Rollup merge of rust-lang#114558 - Enselic:lifetime-diagnostic-fixed, r=cjgillot

Remove FIXME about NLL diagnostic that is already improved

The FIXME was added in rust-lang#46984 when the diagnostic message looked like this:

    // FIXME(rust-lang#46983): error message should be better
    &s.0 //~ ERROR free region `` does not outlive free region `'static`

The message was improved in rust-lang#90667 and now looks like this:

    &s.0 //~ ERROR lifetime may not live long enough

but the FIXME was not removed. The issue rust-lang#46983 about that diagnostics should be improved has been closed. We can remove the FIXME now.

(This PR was made for rust-lang#44366.)

Author: matthiaskrgr

tests/ui/nll/guarantor-issue-46974.rs

@@ -9,7 +9,6 @@ fn foo(s: &mut (i32,)) -> i32 {
 }
 
 fn bar(s: &Box<(i32,)>) -> &'static i32 {
-    // FIXME(#46983): error message should be better
     &s.0 //~ ERROR lifetime may not live long enough
 }
 

tests/ui/nll/guarantor-issue-46974.stderr

@@ -10,11 +10,10 @@ LL |     *x
    |     -- borrow later used here
 
 error: lifetime may not live long enough
-  --> $DIR/guarantor-issue-46974.rs:13:5
+  --> $DIR/guarantor-issue-46974.rs:12:5
    |
 LL | fn bar(s: &Box<(i32,)>) -> &'static i32 {
    |           - let's call the lifetime of this reference `'1`
-LL |     // FIXME(#46983): error message should be better
 LL |     &s.0
    |     ^^^^ returning this value requires that `'1` must outlive `'static`