Prefer type annotations and returns in free region errors

Author: matthewjasper

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

@@ -138,6 +138,8 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 | ConstraintCategory::Boring
                 | ConstraintCategory::BoringNoLocation
                 | ConstraintCategory::Internal => false,
+                ConstraintCategory::TypeAnnotation
+                | ConstraintCategory::Return => true,
                 _ => constraint_sup_scc != target_scc,
             }
         });

src/test/ui/nll/where_clauses_in_functions.stderr

@@ -6,7 +6,7 @@ LL | fn bar<'a, 'b>(x: &'a u32, y: &'b u32) -> (&'a u32, &'b u32) {
    |        |
    |        lifetime `'a` defined here
 LL |     foo(x, y)
-   |     ^^^^^^^^^ argument requires that `'a` must outlive `'b`
+   |     ^^^^^^^^^ function was supposed to return data with lifetime `'b` but it is returning data with lifetime `'a`
 
 error: aborting due to previous error