Rust：如何從函數返回一個迭代器並使用它？

Question

我試圖通過在一個小日曆項目上工作來教自己鏽。

在這裏，我試圖生成一個約定在一個給定日期三個整月的日期列表。 我想返回一個可以迭代這些日期的迭代器。 這是我第一次嘗試：

fn three_months_range(tm: time::Tm) -> std::iter::Iterator<Item=time::Tm> { 
    let fpm: time::Tm = first_of_previous_month(&tm); 
    (0..) 
     .map(|i| fpm + time::Duration::days(i)) 
     .take_while(|&t| t.tm_mon != (tm.tm_mon + 2) % 12) 
} 

不幸的是，這並不編譯，我得到一個錯誤。

src/main.rs:49:40: 49:75 error: the trait `core::marker::Sized` is not implemented for the type `core::iter::Iterator<Item=time::Tm> + 'static` [E0277] 
src/main.rs:49 fn three_months_range(tm: time::Tm) -> std::iter::Iterator <Item=time::Tm> { 
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
src/main.rs:49:40: 49:75 help: run `rustc --explain E0277` to see a detailed explanation 
src/main.rs:49:40: 49:75 note: `core::iter::Iterator<Item=time::Tm> + 'static` does not have a constant size known at compile-time 
src/main.rs:49:40: 49:75 note: the return type of a function must have a statically known size 

「函數的返回類型必須具有靜態已知大小」。好的，經過一番研究，似乎解決方案是通過Box返回迭代器。我仍然想知道標準庫map,filter，take_while ...方法如何管理返回迭代器而不是盒子）。

嗯，這裏是第二次嘗試，成功編譯：

fn three_months_range(tm: time::Tm) -> Box<iter::Iterator<Item=time::Tm>> { 
    let fpm: time::Tm = first_of_previous_month(&tm); 
    Box::new(
     (0..) 
     .map(move |i| fpm + time::Duration::days(i)) 
     .take_while(move |&t| t.tm_mon != (tm.tm_mon + 2) % 12) 
    ) 
} 

不幸的是，我不管理使用這個迭代器。 例如，假設我想構建一個包含每個日期的月份日期（1,2,3，...，31,1,2，...，30,1,2，...）的向量。 31）：

let days_vec: Vec<u64> = 
    (*three_months_range(time::now_utc())) 
    .map(|&t: &time::Tm| t.tm_mday) 
    .collect(); 

→

src/main.rs:14:10: 14:42 error: the trait `core::marker::Sized` is not implemented for the type `core::iter::Iterator<Item=time::Tm> + 'static` [E0277] 
src/main.rs:14   .map(|&t: &time::Tm| t.tm_mday) 
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
src/main.rs:14:10: 14:42 help: run `rustc --explain E0277` to see a detailed explanation 
src/main.rs:14:10: 14:42 note: `core::iter::Iterator<Item=time::Tm> + 'static` does not have a constant size known at compile-time 
src/main.rs:15:10: 15:19 error: no method named `collect` found for type `core::iter::Map<core::iter::Iterator<Item=time::Tm> + 'static, [closure@src/main.rs:14:15: 14:40]>` in the current scope 
src/main.rs:15   .collect(); 
         ^~~~~~~~~ 
src/main.rs:15:10: 15:19 note: the method `collect` exists but the following trait bounds were not satisfied: `core::iter::Iterator<Item=time::Tm> : core::marker::Sized`, `[closure@src/main.rs:14:15: 14:40] : core::ops::FnMut<(time::Tm,)>`, `core::iter::Map<core::iter::Iterator<Item=time::Tm> + 'static, [closure@src/main.rs:14:15: 14:40]> : core::iter::Iterator` 
src/main.rs:14:10: 14:42 error: type mismatch: the type `[closure@src/main.rs:14:15: 14:40]` implements the trait `for<'r> core::ops::FnMut<(&'r time::Tm,)>`, but the trait `core::ops::FnMut<(time::Tm,)>` is required (expected struct `time::Tm`, found &-ptr) [E0281] 
src/main.rs:14   .map(|&t: &time::Tm| t.tm_mday) 
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
src/main.rs:14:10: 14:42 help: run `rustc --explain E0281` to see a detailed explanation 
src/main.rs:14:10: 14:42 error: type mismatch: the type `[closure@src/main.rs:14:15: 14:40]` implements the trait `for<'r> core::ops::FnOnce<(&'r time::Tm,)>`, but the trait `core::ops::FnOnce<(time::Tm,)>` is required (expected struct `time::Tm`, found &-ptr) [E0281] 
src/main.rs:14   .map(|&t: &time::Tm| t.tm_mday) 
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
src/main.rs:14:10: 14:42 help: run `rustc --explain E0281` to see a detailed explanation 
error: aborting due to 4 previous errors 

這是一個很大的錯誤。

那麼我在這裏做錯了什麼？

有沒有一種相對簡單的方法來轉換Rust中的迭代器和/或從函數中返回它們？

Answer 1

問題是您正試圖將Iterator從Box中刪除（因爲它不是特徵對象，因此不是Sized）。但Box是透明的，可以直接使用就可以了map：

let days_vec: Vec<u64> = 
    three_months_range(time::now_utc()) 
    .map(|&t: &time::Tm| t.tm_mday) 
    .collect(); 

注意，map函數需要按值，而不是採取論證的參考。因此，呼叫可以是這樣的：

.map(|t| t.tm_mday)