Simplify two matches.

Matches involving `GenericArgKind` pairs typically use a single `_` for
the impossible case. This commit shortens two verbose matches in this
way.

Author: nnethercote

compiler/rustc_infer/src/infer/at.rs

@@ -381,21 +381,7 @@ impl<'tcx> ToTrace<'tcx> for ty::GenericArg<'tcx> {
                 (GenericArgKind::Const(a), GenericArgKind::Const(b)) => {
                     ValuePairs::Terms(ExpectedFound::new(true, a.into(), b.into()))
                 }
-
-                (
-                    GenericArgKind::Lifetime(_),
-                    GenericArgKind::Type(_) | GenericArgKind::Const(_),
-                )
-                | (
-                    GenericArgKind::Type(_),
-                    GenericArgKind::Lifetime(_) | GenericArgKind::Const(_),
-                )
-                | (
-                    GenericArgKind::Const(_),
-                    GenericArgKind::Lifetime(_) | GenericArgKind::Type(_),
-                ) => {
-                    bug!("relating different kinds: {a:?} {b:?}")
-                }
+                _ => bug!("relating different kinds: {a:?} {b:?}"),
             },
         }
     }

compiler/rustc_middle/src/ty/relate.rs

@@ -212,15 +212,7 @@ impl<'tcx> Relate<TyCtxt<'tcx>> for ty::GenericArg<'tcx> {
             (ty::GenericArgKind::Const(a_ct), ty::GenericArgKind::Const(b_ct)) => {
                 Ok(relation.relate(a_ct, b_ct)?.into())
             }
-            (ty::GenericArgKind::Lifetime(unpacked), x) => {
-                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
-            }
-            (ty::GenericArgKind::Type(unpacked), x) => {
-                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
-            }
-            (ty::GenericArgKind::Const(unpacked), x) => {
-                bug!("impossible case reached: can't relate: {:?} with {:?}", unpacked, x)
-            }
+            _ => bug!("impossible case reached: can't relate: {a:?} with {b:?}"),
         }
     }
 }