutable" - just wrong. "Type Safety" - every function is type-safe. "they cannot perform dynamic computations that depend on runtime inputs" - wrong. "These restrictions ensure that the computations are deterministic and can be performed at compile time." - no, these restrictions (most of them at least) are artifical and expected to be lifted eventually.

“函数的代码在编译期间执行,结果直接嵌入生成的二进制文件”--并不总是这样。“由const fn计算的值是不变的”--这是错误的。“类型安全”--每个函数都是类型安全的。“它们不能执行依赖于运行时输入的动态计算”--错误。“这些限制确保了计算是确定性的,并且可以在编译时执行。”-不,这些限制(至少大部分)是人为的,预计最终会被取消。

"Recursive const fn functions can be tricky to write" - not anymore than normal recursive functions. "and might lead to significant compile-time overhead" - not more than loops.

“递归const fn函数可能很难写”--这并不比普通递归函数更难写。“并可能导致显著的编译时开销”--不超过循环。

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