Lifetime error messages should have better spans

[mdinger]

This error has an arrow pointing where the lifetime issue is (though I don't know why it isn't more precise):

struct Mine<'a> { s: &'a str }

impl <'a>Iterator<&'a str> for Mine<'a> {
    fn next(&mut self) -> Option<&str> { Some("h") }
}

fn main() {}

<anon>:4:5: 4:53 error: method `next` has an incompatible type for trait: expected concrete lifetime, found bound lifetime parameter  [E0053]
<anon>:4     fn next(&mut self) -> Option<&str> { Some("h") }
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:4:40: 4:53 note: expected concrete lifetime is the lifetime 'a as defined on the block at 4:39
<anon>:4     fn next(&mut self) -> Option<&str> { Some("h") }
                                                ^~~~~~~~~~~~~
error: aborting due to previous error
playpen: application terminated with error code 101

---

This won't because next() is multiline:

struct Mine<'a> { s: &'a str }

impl <'a>Iterator<&'a str> for Mine<'a> {
    fn next(&mut self) -> Option<&str> {
        Some("h")
    }
}

fn main() {}

<anon>:4:5: 6:6 error: method `next` has an incompatible type for trait: expected concrete lifetime, found bound lifetime parameter  [E0053]
<anon>:4     fn next(&mut self) -> Option<&str> {
<anon>:5         Some("h")
<anon>:6     }
<anon>:4:40: 6:6 note: expected concrete lifetime is the lifetime 'a as defined on the block at 4:39
<anon>:4     fn next(&mut self) -> Option<&str> {
<anon>:5         Some("h")
<anon>:6     }
error: aborting due to previous error
playpen: application terminated with error code 101

---

Best would be:

<anon>:4:40: 4:53 note: expected concrete lifetime is the lifetime 'a as defined on the block at 4:39
<anon>:4     fn next(&mut self) -> Option<&str> { Some("h") }
                                           ^~~~~~~~~~~~~

Someone might easily put the lifetime on next(&'a self) instead of Option<&'a str> because it didn't point and they didn't understand the message.

[steveklabnik]

Triage: updated code:

struct Mine<'a> { s: &'a str }

impl <'a>Iterator for Mine<'a> {
    type Item = &'a str;
    fn next(&mut self) -> Option<&str> { Some("h") }
}

fn main() {}

same error, same problem.

[estebank]

Current output (updated 2018-02-11):

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 5:5...
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...so that the method type is compatible with trait:
          expected fn(&mut Mine<'a>) -> std::option::Option<&str>
             found fn(&mut Mine<'a>) -> std::option::Option<&str>
note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 3:1...
 --> src/main.rs:3:1
  |
3 | impl <'a>Iterator for Mine<'a> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...so that the types are compatible:
          expected std::iter::Iterator
             found std::iter::Iterator

---

This case should look more like the following:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: the method `next` is defined in the trait as returning `Option<Self::Item>`, which is defined in the impl as `&'a str`...
 --> src/main.rs:4:5
  |
4 |     type Item = &'a str;
  |     ^^^^^^^^^^^^^^^^^^^^
note: ...but the implementation of `next` returns `Option<&str>`:
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |                           ^^^^^^^^^^^^
help: change the return type for `next` to match the trait:
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&'a str> { Some("h") }
  |                           ^^^^^^^^^^^^^^^

[estebank]

Current output, few changes:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in generic type due to conflicting requirements
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime defined here...
 --> src/main.rs:5:13
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |             ^^^^^^^^^
note: ...so that the method type is compatible with trait
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: expected `fn(&mut Mine<'a>) -> Option<&str>`
             found `fn(&mut Mine<'a>) -> Option<&str>`
note: but, the lifetime must be valid for the lifetime `'a` as defined here...
 --> src/main.rs:3:7
  |
3 | impl <'a>Iterator for Mine<'a> {
  |       ^^
note: ...so that the types are compatible
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: expected `<Mine<'a> as Iterator>`
             found `<Mine<'_> as Iterator>`

[estebank]

Current output:

error: `impl` item signature doesn't match `trait` item signature
 --> src/main.rs:5:5
  |
5 |     fn next(&mut self) -> Option<&str> { Some("h") }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 mut Mine<'a>) -> Option<&'1 str>`
 --> /rustc/42b1224e9eb37177f608d3f6a6f2be2ee13902e4/library/core/src/iter/traits/iterator.rs:112:5
  |
  = note: expected `fn(&'1 mut Mine<'a>) -> Option<&'a str>`
  |
  = note: expected signature `fn(&'1 mut Mine<'a>) -> Option<&'a str>`
             found signature `fn(&'1 mut Mine<'a>) -> Option<&'1 str>`
  = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
  = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output