Reduce boilerplate around infallible folders

compiler/rustc_const_eval/src/lib.rs

@@ -23,7 +23,6 @@ Rust MIR: a lowered representation of Rust.
 #![feature(trusted_len)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
-#![feature(unwrap_infallible)]
 #![recursion_limit = "256"]
 
 #[macro_use]

compiler/rustc_const_eval/src/transform/validate.rs

@@ -94,8 +94,7 @@ pub fn equal_up_to_regions(
                 // Leave consts and types unchanged.
                 ct_op: |ct| ct,
                 ty_op: |ty| ty,
-            })
-            .into_ok(),
+            }),
         )
     };
     tcx.infer_ctxt().enter(|infcx| infcx.can_eq(param_env, normalize(src), normalize(dest)).is_ok())