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.

compiler/rustc_borrowck/src/constraints/mod.rs

@@ -115,13 +115,13 @@ impl<'tcx> fmt::Debug for OutlivesConstraint<'tcx> {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "OutlivesConstraintIndex({})"]
     pub struct OutlivesConstraintIndex {
-        DEBUG_FORMAT = "OutlivesConstraintIndex({})"
     }
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "ConstraintSccIndex({})"]
     pub struct ConstraintSccIndex {
-        DEBUG_FORMAT = "ConstraintSccIndex({})"
     }
 }

compiler/rustc_borrowck/src/dataflow.rs

@@ -108,8 +108,8 @@ impl_visitable! {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "bw{}"]
     pub struct BorrowIndex {
-        DEBUG_FORMAT = "bw{}"
     }
 }
 

compiler/rustc_borrowck/src/location.rs

@@ -20,8 +20,8 @@ pub struct LocationTable {
 }
 
 rustc_index::newtype_index! {
+    #[debug_format = "LocationIndex({})"]
     pub struct LocationIndex {
-        DEBUG_FORMAT = "LocationIndex({})"
     }
 }
 

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({})"
     }
 }
 

compiler/rustc_borrowck/src/region_infer/values.rs

@@ -90,12 +90,14 @@ impl RegionValueElements {
 rustc_index::newtype_index! {
     /// A single integer representing a `Location` in the MIR control-flow
     /// graph. Constructed efficiently from `RegionValueElements`.
-    pub struct PointIndex { DEBUG_FORMAT = "PointIndex({})" }
+    #[debug_format = "PointIndex({})"]
+    pub struct PointIndex {}
 }
 
 rustc_index::newtype_index! {
     /// A single integer representing a `ty::Placeholder`.
-    pub struct PlaceholderIndex { DEBUG_FORMAT = "PlaceholderIndex({})" }
+    #[debug_format = "PlaceholderIndex({})"]
+    pub struct PlaceholderIndex {}
 }
 
 /// An individual element in a region value -- the value of a