NLL produces ungreat error messages with free regions

[sapphire-arches]

NLL produces the following ungreat diagnostic message:

error: free region `` does not outlive free region `'a`
   --> src/main.rs:124:17
    |
124 |             let a = self.img.sub_image();
    |                 ^

Note the completely unnamed free region. I'm not sure what a good solution to this is, but it's probably not what is produced right now =)

When compiling the code here https://play.rust-lang.org/?gist=b7d7c69a4eb38a108409bb34a0a6ae46&version=nightly

[sapphire-arches]

The unnamed region here is the automatic region created for the lifetime of the parameters.
Naming it as so:

 fn next<'b>(&'b mut self) -> Option<Self::Item> {

Produces the much better error message:

error: free region `'b` does not outlive free region `'gen_parent`
   --> src/main.rs:125:17
    |
125 |             let a = self.img.sub_image();
    |                 ^

[nikomatsakis]

@bobtwinkles yeah so the empty string comes from the fact that it's an anoymous region, but what worries me more is that we highlight such random code. This is particularly evident in #49493.

[nikomatsakis]

We have a lot of issues about this. I'm going to close this as a dup of #51027