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如何在稳定的Rust中编写返回RangeInclusive或其反向迭代器的函数?

traits rust

Richard Neumann · 技术社区 · 3 年前

我有这个代码

use std::iter::Step;

fn main() {
    for i in mkiter(25, 20) {
        println!("{}", i);
    }
    for i in mkiter(10, 23) {
        println!("{}", i);
    }
}

fn mkiter<T>(start: T, end: T) -> Box<dyn Iterator<Item = T>>
where
    T: Step,
{
    if start > end {
        Box::new((end..=start).rev())
    } else {
        Box::new(start..=end)
    }
}

然而,编译器告诉我正在使用一个不稳定的功能:

error[E0658]: use of unstable library feature 'step_trait': recently redesigned
 --> main.rs:1:5
  |
1 | use std::iter::Step;
  |     ^^^^^^^^^^^^^^^
  |
  = note: see issue #42168 <https://github.com/rust-lang/rust/issues/42168> for more information

error[E0658]: use of unstable library feature 'step_trait': recently redesigned
  --> main.rs:13:10
   |
13 | where T: Step
   |          ^^^^
   |
   = note: see issue #42168 <https://github.com/rust-lang/rust/issues/42168> for more information

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0658`.

但当我去掉这个特质时。。。

fn main() {
    for i in mkiter(25, 20) {
        println!("{}", i);
    }
    for i in mkiter(10, 23) {
        println!("{}", i);
    }
}

fn mkiter<T>(start: T, end: T) -> Box<dyn Iterator<Item = T>> {
    if start > end {
        Box::new((end..=start).rev())
    } else {
        Box::new(start..=end)
    }
}

…它想让我添加它,这让我很困惑:

error[E0369]: binary operation `>` cannot be applied to type `T`
  --> main.rs:11:14
   |
11 |     if start > end {
   |        ----- ^ --- T
   |        T
   |
help: consider restricting type parameter `T`
   |
10 | fn mkiter<T: std::cmp::PartialOrd>(start: T, end: T) -> Box<dyn Iterator<Item = T>> {
   |            ++++++++++++++++++++++

error[E0599]: the method `rev` exists for struct `RangeInclusive<T>`, but its trait bounds were not satisfied
   --> main.rs:12:32
    |
12  |         Box::new((end..=start).rev())
    |                                ^^^ method cannot be called on `RangeInclusive<T>` due to unsatisfied trait bounds
    |
   ::: /home/rne/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/range.rs:345:1
    |
345 | pub struct RangeInclusive<Idx> {
    | ------------------------------ doesn't satisfy `RangeInclusive<T>: Iterator`
    |
    = note: the following trait bounds were not satisfied:
            `T: Step`
            which is required by `RangeInclusive<T>: Iterator`
            `RangeInclusive<T>: Iterator`
            which is required by `&mut RangeInclusive<T>: Iterator`
help: consider restricting the type parameter to satisfy the trait bound
    |
10  | fn mkiter<T>(start: T, end: T) -> Box<dyn Iterator<Item = T>> where T: Step {
    |                                                               +++++++++++++

error[E0277]: the trait bound `T: Step` is not satisfied
  --> main.rs:14:9
   |
14 |         Box::new(start..=end)
   |         ^^^^^^^^^^^^^^^^^^^^^ the trait `Step` is not implemented for `T`
   |
   = note: required for `RangeInclusive<T>` to implement `Iterator`
   = note: required for the cast from `RangeInclusive<T>` to the object type `dyn Iterator<Item = T>`
help: consider restricting type parameter `T`
   |
10 | fn mkiter<T: std::iter::Step>(start: T, end: T) -> Box<dyn Iterator<Item = T>> {
   |            +++++++++++++++++

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0277, E0369, E0599.
For more information about an error, try `rustc --explain E0277`.

如何正确实现返回 RangeInclusive 或其 Rev 分别用于稳定Rust?中的泛型(整数)类型的迭代器?

# rustc --version                                                                                                                                                 2023-05-24 03:34:14
rustc 1.69.0 (84c898d65 2023-04-16)
# rustup update stable                                                                                                                                            2023-05-24 03:34:04
info: syncing channel updates for 'stable-x86_64-unknown-linux-gnu'

  stable-x86_64-unknown-linux-gnu unchanged - rustc 1.69.0 (84c898d65 2023-04-16)

info: self-update is disabled for this build of rustup
info: any updates to rustup will need to be fetched with your system package manager

1 回复 | 直到 3 年前

kmdreko 3 年前

要求 T: Step 只是达到目的的一种手段,你其实并不在乎 T 机具 Step ,你只在乎 RangeInclusive<T> 是一个迭代器,它产生 t 。你可以准确地表达:

fn mkiter<T>(start: T, end: T) -> Box<dyn Iterator<Item = T>>
where
    RangeInclusive<T>: Iterator<Item = T>,
...

编译器仍然抱怨 t 和 步 ,但这只是因为你需要遵循相同的原则 .rev() 通过约束工作 DoubleEndedIterator 以及:

fn mkiter<T>(start: T, end: T) -> Box<dyn Iterator<Item = T>>
where
    RangeInclusive<T>: Iterator<Item = T> + DoubleEndedIterator,
...

然后就是松散的部分,比如添加 T: PartialOrd 所以 start > end 工作,并添加一个生命周期以绕过 'static -默认返回 Box<dyn ...> 。这是最终代码:

fn mkiter<'a, T: 'a>(start: T, end: T) -> Box<dyn Iterator<Item = T> + 'a>
where
    T: PartialOrd,
    RangeInclusive<T>: Iterator<Item = T> + DoubleEndedIterator,
{
    if start > end {
        Box::new((end..=start).rev())
    } else {
        Box::new(start..=end)
    }
}