如何在返回类型中使用带有匿名闭包的更高等级特征边界

Question

Is it possible to return FnMut closure that takes reference and return reference with same lifetime as it takes?

fn fun(buf: &mut [f32], mut idx: usize) -> impl FnMut(&[i16]) -> &[i16] {
    |input| {
        buf[idx] = input[0] as f32;
        idx += 1;
        &input[1..]
    }
}

I've tried things like impl for<'a> FnMut(&'a [i16]) -> &'a [i16]) and it gives

error[E0482]: lifetime of return value does not outlive the function call
 --> src/main.rs:1:44
  |
1 | fn fun(buf: &mut [f32], mut idx: usize) -> impl for<'a> FnMut(&'a [i16]) -> &'a [i16] {
  |                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: the return value is only valid for the anonymous lifetime defined on the function body at 1:13
 --> src/main.rs:1:13
  |
1 | fn fun(buf: &mut [f32], mut idx: usize) -> impl for<'a> FnMut(&'a [i16]) -> &'a [i16] {
  |             ^^^^^^^^^^

Answer 1

• The returned function should capture buf by value (ie, use move )
• The returned function must not outlive buf (having the lifetime 'buf in the below snippet):

So:

fn fun<'buf>(buf: &'buf mut [f32], mut idx: usize) -> impl FnMut(&[i16]) -> &[i16] + 'buf {
    move |input| {
        buf[idx] = input[0] as f32;
        idx += 1;
        &input[1..]
    }
}