Poor labels for anonymous lifetimes in diagnostics

[jsgf]

Given this (incorrect) code:

use std::collections::HashMap;

type MyMap<'a, 'b> = HashMap<&'a str, &'b str>;

fn foo(iter: &mut Iterator<Item=&str>, mut map: MyMap) {
    map.insert(iter.next().unwrap(), "value");
}

fn main() {
}

rustc reports:

   Compiling playground v0.0.1 (file:///playground)
error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> src/main.rs:6:28
  |
6 |     map.insert(iter.next().unwrap(), "value");
  |                            ^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the function body at 5:1...
 --> src/main.rs:5:1
  |
5 | / fn foo(iter: &mut Iterator<Item=&str>, mut map: MyMap) {
6 | |     map.insert(iter.next().unwrap(), "value");
7 | | }
  | |_^
note: ...so that types are compatible (expected std::option::Option<&str>, found std::option::Option<&str>)
 --> src/main.rs:6:28
  |
6 |     map.insert(iter.next().unwrap(), "value");
  |                            ^^^^^^
note: but, the lifetime must be valid for the anonymous lifetime #3 defined on the function body at 5:1...
 --> src/main.rs:5:1
  |
5 | / fn foo(iter: &mut Iterator<Item=&str>, mut map: MyMap) {
6 | |     map.insert(iter.next().unwrap(), "value");
7 | | }
  | |_^
note: ...so that reference does not outlive borrowed content
 --> src/main.rs:6:16
  |
6 |     map.insert(iter.next().unwrap(), "value");
  |                ^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

error: Could not compile `playground`.

The lifetime #1 notation is not very clear or helpful. Assuming it's referring to the lifetimes of the reference parameters, it would be better if it gave them names and used those, along the lines of:

Given:
fn foo<'a, 'b, 'c, 'd>(iter: &'a mut Iterator<Item=&'b str>, mut map: MyMap<'c, 'd>) {

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> src/main.rs:6:28
  |
6 |     map.insert(iter.next().unwrap(), "value");
  |                            ^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime 'b defined on the function body at 5:1...

(etc)

[estebank]

Current output:

error[E0623]: lifetime mismatch
 --> src/main.rs:6:16
  |
5 | fn foo(iter: &mut Iterator<Item=&str>, mut map: MyMap) {
  |                                 ----            -----
  |                                 |
  |                                 these two types are declared with different lifetimes...
6 |     map.insert(iter.next().unwrap(), "value");
  |                ^^^^^^^^^^^^^^^^^^^^ ...but data from `iter` flows into `map` here

The correct code would be

5 | fn foo<'a, 'b>(iter: &mut Iterator<Item=&'a str>, mut map: MyMap<'a, 'b>) {

[Mark-Simulacrum]

I personally think that the current diagnostic is good enough here so I'm going to close this; the only improvement I could think of would be to indicate that MyMap has lifetimes attached to it via MyMap<'1, '2> but I believe we're already tracking that elsewhere and it'll presumably become increasingly common to specify them implicitly via '_ which'll make the diagnostic even clearer.