Begin nightly-ifying rustc_type_ir

compiler/rustc_index/src/vec/tests.rs

@@ -1,7 +1,7 @@
 // Allows the macro invocation below to work
 use crate as rustc_index;
 
-rustc_macros::newtype_index! {
+crate::newtype_index! {
     #[max = 0xFFFF_FFFA]
     struct MyIdx {}
 }