Introduce constraint category for yields.

This commit adds a new `ConstraintCategory` for yield points - this
allows for differentiation between a normal return and a yield in the
diagnostics.

Author: davidtwco

src/librustc/ich/impls_mir.rs

@@ -503,6 +503,7 @@ impl_stable_hash_for!(struct mir::ClosureOutlivesRequirement<'tcx> {
 
 impl_stable_hash_for!(enum mir::ConstraintCategory {
     Return,
+    Yield,
     UseAsConst,
     UseAsStatic,
     TypeAnnotation,

src/librustc/mir/mod.rs

@@ -2894,6 +2894,7 @@ pub struct ClosureOutlivesRequirement<'tcx> {
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
 pub enum ConstraintCategory {
     Return,
+    Yield,
     UseAsConst,
     UseAsStatic,
     TypeAnnotation,

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

@@ -38,6 +38,7 @@ impl ConstraintDescription for ConstraintCategory {
         match self {
             ConstraintCategory::Assignment => "assignment ",
             ConstraintCategory::Return => "returning this value ",
+            ConstraintCategory::Yield => "yielding this value ",
             ConstraintCategory::UseAsConst => "using this value as a constant ",
             ConstraintCategory::UseAsStatic => "using this value as a static ",
             ConstraintCategory::Cast => "cast ",
@@ -133,11 +134,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
 
             match categorized_path[i].0 {
-                ConstraintCategory::OpaqueType
-                | ConstraintCategory::Boring
-                | ConstraintCategory::BoringNoLocation
-                | ConstraintCategory::Internal => false,
-                ConstraintCategory::TypeAnnotation | ConstraintCategory::Return => true,
+                ConstraintCategory::OpaqueType | ConstraintCategory::Boring |
+                ConstraintCategory::BoringNoLocation | ConstraintCategory::Internal => false,
+                ConstraintCategory::TypeAnnotation | ConstraintCategory::Return |
+                ConstraintCategory::Yield => true,
                 _ => constraint_sup_scc != target_scc,
             }
         });

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1467,7 +1467,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                             value_ty,
                             ty,
                             term_location.to_locations(),
-                            ConstraintCategory::Return,
+                            ConstraintCategory::Yield,
                         ) {
                             span_mirbug!(
                                 self,