"only named lifetimes allowed in impl Trait" could have a better error message

[Manishearth]

struct A {
    x: [(u32, u32); 10]
}

impl A {
    
    fn iter_values(&self) -> impl Iterator<Item=u32>{
        self.x.iter().map(|a| a.0)
    }
}

gives the error:

error[E0564]: only named lifetimes are allowed in `impl Trait`, but `` was found in the type `std::iter::Map<std::slice::Iter<'_, (u32, u32)>, [closure@src/main.rs:12:27: 12:34]>`
  --> src/main.rs:11:30
   |
11 |     fn iter_values(&self) -> impl Iterator<Item=u32>{
   |                              ^^^^^^^^^^^^^^^^^^^^^^^

The correct fix is fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> + 'a (or use into_iter, but that's not always an option), however this error message has no indication that this is the way to go in, in fact not many people are aware of the Trait + 'a syntax. We probably can do better.

[gnzlbg]

Looks like a duplicate of #38615

[estebank]

Current output:

error[E0495]: cannot infer an appropriate lifetime for autoref due to conflicting requirements
  --> src/main.rs:10:16
   |
10 |         self.x.iter().map(|a| a.0)
   |                ^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 9:5...
  --> src/main.rs:9:5
   |
9  | /     fn iter_values(&self) -> impl Iterator<Item=u32>{
10 | |         self.x.iter().map(|a| a.0)
11 | |     }
   | |_____^
note: ...so that reference does not outlive borrowed content
  --> src/main.rs:10:9
   |
10 |         self.x.iter().map(|a| a.0)
   |         ^^^^^^
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that return value is valid for the call
  --> src/main.rs:9:30
   |
9  |     fn iter_values(&self) -> impl Iterator<Item=u32>{
   |                              ^^^^^^^^^^^^^^^^^^^^^^^