Confusing "expected reference, found ..." error when trying to chain Iterator<T> and Iterator<&T>

[jstasiak]

I'm running rustc 1.40.0.

The code:

fn main() {
    let a = (0..3).chain([3, 4, 5].iter());
}

Trying to compile it results in:

% rustc test1.rs           
error[E0271]: type mismatch resolving `<std::slice::Iter<'_, {integer}> as std::iter::IntoIterator>::Item == {integer}`
 --> test1.rs:2:20
  |
2 |     let a = (0..3).chain([3, 4, 5].iter());
  |                    ^^^^^ expected reference, found integer
  |
  = note: expected type `&{integer}`
             found type `{integer}`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.

If one focues one the ^^^^^ expected reference, found integer line (as I did) one is faced with the following difficulties:

1. It's the chain identifier that's underlined/pointed towards here which is (to me) confusing
2. If I follow the "expected reference, found integer" advice naively and without trying to get to the bottom of things I may think "ok, the problem is with the chain call, it expects references but I give it integers, let's add references" and produce the following:

fn main() {
    let a = (0..3).chain([&3, &4, &5].iter());
}

And the error gets even more confusing:

% rustc test2.rs       
error[E0271]: type mismatch resolving `<std::slice::Iter<'_, &{integer}> as std::iter::IntoIterator>::Item == {integer}`
 --> test2.rs:2:20
  |
2 |     let a = (0..3).chain([&3, &4, &5].iter());
  |                    ^^^^^ expected reference, found integer
  |
  = note: expected type `&&{integer}`
             found type `{integer}`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.

That attempt to fix things was of course totally incorrect, but the error messages don't help here.

[sugar700]

The issue is caused by item types in iterators being different {integer} vs &{integer}, but there are so many different layers involved it would be tricky to improve this error message.

Ideally, Rust would suggest using copied if something requires impl Iterator<Item = T> but got impl Iterator<Item = &T> where T: Copy.

[enterprisey]

I ran into this today with this code (also below), which also mistakenly highlights the chain:

fn main() {
    let _ = [1].iter().chain(vec![1]);
}

I think it would be less confusing to highlight anything else in the expression besides the chain. Ideally, in this case, I would like the argument of the chain to be highlighted (in which case the expected/found order would flip to "expected &{integer}, found {integer}"), but highlighting the slice would also work.

[estebank]

Current output for the original report is better now (note the note):

error[E0271]: type mismatch resolving `<Iter<'_, {integer}> as IntoIterator>::Item == {integer}`
 --> src/main.rs:2:26
  |
2 |     let a = (0..3).chain([3, 4, 5].iter());
  |                    ----- ^^^^^^^^^^^^^^^^ expected integer, found `&{integer}`
  |                    |
  |                    required by a bound introduced by this call
  |
note: the method call chain might not have had the expected associated types
 --> src/main.rs:2:36
  |
2 |     let a = (0..3).chain([3, 4, 5].iter());
  |                          --------- ^^^^^^ `IntoIterator::Item` is `&{integer}` here
  |                          |
  |                          this expression has type `[{integer}; 3]`
note: required by a bound in `std::iter::Iterator::chain`
 --> /rustc/f3126500f25114ba4e0ac3e76694dd45a22de56d/library/core/src/iter/traits/iterator.rs:505:5

While the case in the previous comment is unchanged

error[E0271]: type mismatch resolving `<Vec<{integer}> as IntoIterator>::Item == &{integer}`
 --> src/main.rs:2:30
  |
2 |     let _ = [1].iter().chain(vec![1]);
  |                        ----- ^^^^^^^ expected `&{integer}`, found integer
  |                        |
  |                        required by a bound introduced by this call
  |
note: required by a bound in `std::iter::Iterator::chain`
 --> /rustc/f3126500f25114ba4e0ac3e76694dd45a22de56d/library/core/src/iter/traits/iterator.rs:505:5