Make MIR pass name a compile-time constant.

compiler/rustc_mir_transform/src/lib.rs

@@ -2,9 +2,11 @@
 #![deny(rustc::diagnostic_outside_of_impl)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
+#![feature(const_type_name)]
 #![feature(cow_is_borrowed)]
 #![feature(decl_macro)]
 #![feature(impl_trait_in_assoc_type)]
+#![feature(inline_const)]
 #![feature(is_sorted)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]