Comment updates

Author: oli-obk

compiler/rustc_codegen_gcc/src/common.rs

@@ -283,6 +283,9 @@ impl<'gcc, 'tcx> ConstCodegenMethods for CodegenCx<'gcc, 'tcx> {
                     }
                     GlobalAlloc::TypeId { .. } => {
                         let val = self.const_usize(offset.bytes());
+                        // This is still a variable of pointer type, even though we only use the provenance
+                        // of that pointer in CTFE and Miri. But to make LLVM's type system happy,
+                        // we need an int-to-ptr cast here (it doesn't matter at all which provenance that picks).
                         return self.context.new_cast(None, val, ty);
                     }
                     GlobalAlloc::Static(def_id) => {

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -93,11 +93,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.write_type_id(tp_ty, dest)?;
             }
             sym::type_id_eq => {
+                // Both operands are `TypeId`, which is a newtype around an array of pointers.
+                // Project until we have the array elements.
                 let a_fields = self.project_field(&args[0], FieldIdx::ZERO)?;
                 let b_fields = self.project_field(&args[1], FieldIdx::ZERO)?;
 
                 let mut a_fields = self.project_array_fields(&a_fields)?;
-
                 let mut b_fields = self.project_array_fields(&b_fields)?;
 
                 let (_idx, a) = a_fields