Make `#[debug_format]` an attribute in `newtype_index`

This removes the `custom` format functionality as its only user was
trivially migrated to using a normal format.

If a new use case for a custom formatting impl pops up, you can add it
back.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/rustc_borrowck/src/member_constraints.rs b/compiler/rustc_borrowck/src/member_constraints.rs
index b5e00f471d2..ccf6fcac9d2 100644
--- a/compiler/rustc_borrowck/src/member_constraints.rs
+++ b/compiler/rustc_borrowck/src/member_constraints.rs
@@ -55,8 +55,8 @@ pub(crate) struct NllMemberConstraint<'tcx> {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "MemberConstraintIndex({})"]
     pub(crate) struct NllMemberConstraintIndex {
-        DEBUG_FORMAT = "MemberConstraintIndex({})"
     }
 }