help, pass closure variable to Ref::map function failed

[code-brewer]

The demo code is:

#![feature(refcell_map_split)]
use std::cell::{Ref, RefCell};    

fn main() { 
    let cell = RefCell::new([1, 2, 3, 4,5]);
    let borrow = cell.borrow(); 
     //    let (begin, end) = Ref::map_split(borrow, |s| s.split_at(3));   // OK

    // extract closure as a variable then pass as argument, raise error.
    let lambda = |s: &[i32;5]| s.split_at(3);
    let (begin, end) = Ref::map_split(borrow, lambda);

    assert_eq!(*begin, [1, 2, 3]);
    assert_eq!(*end, [4, 5]);
}

It complains error when run:

error[E0271]: type mismatch resolving `for<'r> <[closure@examples/dddd.rs:10:18: 10:45] as std::ops::FnOnce<(&'r [i32; 5],)>>::Output == (&'r _, &'r _)`
  --> examples/dddd.rs:11:24
   |
11 |     let (begin, end) = Ref::map_split(borrow, lambda);
   |                        ^^^^^^^^^^^^^^ expected bound lifetime parameter, found concrete lifetime
   |
   = note: required by `<std::cell::Ref<'b, T>>::map_split`

How to fix this while keep passing closure as an argument to Ref::map_split ?

[jonas-schievink]

This works:

#![feature(refcell_map_split)]
use std::cell::{Ref, RefCell};    

fn main() { 
    let cell = RefCell::new([1, 2, 3, 4,5]);
    let borrow = cell.borrow(); 
 //    let (begin, end) = Ref::map_split(borrow, |s| s.split_at(3));   // OK

// extract closure as a variable then pass as argument, raise error.
let lambda: fn(&[i32; 5]) -> (&[i32], &[i32]) = |s: &[i32;5]| s.split_at(3);
let (begin, end) = Ref::map_split(borrow, lambda);

assert_eq!(*begin, [1, 2, 3]);
assert_eq!(*end, [4, 5]);
}

But the current error messages around function and closure type mismatches are very bad.

[estebank]

The current error no longer mentions

   = note: required by `<std::cell::Ref<'b, T>>::map_split`

[estebank]

Current output:

error: lifetime may not live long enough
  --> src/main.rs:10:28
   |
10 | let lambda = |s: &[i32;5]| s.split_at(3);
   |                  -       - ^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'2`
   |                  |       |
   |                  |       return type of closure is (&'2 [i32], &[i32])
   |                  let's call the lifetime of this reference `'1`

error[E0308]: mismatched types
  --> src/main.rs:11:20
   |
11 | let (begin, end) = Ref::map_split(borrow, lambda);
   |                    ^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected tuple `(&[i32], &[i32])`
              found tuple `(&[i32], &[i32])`