remove the unused ConstraintCategory::ClosureBounds

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -50,7 +50,6 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
             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 ",

compiler/rustc_middle/src/mir/query.rs

@@ -241,11 +241,6 @@ pub enum ConstraintCategory<'tcx> {
         unsize_to: Option<Ty<'tcx>>,
     },
 
-    /// A constraint that came from checking the body of a closure.
-    ///
-    /// We try to get the category that the closure used when reporting this.
-    ClosureBounds,
-
     /// Contains the function type if available.
     CallArgument(#[derive_where(skip)] Option<Ty<'tcx>>),
     CopyBound,