Diagnostics for an immutable reference to an iterator

[kornelski]

This problem comes up in generic contexts, but the simplest code to reproduce is:

let iter = &[1,2,3].iter();
for i in iter {
}

Results in:

error[E0277]: &std::slice::Iter<'_, {integer}> is not an iterator

 | for i in iter {
 |          ^^^^ `&std::slice::Iter<'_, {integer}>` is not an iterator
 |

= help: the trait std::iter::Iterator is not implemented for &std::slice::Iter<'_, {integer}>
= note: required by std::iter::IntoIterator::into_iter

At first glance it looks wrong, because it says that Iter is not an iterator. It is not obvious that immutability of the shared reference is what breaks the code.

Interestingly, an only a slightly different code produces a better suggestion:

let iter = [1,2,3].iter();
for i in &iter {
}

help: consider removing 1 leading &-references

So possibly the condition for the help text is too narrow, and could be extended to work in general case. It could also suggest using &mut as another alternative.

(#37914 has a similar problem, but a different root cause)