Add better code comments for peculiarities

Author: Jamesbarford

compiler/rustc_middle/src/ty/context.rs

@@ -1730,9 +1730,7 @@ impl<'a, 'tcx> Lift<TyCtxt<'tcx>> for Interned<'a, RegionKind<'a>> {
     }
 }
 
-// `rustc_type_ir::Const<I>` is only the generic wrapper; lifting it delegates
-// to `I::InternedConstKind`, so the concrete interned const representation
-// must itself implement `Lift`.
+// FIXME: unclear why exactly the macro doesn't work.
 impl<'a, 'tcx> Lift<TyCtxt<'tcx>> for Interned<'a, WithCachedTypeInfo<ConstKind<'a>>> {
     type Lifted = Interned<'tcx, WithCachedTypeInfo<ConstKind<'tcx>>>;
 

compiler/rustc_type_ir/src/interner.rs

@@ -177,6 +177,10 @@ pub trait Interner:
     type Symbol: Symbol<Self>;
 
     // Things stored inside of tys
+
+    // `TypeVisitable<Self>` is required to prevent a problem where the flags
+    // on `Const` indicated that an error was present, as the visitor could no
+    // longer reach the corresponding ErrorGuaranteed
     type ErrorGuaranteed: Copy + Debug + Hash + Eq + TypeVisitable<Self>;
     type BoundExistentialPredicates: BoundExistentialPredicates<Self>;
     type AllocId: Copy + Debug + Hash + Eq;