Error messages about lifetimes for mutable Iterator are cryptic.

Example: http://is.gd/8dVXWc

<anon>:42:14: 42:33 error: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
<anon>:42         Some(&mut self.cont.item)
                       ^~~~~~~~~~~~~~~~~~~
<anon>:39:5: 43:6 help: consider using an explicit lifetime parameter as shown: fn next(&'a mut self) -> Option<&'a mut V>
<anon>:39     fn next(&mut self) -> Option<&'a mut V> {
<anon>:40         if self.emitted { return None }
<anon>:41         self.emitted = true;
<anon>:42         Some(&mut self.cont.item)
<anon>:43     }
error: aborting due to previous error
playpen: application terminated with error code 101

Compilation failed.

@bluss gave me a helpful link on IRC, stating that is a classic thing.

I think for every "classic" problem there should be a lint giving a link to some article about the problem.

In this case the link can be emitted for any lifetimes problem when mutable references and Iterator trait involved together.

For reference: link to changed version that does compile.

1. Current output:

   error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
     --> src/main.rs:42:14
      |
   42 |         Some(&mut self.cont.item)
      |              ^^^^^^^^^^^^^^^^^^^
      |
   note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 39:5...
     --> src/main.rs:39:5
      |
   39 | /     fn next(&mut self) -> Option<&'a mut V> {
   40 | |         if self.emitted { return None }
   41 | |         self.emitted = true;
   42 | |         Some(&mut self.cont.item)
   43 | |     }
      | |_____^
   note: ...so that reference does not outlive borrowed content
     --> src/main.rs:42:14
      |
   42 |         Some(&mut self.cont.item)
      |              ^^^^^^^^^^^^^^^^^^^
   note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 37:1...
     --> src/main.rs:37:1
      |
   37 | / impl<'a, V:'a> Iterator for MutableIter<'a,V> {
   38 | |     type Item = &'a mut V;
   39 | |     fn next(&mut self) -> Option<&'a mut V> {
   40 | |         if self.emitted { return None }
   ...  |
   43 | |     }
   44 | | }
      | |_^
   note: ...so that expression is assignable (expected std::option::Option<&'a mut V>, found std::option::Option<&mut V>)
     --> src/main.rs:42:9
      |
   42 |         Some(&mut self.cont.item)
      |         ^^^^^^^^^^^^^^^^^^^^^^^^^
   

   Esteban Kuber at 2018-01-28 22:24:52

2. It is better.

   Hovewer, Iterator-specific error message may still help (it should add to, no replace the current output).

   Vitaly Shukela at 2018-01-29 00:22:34

3. Current output:

   error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
     --> src/main.rs:42:14
      |
   42 |         Some(&mut self.cont.item)
      |              ^^^^^^^^^^^^^^^^^^^
      |
   note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 39:5...
     --> src/main.rs:39:5
      |
   39 | /     fn next(&mut self) -> Option<&'a mut V> {
   40 | |         if self.emitted { return None }
   41 | |         self.emitted = true;
   42 | |         Some(&mut self.cont.item)
   43 | |     }
      | |_____^
   note: ...so that reference does not outlive borrowed content
     --> src/main.rs:42:14
      |
   42 |         Some(&mut self.cont.item)
      |              ^^^^^^^^^^^^^^^^^^^
   note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 37:1...
     --> src/main.rs:37:1
      |
   37 | impl<'a, V:'a> Iterator for MutableIter<'a,V> {
      | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      = note: ...so that the expression is assignable:
              expected std::option::Option<&'a mut V>
                 found std::option::Option<&mut V>
   

   Esteban Kuber at 2018-02-12 05:27:28

4. Triage: no change.

   Esteban Kuber at 2020-01-22 18:52:35

5. Current output, few changes:

   error[E0495]: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements
     --> src/main.rs:42:14
      |
   42 |         Some(&mut self.cont.item)
      |              ^^^^^^^^^^^^^^^^^^^
      |
   note: first, the lifetime cannot outlive the anonymous lifetime defined here...
     --> src/main.rs:39:13
      |
   39 |     fn next(&mut self) -> Option<&'a mut V> {
      |             ^^^^^^^^^
   note: ...so that reference does not outlive borrowed content
     --> src/main.rs:42:14
      |
   42 |         Some(&mut self.cont.item)
      |              ^^^^^^^^^^^^^^^^^^^
   note: but, the lifetime must be valid for the lifetime `'a` as defined here...
     --> src/main.rs:37:6
      |
   37 | impl<'a, V:'a> Iterator for MutableIter<'a,V> {
      |      ^^
   note: ...so that the types are compatible
     --> src/main.rs:42:9
      |
   42 |         Some(&mut self.cont.item)
      |         ^^^^^^^^^^^^^^^^^^^^^^^^^
      = note: expected `Option<&'a mut V>`
                 found `Option<&mut V>`
   

   Esteban Kuber at 2022-04-23 01:27:30

6. Current output

   error: lifetime may not live long enough
     --> src/main.rs:42:9
      |
   37 | impl<'a, V:'a> Iterator for MutableIter<'a,V> {
      |      -- lifetime `'a` defined here
   38 |     type Item = &'a mut V;
   39 |     fn next(&mut self) -> Option<&'a mut V> {
      |             - let's call the lifetime of this reference `'1`
   ...
   42 |         Some(&mut self.cont.item)
      |         ^^^^^^^^^^^^^^^^^^^^^^^^^ associated function was supposed to return data with lifetime `'a` but it is returning data with lifetime `'1`
   

   Esteban Kuber at 2022-12-28 22:55:32