rust - "Cannot infer type - type annotations needed"，但类型已知

Question

我一直在 LC 上练习，我打了 following issue .

pub fn largest_rectangle_area(heights: Vec<i32>) -> i32 {
    let mut widths = vec![(0usize, heights.len()); heights.len()];
    let mut stack = vec![];

    for (idx, &x) in heights.iter().enumerate() {
        while let Some(&pos) = stack.last() {
            if x >= heights[pos] {
                break;
            }

            widths[pos].1 = idx;
            stack.pop();
        }

        stack.push(idx);
    }

    todo!()
}

error[E0282]: type annotations needed
  --> src/lib.rs:11:13
   |
11 |             widths[pos].1 = idx;
   |             ^^^^^^^^^^^ cannot infer type
   |
   = note: type must be known at this point

更奇怪的是，如果我先执行 widths[pos] = (0, idx);，then the error disappears :

pub fn largest_rectangle_area(heights: Vec<i32>) -> i32 {
    let mut widths = vec![(0, heights.len()); heights.len()];
    let mut stack = vec![];

    for (idx, &x) in heights.iter().enumerate() {
        while let Some(&pos) = stack.last() {
            if x >= heights[pos] {
                break;
            }

            widths[pos] = (0, idx); // do a full update
            stack.pop();
        }

        stack.push(idx);
    }
    stack.clear();

    for (idx, &x) in heights.iter().enumerate().rev() {
        while let Some(&pos) = stack.last() {
            if x >= heights[pos] {
                break;
            }

            widths[pos].0 = idx + 1; // now it's fine to use `.0`
            stack.pop();
        }

        stack.push(idx);
    }
    todo!()
}

我是不是遗漏了什么，或者这是编译器中的错误？

更新

@ Ömer Erden发现，它无法推断出应该是 usize 的 pos 类型，因为它间接来自 enumerate()。

这看起来很奇怪，因为它在执行 widths[pos].1 = idx; 时无法推断出 pos 的类型，但在执行 widths 时却成功了[pos] = (0, idx);

He also managed to create more minimal MCVE:

fn _some_fn() {
    let mut widths = vec![(0usize, 0usize); 100];
    let mut stack = vec![];
    //let mut stack:Vec<usize> = vec![]; //why explicit type definition needed

    let a = stack.pop().unwrap();
    let idx = 0usize;
    widths[a].1 = idx;

    stack.push(idx);
}

错误报告

https://github.com/rust-lang/rust/issues/90252

Answer 1

我不知道为什么编译器无法省略类型，但您可以通过一些简单的更改手动注释它:

pub fn largest_rectangle_area(heights: Vec<i32>) -> i32 {
    let mut widths = vec![(0usize, heights.len()); heights.len()];
    let mut stack = vec![];

    for (idx, &x) in heights.iter().enumerate() {
        while let Some(&pos) = stack.last() {
            if x >= heights[pos] {
                break;
            }
            let at_pos: &mut (usize, usize) = &mut widths[pos];
            at_pos.1 = idx;
            stack.pop();
        }

        stack.push(idx);
    }

    todo!()
}

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