Streamline ConstraintCategory::description.

Author: nnethercote

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -40,28 +40,25 @@ use crate::{MirBorrowckCtxt, borrowck_errors, fluent_generated as fluent};
 
 impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
     fn description(&self) -> &'static str {
+        use ConstraintCategory::*;
         // Must end with a space. Allows for empty names to be provided.
         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 { is_implicit_coercion: false, .. } => "cast ",
-            ConstraintCategory::Cast { is_implicit_coercion: true, .. } => "coercion ",
-            ConstraintCategory::CallArgument(_) => "argument ",
-            ConstraintCategory::TypeAnnotation => "type annotation ",
-            ConstraintCategory::ClosureBounds => "closure body ",
-            ConstraintCategory::SizedBound => "proving this value is `Sized` ",
-            ConstraintCategory::CopyBound => "copying this value ",
-            ConstraintCategory::OpaqueType => "opaque type ",
-            ConstraintCategory::ClosureUpvar(_) => "closure capture ",
-            ConstraintCategory::Usage => "this usage ",
-            ConstraintCategory::Predicate(_)
-            | ConstraintCategory::Boring
-            | ConstraintCategory::BoringNoLocation
-            | ConstraintCategory::Internal
-            | ConstraintCategory::IllegalUniverse => "",
+            Assignment => "assignment ",
+            Return(_) => "returning this value ",
+            Yield => "yielding this value ",
+            UseAsConst => "using this value as a constant ",
+            UseAsStatic => "using this value as a static ",
+            Cast { is_implicit_coercion: false, .. } => "cast ",
+            Cast { is_implicit_coercion: true, .. } => "coercion ",
+            CallArgument(_) => "argument ",
+            TypeAnnotation => "type annotation ",
+            ClosureBounds => "closure body ",
+            SizedBound => "proving this value is `Sized` ",
+            CopyBound => "copying this value ",
+            OpaqueType => "opaque type ",
+            ClosureUpvar(_) => "closure capture ",
+            Usage => "this usage ",
+            Predicate(_) | Boring | BoringNoLocation | Internal | IllegalUniverse => "",
         }
     }
 }