Fix lifetime error to print 'a instead of &'a

[ftxqxd]

This changes certain error messages about lifetimes so that they display lifetimes without an &.

Fixes #10291.

[lilyball]

.to_strbuf() here is unnecessary allocation.

[lilyball]

Do we really want to print '<anon> everywhere? That seems likely to be unnecessarily noisy. I feel like we should only be printing anonymous lifetimes when there's a lifetime mismatch, and we should be generating unique names for them then (to stress that they're different).

[ftxqxd]

@kballard: I think it would indeed be better to only print it on errors relating to lifetimes. The only reason I didn’t do so was because I thought it would be slightly more complex to implement, and I wanted to see others’ opinion on this.

Regarding naming, originally in my code I made it print something like &'#1 instead of &<anon> to show that they are different — do you think that I could reinstate that? Anonymous lifetimes are given numbers in other errors, too (not sure which ones, but I’ve seen it before & it's in the source).

[lilyball]

Something like &'#1 seems better, but again, I think it only makes sense to print this when it's important to indicate that the lifetime of two references is different. As opposed to what you're doing right now, which I assume is unconditionally printing the anonymous lifetime for all references.

[alexcrichton]

I agree that these error messages have predominately taken a step backwards. If this is fixing a problem where the compiler says it found &int but expected &int, is this making it clearer by printing the same string as the lifetime?

[ftxqxd]

@alexcrichton I’m not sure I understand what you mean by “the same string as the lifetime”.

As I’m still finding my way around the compiler code, I’m struggling to locate the actual code relating to the lifetime errors so that I can specialise the display of anonymous lifetimes for them only. Given that the naming of anonymous lifetimes isn’t actually part of #10291 (it only properly concerns a confusing & in an error message), and the indecision relating to how anonymous lifetimes should be displayed, would it be OK if I could simply omit that bug from this PR and leave that for a later date if I don’t find a solution soon? (Perhaps a separate issue could also be created in that case regarding the anonymous lifetime error message confusion.)

[alexcrichton]

In the compiler message where it says that it found &int but expected &int (I'm still not sure if this is exactly what you're fixing), the new message would be that it found &'<anon> int but expected &'<anon> int which doesn't seem that much different than before.

It'd be fine to leave out '<anon> from this PR for a later date.

[ftxqxd]

I’ve updated the PR to remove the code about anonymous lifetimes. (I also fixed a typo in the title/commit message.)

[alexcrichton]

Would it be possible to add a test for this as well? (testing the error message)

[ftxqxd]

I’ve added a test, and it passes locally, but I’m not sure if it’s extensive enough. Should that be good enough?