Remove FIXME after rebase and add test

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -675,12 +675,6 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         if self.to_error_region(outlived_fr) != Some(tcx.lifetimes.re_static) {
             return;
         }
-        // FIXME: there's a case that's yet to be handled: `impl dyn Trait + '_ where Self: '_`
-        // causes *two* errors to be produded, one about `where Self: '_` not being allowed,
-        // and the regular error with no additional information about "lifetime may not live
-        // long enough for `'static`" without mentioning where it came from. This is because
-        // our error recovery fallback is indeed `ReStatic`. We should at some point introduce
-        // a `ReError` instead to avoid this and other similar issues.
 
         // Look for `'static` bounds in the generics of the method and the `impl`.
         // ```

tests/ui/lifetimes/static-impl-obligation.rs

@@ -276,4 +276,16 @@ mod w {
         y.hello(); //~ ERROR lifetime may not live long enough
     }
 }
+mod x {
+    trait Foo {}
+    impl<'a> Foo for &'a u32 {}
+    impl dyn Foo + '_ where Self: '_ { //~ ERROR `'_` cannot be used here
+        fn hello(&self) {}
+
+    }
+    fn convert<'a>(x: &'a &'a u32) {
+        let y: &dyn Foo = x; //~ ERROR lifetime may not live long enough
+        y.hello();
+    }
+}
 fn main() {}

tests/ui/lifetimes/static-impl-obligation.stderr

@@ -1,3 +1,9 @@
+error[E0637]: `'_` cannot be used here
+  --> $DIR/static-impl-obligation.rs:282:35
+   |
+LL |     impl dyn Foo + '_ where Self: '_ {
+   |                                   ^^ `'_` is a reserved lifetime name
+
 error[E0478]: lifetime bound not satisfied
   --> $DIR/static-impl-obligation.rs:222:10
    |
@@ -453,7 +459,15 @@ note: the `impl` on `dyn w::Foo` has a `'static` lifetime requirement
 LL |     trait Trait where Self: 'static { fn hello(&self) {} }
    |                             ^^^^^^^
 
-error: aborting due to 25 previous errors
+error: lifetime may not live long enough
+  --> $DIR/static-impl-obligation.rs:287:27
+   |
+LL |     fn convert<'a>(x: &'a &'a u32) {
+   |                -- lifetime `'a` defined here
+LL |         let y: &dyn Foo = x;
+   |                           ^ cast requires that `'a` must outlive `'static`
+
+error: aborting due to 27 previous errors
 
-Some errors have detailed explanations: E0478, E0521.
+Some errors have detailed explanations: E0478, E0521, E0637.
 For more information about an error, try `rustc --explain E0478`.